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posted on 19 December 2017

December 18, 2017 Weather and Climate Report - Cool?, Cooler?, Coolest?

Written by Sig Silber

There definitely was a pattern change. We are now waiting to see the extent to which Arctic Air will intrude into CONUS. Will it be just the Northern Tier or will it drop down into Texas?

The factors that will determine the outcome will be discussed in our report and the maps which auto-update may be useful for people with Holiday Travel Plans. I think you will find this report worth reading.

  Let it Snow https://pixabay.com/en/service/terms/#usage


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Some housekeeping: On Saturday November 18, 2018 we published our monthly Seasonal Outlook Update and you can access that here. Also on Friday December 2 we published the December Update and you can access it here. Remember if you leave this page to go to links or if you have clicked to enlarge a graphic, in order to get back you need to hit the return arrow usually located on the left of your URL box on the upper left of the screen. If you have additional suggestions for improvement, please make them in the comments section following the article.

 NOAA Snow Forecast looking ahead to Days 4,5 (top Row) ,6 and 7 (bottom row). When you view these graphics you can click on them to enlarge them.

Day 4 Day 5
Day 6 Snow Forecast Day 7 Snow

 

I think they are only showing large areas so locally there could be other areas especially at high elevation that also receive significant snowfall. The pattern is clearly across the northern tier of CONUS but extending to the south as the cool anomaly moves east. I  think I have now figured out how to have these images update. But if the issued date on the graphics do not match the date when you view them, you can obtain updated images plus short term snow forecasts here.

As we wondered last week there has been a change in the pattern.

Atmospheric Anomalies.

The High Pressure is now way north. It is kind of a blocking pattern. It may cause warmer than usual temperatures to the north but colder than usual along the northern tier of  CONUS. Winter is here!

A. Now we return to our  regular approach and focus on Alaska and CONUS (all U.S.. except Hawaii) 

I am starting with a summary first for temperature and then for precipitation of small images of the three short-term maps. You can click on these maps to see larger versions. The easiest way to return to this report is by using the "Back Arrow" usually found top left corner of your screen to the left of the URL Box. Larger maps are available later in the article with the discussion and analysis.

For most people, the summary with the small images will be sufficient. Later in the article for those with sufficient interest there is a full description of the factors determining the maps shown here with a detailed analysis of the ENSO situation which so dramatically impacts the forecasts below.

First Temperature

6 - 10 Day Forecast Temperature

Transitioning from the 6 to 10 day outlook on the left (also called Week One) to the 8 to 14 day outlook (Week Two) on the right         →

8 - 14 Day Temperature

Week One is a dramatic change from the Southwest being warm to being cold with the Northwest being very cold. The pattern progresses to the east in Week Two.

To the right is the week 3 and 4 Forecast.

There are now three warm anomalies and a Northeast/North Central cool anomaly. →

3 - 4 Week Temperature

                     ↑

←  The transition from the 8 -14 day forecast shown above to the week 3/4 shown to the left seems feasible.
 

 

And then Precipitation

6 - 10 Day Forecast Precipitation

Transitioning from the 6 to 10 day outlook on the left to the 8 to 14 day outlook on the right.         →

8 - 14 Day Precipitation

Mostly a continuation of the pattern into the second week (Days 8 - 14). But the pattern is deamplified..

To the right is the week 3 and 4 Experimental Forecast.          →

Notice the two Northern Tier wet anomalies one on the West Coast and one on the East Coast and the Alaska and Southern Tier dry anomaly or two anomalies depending on how one looks at it.

3 - 4 Week Precipitation

                     ↑

←  The transition from the 8 -14 day forecast shown above to the week 3/4 shown to the left seems unlikely especially for Week 3. It also raises questions about the Week Two Days 6 - 14 Outlook.

 

Let us focus on the Current (Right Now to 5 Days Out) Weather Situation.

Water Vapor.

This view of the past 24 hours provides a lot of insight as to what is happening.

Eastern Pacific Animation

You can see split in the Jet Stream resulting in Northern Tier activity and activity in Mexico that then enters Texas. Remember this is an animation of what has happened not a forecast. 

Below is the same graphic as above but without the animation to show the current situation with respect to water vapor imagery for North America. It also covers more of CONUS.

Water Vapor Imagery

Tonight, Monday evening December 18, 2017, as I am looking at the above graphic, you see for now pretty much the same pattern with Northern Tier and Mexican activity which rises into Texas and proceeds east.

This graphic is about Atmospheric Rivers i.e. thick concentrated movements of water moisture. More explanation on Atmospheric Rivers can be found by clicking here or if you want more theoretical information by clicking here. The idea is that we have now concluded that moisture often moves via narrow but deep channels in the atmosphere (especially when the source of the moisture is over water) rather than being very spread out. This raises the potential for extreme precipitation events. You can convert this graphic into a flexible forecasting tool by clicking here. One can obtain views of different geographical areas by clicking here.

 Center for Western Weather and Water Extremes, Scripps/UCSD.

There is some light activity along the border with Canada and activity that appears to be of Gulf of Mexico origin but is related to the southern branch of the Polar Jet.

Day One CONUS Forecast

Day Two CONUS Forecast

Day 1 Forecast Map

Day 2 Forecast

There is snow forecast for the Northern Tier. Earlier I have provided snow forecasts for day 4 through 7 and a link to earlier days.

These graphics update and can be clicked on to enlarge but my brief comments are only applicable to what I see on Monday night prior to publishing.

 

60 Hour Forecast Animation

Here is a national animation of weather fronts and precipitation forecasts with four 6-hour projections of the conditions that will apply covering the next 24 hours and a second day of two 12-hour projections the second of which is the forecast for 48 hours out and to the extent it applies for 12 hours, this animation is intended to provide coverage out to 60 hours. Beyond 60 hours, additional maps are available at links provided below. The explanation for the coding used in these maps, i.e. the full legend, can be found here although it includes some symbols that are no longer shown in the graphic because they are implemented by color coding.

current highs and lows

You can enlarge the below daily (days 3 - 7) weather maps for CONUS by clicking on Day 3 or Day 4 or Day 5 or Day 6 or Day 7. These maps auto-update so whenever you click on them they will be forecast maps for the number of days in the future shown.

Short term forecasts

What is Behind the Forecasts? Let us try to understand what NOAA is looking at when they issue these forecasts.

Below is a graphic which highlights the forecasted surface Highs and the Lows re air pressure on Day 7. The Day 3 forecast can be found here. the Day 6 Forecast can be found here. Actually all the small graphics below can be clicked on to enlarge them.

Day 7 Highs and Lows

When I look at this Day 7 forecast, there is a weak Low centered on the Bering Sea with surface central pressure of 1004 hPa. The Low is elongated to the southwest where there is another center of 1004 hPa. The location of that Center is critical to how the second half of this week evolves. To the extent it moves to the east it may reduce the southern extension of the Arctic Air intrusion. Further to the west there is Low moving towards Kamchatka with surface central pressure of 992 hPa. There is a very strong High centered over Western Canada with surface central pressure of 1040 hPa. Looking at this graphic it would seem that any significant precipitation would be directed north of CONUS. But the forecast is for moisture entering prior to Day 7 and moisture related to the High because it will be a very cold High.

I provided this K - 12 write up that provides a simple explanation on the importance of semipermanent Highs and Lows and another link that discussed possible changes in the patterns of these highs and lows which could be related to a Climate Shift (cycle) in the Pacific or Global Warming. Remember this is a forecast for Day 6. It is not the current situation.

The table below showing the Day 3, Day 4, Day 5, Day 6 and Day 7 of this graphic can be useful in thinking about how the pattern of Highs and Lows is expect to move during the week.

Day 3 Day 4
Day 5 Day 6

From left to right and then down, Days 3 and 4 top row, Days 5 and 6 second row and Day 7 to the right. These are small images but you can if you want click on them and get larger images but even with the small images you can trace the evolution of the pattern. The graphics update but my commentary below does not so it is just a guide for how to read these graphics.

Day 7

We see the progression to the very strong High centered over Western Canada. Earlier in the period, a storm enters the Great Basin.

Things to look for in general are the position and strength of the Aleutian Low, the Hawaiian High and any troughs especially if they extend far to the south and are over water.

 

Looking at the current activity of the Jet Stream.  The below graphics and the above graphics are very related.

Not all weather is controlled by the Jet Stream (which is a high altitude phenomenon) but it does play a major role in steering storm systems especially in the winter The sub-Jet Stream level intensity winds shown by the vectors in this graphic are also very important in understanding the impacts north and south of the Jet Stream which is the higher-speed part of the wind circulation and is shown in gray on this map. In some cases however a Low-Pressure System becomes separated or "cut off" from the Jet Stream. In that case it's movements may be more difficult to predict until that disturbance is again recaptured by the Jet Stream. This usually is more significant for the lower half of CONUS with the cutoff lows being further south than the Jet Stream. Some basic information on how to interpret the impact of jet streams on weather can be found here and here.  I have not provided the ability to click to get larger images as I believe the smaller  images shown are easy to read.

Current Day 5
Current Jet Stream Jet Stream Five Days Out
You can see that early in the week a trough is anticipated to impact the Great Basin. The meridional extent of that trough is the major forecasting issue right now.

 

Putting the Jet Stream into Motion and Looking Forward a Few Days Also

To see how the pattern is projected to evolve, please click here. In addition to the shaded areas which show an interpretation of the Jet Stream, one can also see the wind vectors (arrows) at the 300 Mb level.

This longer animation shows how the jet stream is crossing the Pacific and when it reaches the U.S. West Coast is going every which way.

When we discuss the jet stream and for other reasons, we often discuss different layers of the atmosphere. These are expressed in terms of the atmospheric pressure above that layer. It is kind of counter-intuitive to me. The below table may help the reader translate air pressure to the usual altitude and temperature one might expect at that level of air pressure. It is just an approximation but useful.

air pressure and altitude

Re the above, H8 is a frequently used abbreviation for the height of the 850 millibar level (which is intended to represent the atmosphere above the Boundary Layer most impacted by surface conditions), H7 is the 700 mb level, H5 is the 500 mb level, H3 is the 300 mb level. So if you see those abbreviations in a weather forecast you will know what they are talking about.

Click here to gain access to a very flexible computer graphic. You can adjust what is being displayed by clicking on "earth" adjusting the parameters and then clicking again on "earth" to remove the menu. Right now it is set up to show the 500 hPa wind patterns which is the main way of looking at synoptic weather patterns. This amazing graphic covers North and South America. It could be included in the Worldwide weather forecast section of this report but it is useful here re understanding the wind circulation patterns.

500 MB Mid-Atmosphere View

The map below is the mid-atmosphere 7-Day chart rather than the surface highs and lows and weather features. In some cases it provides a clearer less confusing picture as it shows only the major pressure gradients. This graphic auto-updates so when you look at it you will see NOAA's latest thinking. The speed at which these troughs and ridges travel across the nation will determine the timing of weather impacts. This graphic auto-updates I think every six hours and it changes a lot. Because "Thickness Lines" are shown by those green lines on this graphic, it is a good place to define "Thickness" and its uses. The 540 Level generally signifies equal chances for snow at sea level locations. Thickness of 600 or more suggests very intensely heat and fire danger. Sometimes Meteorologists work with the 500 mb heights which provide somewhat similar readings to the "Thickness" lines but IMO provide slightly less specific information. Thinking about clockwise movements around High Pressure Systems and counter- clockwise movements around Low Pressure Systems provides a lot of information.

7 Day 500 MB Geopotential Forecast

What we see for Day 7 is a huge trough in the center of CONUS with a ridge to the left and to the right.  That trough controls the weather for the next ten days at least.

Here is the seven-day cumulative precipitation forecast. More information is available here.

Seven Day WPC Quantitative precipitation forecast

We do not see a lot of QPF associated with that huge Trough except in the Gulf States. There is some activity in the Northwest. The weather will be more about temperature than precipitation.

Four Quadrant Jet Streak Model Read more here This is very useful for guessing at weather as a trough passes through. It would apply to the states that are at the apex of the trough.

If the centripetal accelerations owing to flow curvature are small, then we can use the "straight" jet streak model. The schematic figure directly below shows a straight jet streak at the base of a trough in the height field. The core of maximum winds defining the jet streak is divided into four quadrants composed of the upstream (entrance) and downstream (exit) regions and the left and right quadrants, which are defined facing downwind.


Isotachs are shaded in blue for a westerly jet streak (single large arrow). Thick red lines denote geopotential height contours. Thick black vectors represent cross-stream (transverse) ageostrophic winds with magnitudes given by arrow length. Vertical cross sections transverse to the flow in the entrance and exit regions of the jet (J) are shown in the bottom panels along A-A' and B-B', respectively. Convergence and divergence at the jet level are denoted by "CON" and "DIV". "COLD" and "WARM" refer to the air masses defined by the green isentropes.

[Editor's Note: There are many undefined words in the above so here are some brief definitions. Isotachs are lines of equal wind speed. Convergence is when there is an inflow of air which tends to force the air higher with cooling and cloud formation. Divergence is when there is an outflow of air which tends to result in air sinking which causes drying and warming, Confluence is when two streams of air come together. Diffluence is when part of a stream of air splits off.]

Four- Week Outlook: Looking Beyond Days 1 to 5, What is the Forecast for the Following Three + Weeks?

I use "EC" in my discussions although NOAA sometimes uses "EC" (Equal Chances) and sometimes uses "N" (Normal) to pretty much indicate the same thing although "N" may be more definitive.

First - Temperature

6 - 10 Day Temperature Outlook issued today (Note the NOAA Level of Confidence in the Forecast Released on December 18, 2017 was 4 out of 5

6 - 10 Day Temperature Outlook

8 - 14 Day Temperature Outlook issued today (Note the NOAA Level of Confidence in the Forecast Released on December 18, 2017 was 3 out of 5).

8-14 Day Temperature Outlook

Looking further out.

Experimental Week 3-4 Temperature Outlook

Now - Precipitation

6 - 10 Day Precipitation Outlook Issued Today (Note the NOAA Level of Confidence in the Forecast Released on December 18 was 4 out of 5)

6 - 10 Day Precipitation Outlook

8 - 14 Day Precipitation Outlook Issued Today (Note the NOAA Level of Confidence in the Forecast Released on December 18, 2017 was 3 out of 5)

Current 8 - 14 Day Precipitation Outlook

Looking further out.

Weeks 3 and 4 Experimental Forecast..

Here is the 6 - 14 Day NOAA discussion released today December 18, 2017 and the Week 3/4 discussion released Friday December 15, 2017

6-10 DAY OUTLOOK FOR DEC 24 - 28 2017

TODAY'S MODEL FORECASTS ARE IN GOOD AGREEMENT ON A PREDICTED AMPLIFIED 500-HPA CIRCULATION PATTERN FOR THE 6-10 DAY PERIOD. ENSEMBLE PREDICTION SYSTEMS FROM  THE NCEP GEFS, ECMWF, AND ENVIRONMENT CANADA PREDICT A STRONG RIDGE OVER THE NORTH PACIFIC AND EASTERN ALASKA, A DOWNSTREAM TROUGH EXTENDING FROM HUDSON BAY INTO THE CONUS, AND ABOVE NORMAL 500-HPA HEIGHTS OVER THE SOUTHEASTERN CONUS AND NORTH ATLANTIC. THE ECMWF ENSEMBLE PREDICTS ORIENTATION OF THE PREDICTED TROUGH AXIS OVER NORTH AMERICA TO EXTEND FROM THE NORTH-CENTRAL CONUS TO THE SOUTHWEST. THE ENVIRONMENT CANADA ENSEMBLE PREDICTS A SIMILAR BUT LESS AMPLIFIED TROUGH, WHILE THE GEFS INDICATES A LESS AMPLIFIED TROUGH CENTERED OVER THE U.S. MIDWEST. THE 500-HPA FLOW PATTERN OF THE ECMWF ENSEMBLE PREDICTION SYSTEM IS WEIGHTED MORE HEAVILY IN THE OFFICIAL FORECAST. 

ABOVE NORMAL TEMPERATURES ARE LIKELY FOR MOST OF ALASKA, EXCEPT FOR THE SOUTHERN ALASKA PANHANDLE, UNDER LARGE POSITIVE 500-HPA HEIGHT ANOMALIES AND ANOMALOUS SOUTHERLY FLOW, WITH PROBABILITIES EXCEEDING 70 PERCENT FOR NORTHWEST REGIONS. BELOW NORMAL TEMPERATURES ARE LIKELY FOR A LARGE AREA OF THE CONUS, UNDER A PREDICTED AMPLIFIED TROUGH AND AN INFLUX OF ARCTIC AIR. PROBABILITIES EXCEED 70 PERCENT OVER A LARGE AREA OF THE NORTHERN CONUS, AS WELL AS THE CENTRAL AND SOUTHERN GREAT PLAINS STATES. UNDER PREDICTED ANOMALOUS SOUTHERLY FLOW AND ABOVE NORMAL 500-HPA HEIGHTS, ABOVE NORMAL TEMPERATURES ARE LIKELY FOR ATLANTIC COASTAL STATES IN THE SOUTHEAST, INCLUDING FLORIDA.   

TROUGHING TO THE WEST AND PREDICTED SOUTHERLY FLOW OVER ALASKA LEADS TO LIKELY ABOVE NORMAL PRECIPITATION OVER PARTS OF WESTERN AND INTERIOR REGIONS OF THE STATE. THE PREDICTED TROUGH OVER THE WEST-CENTRAL CONUS ENHANCES THE PROBABILITY OF ABOVE NORMAL PRECIPITATION FOR EASTERN AREAS OF COLORADO AND ARIZONA, PARTS OF THE SOUTHERN PLAINS, AS WELL AS ALONG A LINE FROM LOUISIANA ACROSS THE SOUTHEAST INTO THE MID-ATLANTIC AND NEW ENGLAND COASTS, ALONG THE PREDICTED MEAN FRONTAL BOUNDARY. BELOW NORMAL PRECIPITATION IS LIKELY FOR THE NORTHWEST UNDER PREDICTED ANOMALOUS NORTHERLY 500-HPA FLOW. BELOW NORMAL  PRECIPITATION IS LIKELY FROM THE NORTHERN PLAINS INTO THE CENTRAL MISSISSIPPI VALLEY WITH AN INFLUX OF DRY ARCTIC AIR. 

FORECAST CONFIDENCE FOR THE 6-10 DAY PERIOD: ABOVE AVERAGE, 4 OUT OF 5, DUE TO GOOD MODEL AGREEMENT ON A HIGHLY AMPLIFIED FLOW PATTERN.   

8-14 DAY OUTLOOK FOR DEC 26, 2017 - JAN 01, 2018   

MODEL FORECASTS PREDICT A LESS AMPLIFIED TROUGH WITH SOME EASTWARD PROGRESSION OVER THE CONUS IN THE WEEK 2 PERIOD, AND NEGATIVE 500-HPA HEIGHT ANOMALIES  PRIMARILY LOCATED FROM THE NORTHERN PLAINS AND GREAT LAKES REGION INTO THE NORTHEAST. ENSEMBLE PREDICTION SYSTEMS FROM THE NCEP GEFS, ECMWF, AND ENVIRONMENT CANADA ENSEMBLES ARE IN GENERAL AGREEMENT ON THE DEAMPLIFICATION AND EASTWARD PROGRESSION OF THE CIRCULATION PATTERN. ENSEMBLE SPREAD AND UNCERTAINTY, HOWEVER, INCREASE IN THE WEEK 2 PERIOD FORECASTS.

ABOVE NORMAL TEMPERATURES REMAIN VERY LIKELY FOR MOST OF ALASKA, EXCEPT FOR SOUTHEASTERN REGIONS OF THE STATE, UNDER THE PREDICTED RIDGE. BELOW NORMAL TEMPERATURES ARE MOST LIKELY FOR THE ALASKA PANHANDLE, UNDER ANOMALOUS  NORTHERLY FLOW. BELOW NORMAL TEMPERATURES CONTINUE TO BE LIKELY FOR A LARGE AREA OF THE CONUS, UNDER THE PREDICTED TROUGH. ABOVE NORMAL TEMPERATURES ARE LIKELY OVER PARTS OF SOUTHERN CALIFORNIA AND THE SOUTHWEST REGION WITH MORE ZONAL FLOW PREDICTED OVER THE REGION, RELATIVE TO THE MODEL 6-10 DAY PERIOD FORECASTS. THE GEFS ENSEMBLE FORECAST INDICATES GREATER POSITIVE 500-HPA HEIGHT ANOMALIES OVER THE SOUTHWEST, AND WITH IT AN INCREASED LIKELIHOOD OF ABOVE NORMAL TEMPERATURES, COMPARED TO THE ECMWF ENSEMBLE. ABOVE NORMAL TEMPERATURES ARE LESS LIKELY OVER PARTS OF THE SOUTHEAST IN WEEK 2, RELATIVE TO THE 6-10 DAY PERIOD FORECAST, WITH THE EASTWARD PROGRESSION OF THE TROUGH OVER THE CENTRAL CONUS. 

WITH SOME WESTWARD RETROGRESSION OF THE RIDGE OVER ALASKA, ABOVE NORMAL PRECIPITATION IS LIKELY ONLY OVER NORTHERN AREAS OF THE STATE IN WEEK 2. THE PRECIPITATION FORECAST OVER THE CONUS FOR WEEK 2 IS SIMILAR TO THE 6-10 DAY PERIOD FORECAST WITH A COUPLE EXCEPTIONS. AN INCREASED CHANCE OF ABOVE NORMAL PRECIPITATION IS INDICATED OVER CENTRAL MONTANA BY CALIBRATED ENSEMBLE PROBABILITY FORECASTS FROM THE ECMWF AND GEFS ENSEMBLES. NEAR AND BELOW NORMAL PRECIPITATION ARE MORE LIKELY AND ABOVE NORMAL IS LESS LIKELY OVER PARTS OF THE SOUTHEAST AND ATLANTIC COAST, AS THE PREDICTED TROUGH OVER THE CONUS IS LESS AMPLIFIED. 

FORECAST CONFIDENCE FOR THE 8-14 DAY PERIOD: ABOUT AVERAGE, 3 OUT OF 5, DUE TO A LESS AMPLIFIED PREDICTED CIRCULATION PATTERN IN WEEK 2, RELATIVE TO THE 6-10 DAY PERIOD.

THE NEXT SET OF LONG-LEAD MONTHLY AND SEASONAL OUTLOOKS WILL BE RELEASED ON  DECEMBER 21.

Week 3-4 Forecast Discussion Valid Sat Dec 30 2017-Fri Jan 12 2018

With La Nina conditions currently present across the equatorial Pacific Ocean, equatorial sea surface temperatures (SSTs) are below average across the central and eastern Pacific Ocean. The CPC velocity potential based and RMM MJO indices indicate an enhanced MJO signal from the Maritime Continent to the western and central Pacific. Forecasts of the RMM-based MJO indices indicate an increase in amplitude of the signal into the Phase 7 pattern (Western Pacific) and progression eastward in the next week. The Week 3/4 temperature and precipitation outlooks rely primarily on dynamical model forecasts from the NCEP CFS, ECMWF and JMA operational ensemble prediction systems, as well as forecasts from the Subseasonal Experiment (SubX), a multi-model ensemble (MME) of experimental ensemble prediction systems. Consideration is also given to the possible evolution of the predicted circulation, temperature and precipitation patterns for Week 2.

The various model guidance supporting the Week 3-4 outlook is in good agreement today, indicating a large high pressure anomaly and ridging over parts of Alaska, Siberia and the Arctic Ocean, behind a trough and below normal 500-hPa heights over central Canada, extending southward into parts of the northwestern CONUS. Ridging and above normal 500-hPa heights are generally predicted over the southeast and eastern CONUS. The CFS indicates an evolution of the pattern through the Week 3-4 period with ridging building over the northeast. This forecast is somewhat discounted in favor of the ECMWF and JMA model circulation solutions, which are more consistent with evolution of the circulation from the Week 2 forecast. The Canadian ensemble prediction system, as well as the NCEP GEFS, in addition to other dynamical model forecasts from SubX, indicate a similar orientation to the trough over North America as in the ECMWF and JMA models.

The forecast temperature pattern for Weeks 3-4 is based primarily on a skill-weighted consolidation of the CFS, ECMWF, and JMA temperature forecasts, as well as statistical guidance from a multiple linear regression tool indicating ENSO, trend and MJO impacts. In addition an MME average of SubX temperature anomalies was consulted.

Below normal temperatures are most likely for the Alaska Panhandle, as well as a broad area from the Northern Rockies across the northern and central Plains states into the Upper Mississippi Valley and Great Lakes region and the Northeast CONUS. Below normal temperatures over this region result from the troughing and below normal 500-hPa heights over North America. Above normal temperatures are more likely over parts of the western CONUS influenced by warm Pacific air, and over the Southeast, under above normal 500-hPa heights and ridging. Most of Alaska, except for the southeastern regions of the state, are likely to experience above normal temperatures.

Above median precipitation is likely over much of the western CONUS from California into the Rocky Mountains, ahead of warm Pacific flow. Above median precipitation is also most likely for parts of the eastern CONUS from the central Mississippi Valley, across the Ohio Valley and parts of the Southeast, into the Mid-Atlantic states and the Carolinas, following expected tracking of storms ahead of the trough over the central CONUS. This forecast is supported by the CFS, ECMWF and JMA operational models, as well as nearly all SubX model forecasts. Below median precipitation is most likely for areas of the Southwest behind the predicted trough axis and for the Southeast, under above normal 500-hPa heights. Below median precipitation is likely for most of Alaska, with the exception of the North Slope, under anomalous northerly flow and high pressure.

Above normal temperatures are predicted for Hawaii by most operational and experimental dynamical models, with larger positive anomalies for the southeastern islands. There is some model disagreement on the precipitation forecast for Hawaii, among dynamical models. However, most SubX models and a consensus of the operational models indicate below median precipitation is most likely for Hawaii.

Some might find this analysis which you need to click to read interesting as the organization which prepares it focuses on the Pacific Ocean and looks at things from a very detailed perspective and their analysis provides a lot of information on the history and evolution of ENSO events.

Analogs to the Outlook.

Now let us take a detailed look at the "Analogs" which NOAA provides related to the 5 day period centered on 3 days ago and the 7 day period centered on 4 days ago. "Analog" means that the weather pattern then resembles the recent weather pattern and was used in some way to predict the 6 - 14 day Outlook.

Here are today's analogs in chronological order although this information is also available with the analog dates listed by the level of correlation. I find the chronological order easier for me to work with. There is a second set of analogs associated with the Outlook but I have not been regularly analyzing this second set of information. The first set which is what I am using today applies to the 5 and 7 day observed pattern prior to today. The second set, which I am not using, relates to the correlation of the forecasted outlook 6 - 10 days out with similar patterns that have occurred in the past during the dates covered by the 6 - 10 Day Outlook. The second set of analogs may also be useful information but they put the first set of analogs in the discussion with the second set available by a link so I am assuming that the first set of analogs is the most meaningful and I find it so.

Centered

Day

ENSO

Phase

PDO AMO

Other Comments

Dec 26, 1953 El Nino - +  
Dec 27, 1953 El Nino - +  
Dec 28, 1960 Neutral + +  
Dec 29, 1960 Neutral + +  
Dec 17, 1988 La Nina - -  
Dec 28, 1989 Neutral - -  
Dec 30, 1989 Neutral - -  
Dec 28, 1999 (2) La Nina - N  
Dec 12, 2007 La Nina - +  

 

(t) = a month where the Ocean Cycle Index has just changed or does change the following month.

The spread among the analogs from December 12 to December 30 is 18 days which is much tighter than last week. I have not calculated the centroid of this distribution which would be the better way to look at things but the midpoint, which is a lot easier to calculate, is about December 21. These analogs are centered on 3 days and 4 days ago (December 15 or December 16). So the analogs could be considered to be out of sync with respect to weather that we would normally be getting right now being almost a week early which is consistent with the widespread cool anomalies. For more information on Analogs see discussion in the GEI Weather Page Glossary.

Including the duplicates, there are four Neutral Analogs, four La Nina analogs and two El Nino Analogs. The phases of the analogs favor PDO -. The analogs are consistent with all of the McCabe conditions except McCabe A. But McCabe A is consistent with the forecast suggesting that there is significant potential for the forecast to vary from the actual in important ways. A possible reason for this is that we are dealing with a fairly dry but cold intrusion and the latitudinal extension of that trough is viewed differently by some of the forecast models. So the overall pattern seems pretty much known but the exact details may vary in important ways.

The seminal work on the impact of the PDO and AMO on U.S. climate can be found here. Water Planners might usefully pay attention to the low-frequency cycles such as the AMO and the PDO as the media tends to focus on the current and short-term forecasts to the exclusion of what we can reasonably anticipate over multi-decadal periods of time. One of the major reasons that I write this weather and climate column is to encourage a more long-term and World view of weather.

In color Black and White same graphics
McCabe Maps modified to include the subtitles McCabe Conditions from 2007 report with labels corrected with authors permission

 

McCabe Condition Main Characteristics
A Very Little Drought. Southern Tier and Northern Tier from Dakotas East Wet. Some drought on East Coast.
B More wet than dry but Great Plains and Northeast are dry.
C Northern Tier and Mid-Atlantic Drought
D Southwest Drought extending to the North and also the Great Lakes. This is the most drought-prone combination of Ocean Phases.

 

You may have to squint but the drought probabilities are shown on the map and also indicated by the color coding with shades of red indicating higher than 25% of the years are drought years (25% or less of average precipitation for that area) and shades of blue indicating less than 25% of the years are drought years. Thus drought is defined as the condition that occurs 25% of the time and this ties in nicely with each of the four pairs of two phases of the AMO and PDO.

Historical Anomaly Analysis

When I see the same dates showing up often I find it interesting to consult this list.

Recent CONUS Weather

This is provided mainly to see the pattern in the weather that has occurred recently.

And the 30 Days ending December 9, 2017 And the 30 Days ending December 16, 2017

December11, 2017 30 Day Temperature and Precipitation Departures

December 18, 2017 30 Day Temperature and Precipitation Departures
Not much change re precipitation. The warm anomaly rotated just a bit moving away from the Southeast as bit and extending to the north a bit more.

Actually not much change in the pattern for the last 30 days. It will look different by next week.

Remember, these maps are a 30 average so the most distant seven days are removed and the most recent seven days are added.

 

Temperature Rankings Historically By State (Note that Utah, Colorado, Arizona and New Mexico had the warmest November in the 123 years of modern records. What are the odds of four neighboring states all having record for the month average temperatures?)

November 2017 State Average Temperature Historical Rankings.

I presented this graphic last week and forgot to discuss the probability of this happening. Here goes: For any selected four states to all break a record in the same month the math is (I hope) (1/123) to the power 4 or -.00000000043. But a better question might be what were the chances than any four states not just these four in particular would all be record warm? There are 48 states shown on that map. That means there are 48!/[(48-4)!X(4!)] or (48X47X46X45)/4X3X2. That equals 4669920/24 = 194580 meaning there that many combinations of four states.  So the probability is 194580X0.00000000043 =0.00085 which is a little less than 0.1%.Given that there are twelve months in a year that is about one chance in a hundred in a year so this was a hundred year event that four states would break their temperature record during some month in the year.
But then again those four states were adjacent so not independent and history may have ended up with them being a single state. In fact Arizona and New Mexico were originally planned to be one state but the vote was to not do that. So you can see that it is not easy to assess how rare such an event really is and I might not have done the math correctly. But it certainly was an oddity and trying to assess ow unlikely that was is a good exercise in trying to understand some of the issues of attribution relative to Global Warming.

Looking Out Beyond Three Months

On Saturday November 18, 2018 we published our monthly Seasonal Outlook Update and you can access that here. Also on Friday December 2 we published the December Update and you can access it here. There will be a new Seasonal Outlook issued by NOAA on December 21 which we will report on December 23.

Reference Forecasts Full Month and Three Months.

Below are the Temperature followed by the Precipitation Outlooks for the month and three months shown in the Legend. These map are issued on the Third Thursday of the Month. The maps for the following month (but not the three-month maps) are updated on the last day of the month. The 6 - 10 day and 8 - 14 Day update daily and the Week 3/4 Map Updates every Friday so usually these are more up to date. Also the three shorter-term maps will generally cover a slightly different time period since they update daily as the month progresses. But these reference maps are sometimes useful if one wants to understand how the current month was originally forecast to play out.

 
December Temperature Outlook Update on November 16, 2017 To the left is the full month Temperature Outlook. To the right is the three-month Temperature outlook DJF Temperature  Issued on November 16,2017
December 2017 Precipitation Outlook Updated on December, 2017 To the left is the full month Precipitation outlook. To the right is the three-month Precipitation outlook. DJF 2017-2018 Precipitation Outlook Issued on November 16,  2017

 

B. Beyond Alaska and CONUS Let's Look at the World which of Course also includes Alaska and CONUS

It is Useful to Understand the Semipermanent Pattern that Control our Weather and Consider how These Change from Winter to Summer. These two graphics (click on each one to enlarge) are from a much larger set available from the Weather Channel.  They highlight the position of the Bermuda High which they are calling the Azores High in the January graphic and is often called NASH and it has a very big impact on CONUS Southeast weather and also the Southwest. You also see the north/south migration of the Pacific High which also has many names and which is extremely important for CONUS weather and it also shows the change of location of the ITCZ which I think is key to understanding the Indian Monsoon. A lot of things become much clearer when you understand these semi-permanent features some of which have cycles within the year, longer period cycles and may be impacted by Global Warming. We are now moving into late-December and are almost to the set of positions shown above for January. For CONUS, the seasonal repositioning of the Bermuda High and the Pacific High are very significant. Notice the Winter position of the Pacific High.

Same as above but for July

 

Forecast for Today (you can click on the maps to enlarge them)

This graphic is anomalies i.e. deviations from normal not the actual temperatures. This graphic is actuals not anomalies as is the case in the temperature map.
Temperature at 2 Meters Maine Reanalyer

Notice that below the map there is a tabulation of magnitude of the current anomalies by region. Overall it is warm except

primarily northeast Asia

We again see the dry belt stretching from Northern Africa to Eastern Asia but not including Southeast Asia.

 

Additional Maps showing different weather variables can be found here.

Forecast for Day 6 (Currently Set for Day 6 but the reader can change that)

World Weather Forecast produced by the Australian Bureau of Meteorology. Unfortunately I do not know how to extract the control panel and embed it into my report so that you could use the tool within my report. But if you visit it Click Here and you will be able to use the tool to view temperature or many other things for THE WORLD. It can forecast out for a week. Pretty cool. Return to this report by using the "Back Arrow" usually found top left corner of your screen to the left of the URL Box. It may require hitting it a few times depending on how deep you are into the BOM tool. Below are the current worldwide precipitation and temperature forecasts for six days out. They will auto-update and be current for Day 6 whenever you view them. If you want the forecast for a different day Click Here

Temperature

Precipitation

BOM Current Temperature Wedensday

BOM World Preciptation  Wednesday

Please remember this graphic updates every six hours so the diurnal pattern can confuse the reader. As we discussed earlier looking at the Day 1 forecast it is interesting to try to figure how what the Zone Wave Number is in this graphic.

 

And now we have experimental forecasts from the U.S. NAEFS Model.

Temperature Precipitation

NAEFS 8 - 14 Day Temperature

NAEFS Experimental World Precipitation

We see a cold North American You definitely see the La Nina pattern for North America.

 

Looking Out a Few Months

Here is the precipitation forecast from Queensland Australia:

Consistently positive SOI  forecast for January to Febrruary 2017.- 2018

It is kind of amazing that you can make a worldwide forecast based on just one parameter the SOI and changes in the SOI. Notice the continuance of the underlying driver of the SOI having been consistently positive. CONUS now looks like a north south typical La Nina divide with the northern tier wet. Northwest South America is wet. Southern Africa is wet. Eastern Australia is wet.

JAMSTEC Forecasts

One can always find the latest JAMSTEC maps by clicking this link. You will find additional maps that I do not general cover in my monthly Update Report. Remember if you leave this page to visit links provided in this article, you can return by hitting your "Back Arrow", usually top left corner of your screen just to the left of the URL box.

Sea Surface Temperature (SST) Departures from Normal for this Time of the Year i.e. Anomalies

My focus here is sea surface temperature anomalies as they are one of the two largest factors determining weather around the World. If we want to have a good feel for future weather we need to look at the oceans as our weather mostly comes from oceans and we need to look at

  • Surface temperature anomalies (weather develops from the ocean surface  and
  • the changes in the temperature anomalies since that may provide clues as to how the surface anomalies will change based on the current trend of changes. This is not that easy to do since the oceans are deep, there are many currents, winds have an impact etc. Two ways that are available to use are to look at the change in the situation today compared ot the average over a period of time and NOAA also produces a graphic of monthly changes. I use both. The first set of graphics is simply looking at the average compared to today and that is below.
Three Month Average Anomaly Current Anomaly
Three month average anomaly Daily SST Anomaly
La Nina shows up La Nina is more intense

 

And when we look in more detail at the current Sea Surface anomalies below, we see a lot of them not just along the Equator related to ENSO.

                              First the categorization of the SST anomalies.

Mediterranean, Black Sea and Caspian Sea Western Pacific West of North America North and East of North America North Atlantic
Fairly Neutral

Fairly Neutral as mixed no clear pattern

Mostly Neutral

Warm and south small anomalies west of Central America

Warm off Nova Scotia and Newfoundland

Warm north of Scandinavia,

Equator

Pacific cool east of Dateline but more intense east of 160W 

SST Daily Anomalies
Africa West of Australia North, South and East of Australia

West of South America

East of South America

Slightly warm west of North Africa.

Cool southeast of Africa south of  Madagascar

Warm off of Somalia.

Cool off Namibia but away from the coast.

Mostly neutral near shore.

Very warm southeast all the way to and beyond New Zealand

Cool, cool, cool .

Cool east of 30S but offshore.

Warm off 40S

 

Then we look at the change in the anomalies.  Here it gets a little tricky as for this graphic red does not mean a warm anomaly but a warming of the anomaly which could mean more warm or less cool and blue does not mean cool but more cool or less warm.

Mediterranean, Black Sea and Caspian Sea Western North Pacific West of North America East of North America North Atlantic

Mediterranean warming slightly at east end.

Arabian Sea cooling.

Cooling between China and Japan

Cooling along the Aleutians.

Cooling east of the Dateline from 45N to 20N with some warming on the other side of the Dateline

.

.

Warming off Nova Scotia

Cooling south of Greenland

Equator Eastern Pacific mixed with warming north of the Equator from 150W to 110W. Cooling for Maritime Continent.
Change in weekly anomalies December 11, 2017
Africa West of Australia North, South and East of Australia West of South America East of South America

Warming west of Gulf of Guinea.

Cooling west of Africa from Gulf of Guinea to and around Cape of Good Hope.

Neutral.

Warming around New Zealand

Reduced band of cooling 30S.

More extreme band further offshore west of 30S to 40S

Cooling 20S to Cape Horn

 

This may be a good time to show the recent values to the indices most commonly used to describe the overall spacial pattern of temperatures in the (Northern Hemisphere) Pacific and the (Northern Hemisphere) Atlantic and the Dipole Pattern in the Indian Ocean.

Most Recent Six Months of Index Values   PDO Click for full list

AMO click for full list.

Indian Ocean Dipole (Values read off graph)
October   -0.68 +0.39 -0.3
November   +0.84 +0.40 0.0
December   +0.55 +0.34 -0.1
January   +0.10 +0.23 0.0
February   +0.04 +0.23 +0.2
March   +0.12 +0.17 +0.0
April   +0.52 +0.29 +0.2
May   +0.30 +0.32 +0.2
June   +0.19 +0.31 0.0
July   -0.54 +0.31 0.0
August   -0.64 +0.31 +0.4
September   -0.26 +0.35 +0.2
October   -0.64 +0.44  0.0
November   -0.52 +0.35  0.0

 

Switching gears, below is an analysis of projected tropical hazards and benefits over an approximately two-week period.

Tropical Hazards

This graphic updates on Tuesdays and I post on Monday which is almost a week later than when this graphic was last updated. So Week Two applies at the time I write this article on Monday but by the time you read it on Tuesday, the Week Two that I am looking at is updated and becomes Week One. Mostly I see as I look at this on December 18 for what is shown as Week Two, the period December 20 to December 26, 2017, we see wet conditions* and possible* cyclonic development for the Philippines and dry conditions* at the eastern and western ends of the Maritime Continent. There is also an indication of wet conditions* for northern Madagascar.  .

 * Moderate Confidence that the indicated anomaly will be in the upper or lower third of the historical range as indicated in the Legend. ** High Confidence that the indicated anomaly will be in the upper or lower third of the historical range as indicated in the Legend.

Tropical Activity Possibly Impacting CONUS.

Eastern Pacific Two Day Tropical Weather Outlook

When there is activity and I have not provided the specific links to the storm of "immediate" interest, one can obtain that information at this link. At this point in time, no (new) tropical events are expected to appear in this graphic during the next 48 hours. If that changes, we will provide an update.

Now let us look at the Western Pacific in Motion.

Western Pacific Tropical Activity

The above graphic which I believe covers the area from the Dateline west to 100E and from the Equator north to 45N normally shows the movement of tropical storms towards Asia in the lower latitudes (Trade Winds) and the return of storms towards CONUS in the mid-latitudes (Prevailing Westerlies). This is recent data not a forecast. But, it ties in with the Week 1 forecast in the graphic just above this graphic. Information on Western Pacific storms can be found by clicking here. This (click here to read) is an unofficial private source but one that is easy to read.

C. Progress of ENSO

A major driver of weather is Surface Ocean Temperatures. Evaporation only occurs from the Surface of Water. So we are very interested in the temperatures of water especially when these temperatures deviate from seasonal norms thus creating an anomaly. The geographical distribution of the anomalies is very important. To a substantial extent, the temperature anomalies along the Equator have disproportionate impact on weather so we study them intensely and that is what the ENSO (El Nino - Southern Oscillation) cycle is all about. Subsurface water can be thought of as the future surface temperatures. They may have only indirect impacts on current weather but they have major impacts on future weather by changing the temperature of the water surface. Winds and Convection (evaporation forming clouds) is weather and is a result of the Phases of ENSO and also a feedback loop that perpetuates the current Phase of ENSO or changes it. That is why we monitor winds and convection along or near the Equator especially the Equator in the Eastern Pacific.

Starting with Surface Conditions.

TAO/TRITON GRAPHIC (a good way of viewing data related to the part of the Equator and the waters close to the Equator in the Eastern Pacific where we monitor to determining the current phase of ENSO. It is probably not necessary in order to follow the discussion below, but here is a link to TAO/TRITON terminology.

And here is the current version of the TAO/TRITON Graphic. The top part shows the actual temperatures, the bottom part shows the anomalies i.e. the deviation from normal.

Current SST and wind anomalies

Location Bar for Nino 3.4 Area Above and Below

------------------------------------------------ A B C D E -----------------

 

The pattern now is

The below table only looks at the Equator (and starting this week I am including large anomalies just off the Equator also) and shows the extent of anomalies along the Equator. The ONI Measurement Area is the 50 degrees of Longitude between 170W and 120W and extends 5 degrees of Latitude North and South of the Equator so the above table is just a guide and a way of tracking the changes. The top rows show El Nino anomalies. The two rows just below that break point contribute to ENSO Neutral.

Subareas of the Anomaly

Westward Extension

 

Eastward Extension

 

Degrees of Coverage Total by ENSO Phase

Total

Portion in Nino 3.4 Measurement Area

These Rows below show the Extent of El Nino Impact on the Equator  

1C to 1.5C (strong)

NA

NA

0

0

0

+0.5C to +1C (marginal)

NA

NA

0

0

These Rows Below Show the Extent of ENSO Neutral Impacts on the Equator  
0.5C or cooler Anomaly (warmish neutral)

160E

170E

10

0

0

0C or cooler Anomaly (coolish neutral)

170E

DATELINE

10

0

These Rows Below Show the Extent of La Nina Impacts on the Equator.  
-0.5C or cooler Anomaly

DATELINE

150W

30

20

50

-1.0C or cooler Anomaly

150W

120W

30

30

-1.5C or cooler Anomaly

120W

110W

10

0

-2.0C or cooler Anomaly

110W

105W

5

0

-2.5C or cooler Anomaly

105W

LAND

10

0

This week the entire Equator in the Nino 3.4 Measurement Area registers La Nina values. That is not the case for the full +5N and +5S width of the Nino 3.4 Measurement Area but in this analysis we are just looking at the Equator.

 

My Calculation of the Nino 3.4 Index

I calculate the current value of the Nino 3.4 Index each Monday using a method that I have devised. To refine my calculation, I have divided the 170W to 120W Nino 3.4 measuring area into five subregions (which I have designated from west to east as A through E) with a location bar shown under the TAO/TRITON Graphic). I use a rough estimation approach to integrate what I see below and record that in the table I have constructed. Then I take the average of the anomalies I estimated for each of the five subregions.

So as of Monday December 18, in the afternoon working from the December 17 TAO/TRITON report [Although the TAO/TRITON Graphic appears to update once a day, in reality it updates more frequently.], this is what I calculated.

Calculation of Nino 3.4 from TAO/TRITON Graphic

Anomaly Segment Estimated Anomaly
  Last Week This Week
A. 170W to 160W -0.2 -0.7
B. 160W to 150W -0.2 -0.8
C. 150W to 140W -0.5 -1.0
D. 140W to 130W -0.8 -1.1
E. 130W to 120W -1.1 -1.4
Total -2.8 -5.0
Total divided by five i.e. the Daily Nino 3.4 Index (-2.8)/5 = -0.6 (-5.0)/5 = -1.0

 

My estimate of the daily Nino 3.4 SST anomaly tonight is -1.0 which is a moderate ENSO La Nina value. NOAA has reported the weekly Nino 3.4 to be -0.8 which is La Nina value. Nino 4 is reported the same this week at -0.3. Nino 3 is almost the same at -1.1. Nino 1 + 2 which extends from the Equator south rather than being centered on the Equator is reported less cool at -1.3. It was up there close to 3 at one time so this index has been declining quite a bit and also fluctuating quite a bit which is not surprising as it is the area most impacted by the Upwelling off the coast. So it is an indication of the interaction between surface water and rising cool water. Thus it is subject to larger changes. I am only showing the currently issued version of the NINO SST Index Table as the prior values are shown in the small graphics on the right with this graphic. The same data in table form but going back a couple of more years can be found here.

The cool anomaly pretty much  remained in the same locating this week and again records as La Nina.

December 18, 2017 Nino Readings

This is probably the best place to AGAIN express the thought that this way of measuring an ENSO event leaves a lot to be desired. Only the surface interacts with the atmosphere and is able to influence weather. The subsurface tells us how long the surface will remain cool (or warm). Anomalies are deviations from "Normal". NOAA calculates and determines what is "Normal" which changes due to long ocean cycles and Global Warming. So to some extent, the system is "rigged". Hopefully it is rigged to assist in providing improved weather forecasts. But to assume that any numbers reported can be assumed to be accurate to a high level of precision is foolhardy.

This overlaps with the next topic but I will show it here.

Equatorial (0 - 300) meter heat content As reported December 18, 2017

The discussion in this slide says it better than I could. One might compare the current reading to Oct/Nov 2016. We may be at Peak La Nina but it has now only a few months to run and we are starting our La Nina Demise Count Down.  But we are not yet ready to predict the end of La Nina readings.

Sea Surface Temperature and Anomalies

It is the ocean surface that interacts with the atmosphere and causes convection and also the warming and cooling of the atmosphere. So we are interested in the actual ocean surface temperatures and the departure from seasonal normal temperatures which is called "departures" or "anomalies". Since warm water facilitates evaporation which results in cloud convection, the pattern of SST anomalies suggests how the weather pattern east of the anomalies will be different than normal.

December 18, 2017 Equatorial Pacific SST Anomalies

A major advantage of the Hovmoeller method of displaying information is that it shows the history so I do not need to show a sequence of snapshots of the conditions at different points in time. This Hovmoeller provides a good way to visually see the evolution of this ENSO event. I have decided to use the prettied-up version that comes out on Mondays rather that the version that auto-updates daily because the SST Departures on the Equator do not change rapidly and the prettied-up version is so much easier to read. The bottom of the Hovmoeller shows the current readings. Remember the +5, -5 degree strip around the Equator that is being reported in this graphic. So it is the surface but not just the Equator.

Not much changed from last week. You can see where La Nina took a vacation but it was a short vacation.

This next graphic is more focused on the Equator and looks down to 300 meters rather than just being the surface.

December 18, 2017 Upper Ocean Heat Anoma

The bottom of the Hovmoeller shows the current situation.

The pattern this week is very similar to last week. A slight westward shift but the Nino readings do not confirm that. You can see how we have two centers of the coldest water. Not sure of the implications of that are but it is clear that it is the case. 

Let us look in more detail at the Equatorial Water Temperatures.

We are now going to look at a three-dimensional view of the Equator and move from the surface view and an average of the subsurface heat content to a more detailed view from the surface down This graphic provides both a summary perspective and a history (small images on the right).

.December 18, 2017 Kelvin Wave Analysis.

Not much change from last week. The subsurface warm anomaly west of 165W is stronger.

Anomalies are strange. You can not really tell for sure if the blue area is colder or warmer than the water above or below. All you know is that it is cooler than usual for this time of the year. A later graphic will provide more information. Aside from buoyancy the currents tend to bring water from that depth up to the surface mostly farther east.

Now for a more detailed look. Below is the pair of graphics that I regularly provide. The date shown is the midpoint of a five-day period with that date as the center of the five-day period. The bottom graphic shows the absolute values, the upper graphic shows anomalies compared to what one might expect at this time of the year in the various areas both 130E to 90W Longitude and from the surface down to 450 meters. At different times I have discussed the difference between the actual values and the deviation of the actual values from what is defined as current climatology (which adjusts every ten years except along the Equator where it is adjusted every five years) and how both measures are useful for other purposes.

There is cold water from 170W to Land. At  the west end of the cool anomaly it is now not quite 200 meters deep. We now have warm water developing west of the Dateline and starting to cross the Dateline. It is now intruding into the Eastern Pacific Nino 3.4 Measurement Area but at depth not at the surface. La Nina's days are numbered but in terms of months not weeks or days.
Subsurface temperature Anomalies
The 28C Isotherm is at the Dateline, the 27C Isotherm is at 170W, the 25C Isotherm is at 150W and the 20C Isotherm has reached the surface at about 110W which is not in the Nino 3.4 Measurement area.

 

The flattening of the Isotherm Pattern is an indication of ENSO Neutral just as the steepening of the pattern indicates La Nina or El Nino depending on where the slope shows the warm or cool pool to be. That flattening has occurred and we have gone to a Weak La Nina thermocline.

Tracking the change.

Sepember 15, 2017 Subsurface Water Temperatures Equatorial Ocean Subsurface as of December 14, 2017

 

I have "frozen" the graphic on the left side above which shows the situation as reported for September 15, 2017. The one on the right which is top half of the above graphic is updated by NOAA every five days but I have to update this snipped portion of that graphic into this article manually so the two can get out of phase by five days. It shows the situation now.  The situation is not much different east of the Dateline from the situation as reported for September 15, 2017. But west of the Dateline it looks a lot different  i.e. warmer. We use the graphic on the left as a reference to see how the current situation changes over time. September 15  to December 14, 2017, the subsurface warm anomaly has progressed to the east and strengthened substantially. It is not ready to displace or dilute the Eastern Pacific Cool Anomaly just yet but it will not be long. We now see the warm anomaly appearing below the cool anomaly at just east of 165W..

Here are the above graphics as a time sequence animation. You may have to click on them to get the animation going.

   

Equatorial Temperature Simulation

Isotherm Simulation

   

 

And now Let us look at the Atmosphere.
 

This graphic shows the Low-Level Wind Anomalies near the Equator.

The 850 hPa level is above the surface but close to the surface.

And now the Outgoing Long-wave Radiation (OLR) Anomalies which tells us where convection has been taking place. The bottom of a Hovmoeller graphic shows the most recent readings.
Low Level Wlind Anomalies

OLR Anomalies Along the Equator

Reds and browns would be suppressed easterlies or enhanced westerlies and are typical of El Nino. We have the opposite. We see suppressed Outgoing Long Wave Radiation (OLR) at the Dateline i.e (dry) and we  see enhanced OLR at 120E ( wet)

 

And Now the Air Pressure to Confirm that the Atmosphere is Reacting to the Sea Surface Temperature Pattern. The most Common way to do that is to use an Index called the SOI.

This index provides an easy way to assess the location of and the relative strength of the Convection (Low Pressure) and the Subsidence (High Pressure) near the Equator. Experience shows that the extent to which the Atmospheric Air Pressure at Tahiti exceeds the Atmospheric Pressure at Darwin Australia when normalized is substantially correlated with the Precipitation Pattern of the entire World. At this point there seems to be no need to show the daily preliminary values of the SOI but we can work with the 30 day and 90 day values.

Current SOI Readings

The 30 Day Average on December18 was reported as +5.00 which is NOT a La Nina value. The 90 Day Average was reported at +8.04 which is a marginal La Nina value. Looking at both the 30 and 90 day averages is useful and right now both are in agreement with the 90 day lagging the 30 day as one would expect.

 

SOI = 10 X [ Pdiff - Pdiffav ]/ SD(Pdiff) where Pdiff = (average Tahiti MSLP for the month) - (average Darwin MSLP for the month), Pdiffav = long term average of Pdiff for the month in question, and SD(Pdiff) = long term standard deviation of Pdiff for the month in question. So really it is comparing the extent to which Tahiti is more cloudy than Darwin, Australia. During El Nino we expect Darwin Australia to have lower air pressure and more convection than Tahiti (Negative SOI especially lower than -7 correlates with El Nino Conditions). During La Nina we expect the Warm Pool to be further east resulting in Positive SOI values greater than +7).

To some extent it is the change in the SOI that is of most importance. The MJO or Madden Julian Oscillation is an important factor in regulating the SOI and Ocean Equatorial Kelvin Waves and other tropical weather characteristics. More information on the MJO can be found here. Here is another good resource.

Forecasting the Evolution of ENSO

Here is the primary NOAA model for forecasting the ENSO Cycle. The CDAS model is a legacy "frozen" NOAA system meaning the software is maintained but not updated. We find it convenient to obtain this graphic from Tropical Tidbits.com

CFSv2 spread and bias correct ENSO forecast

CDAS Legacy System

This model is forecasting a La Nina. It probably is the most aggressive model re being so definitive about the ENSO Phase for this Fall and Winter. Click here to see a month by month version of the same model but without some of the correction methodologies applied. It gives us a better picture of the further out months as we are looking at monthly estimates versus three-month averages. Notice that since mid-October, the Nino 3.4 Index has been in a declining channel. It is not in conflict with the primary NOAA model but shows daily values rather then smoothing them out like the CFSv2 Model does.

 

The CFS.v2 is not the only forecast tool used by NOAA. The CPC/IRI Analysis which is produced out of The International Research Institute (IRI) for Climate and Society at Columbia University is also very important to NOAA. Below are the December 9 and November 20 CPC/IRI ENSO Forecasts

December 14, 2017 CPC/IRI ENSO 3.4 forecast.

And here is a larger version of the new forecast. NOAA has changed their format slightly and this is a transition month so that is why there are two sizes of graphics above.

Early December 2017 ENSO Forecast

Looks like the new forecast shows La Nina lasting a month longer than in the November forecasts. .

This is the discussion

CPC/IRI ENSO Update Published: December 14, 2017

El Niño/Southern Oscillation (ENSO) Diagnostic Discussion issued jointly by the Climate Prediction Center/NCEP/NWS and the International Research Institute for Climate and Society

ENSO Alert System Status: La Niña Advisory

Synopsis: La Niña is likely (exceeding ~80%) through the Northern Hemisphere winter 2017-18, with a transition to ENSO-neutral most likely during the mid-to-late spring.

La Niña strengthened during the past month, as indicated by an increasingly prominent pattern of below-average sea surface temperatures (SSTs) across the central and eastern equatorial Pacific Ocean (Fig. 1). The latest weekly Niño-3.4 index value was -0.8°C, with the easternmost Niño-3 and Niño-1+2 indices at or below -1.0°C during much of the month (Fig. 2). Sub-surface temperature anomalies weakened slightly during November, but remained significantly negative (Fig. 3) due to the anomalously shallow depth of the thermocline across the central and eastern Pacific (Fig. 4). The atmospheric circulation over the tropical Pacific Ocean also reflected La Niña, with convection suppressed near the International Date Line and enhanced over Indonesia (Fig. 5). The low-level trade winds were stronger than average over the western and central Pacific, with anomalous westerly winds at upper-levels.  Overall, the ocean and atmosphere system reflects La Niña.

La Niña is predicted to persist through the Northern Hemisphere winter 2017-18 by nearly all models in the IRI/CPC plume (Fig. 6) and in the North American Multi-Model Ensemble (NMME; Fig. 7). Based on the latest observations and forecast guidance, forecasters favor the peak of a weak-to-moderate La Niña during the winter (3-month Niño-3.4 values between 0.5°C and 1.5°C).  In summary, La Niña is likely (exceeding ~80%) through the Northern Hemisphere winter 2017-18, with a transition to ENSO-neutral most likely during the mid-to-late spring (click CPC/IRI consensus forecast for the chance of each outcome for each 3-month period).

La Niña is anticipated to affect temperature and precipitation across the United States during the upcoming months (the 3-month seasonal temperature and precipitation outlooks will be updated on Thursday December 21st). The outlooks generally favor above-average temperatures and below-median precipitation across the southern tier of the United States, and below-average temperatures and above-median precipitation across the northern tier of the United States

This discussion is a consolidated effort of the National Oceanic and Atmospheric Administration (NOAA), NOAA’s National Weather Service, and their funded institutions. Oceanic and atmospheric conditions are updated weekly on the Climate Prediction Center web site (El Niño/La Niña Current Conditions and Expert Discussions). Forecasts are also updated monthly in the Forecast Forum section of CPC’s Climate Diagnostics Bulletin. Additional perspectives and analysis are also available in an ENSO blog.

The next ENSO Diagnostics Discussion is scheduled for 11 January 2018. To receive an e-mail notification when the monthly ENSO Diagnostic Discussions are released, please send an e-mail message to: ncep.list.enso-update@noaa.gov.

Forecasts from Other Meteorological Agencies.

Here is the Nino 3.4 report from the Australian BOM (it updates every two weeks)

Australia POAMA ENSO model run

And the ENSO Outlook Discussion Issued on December 19, 2017:

La Niña conditions continue in the tropical Pacific. However, the event is expected to be short-lived, and is likely to end in the southern autumn of 2018.

Latest sea surface temperature observations in the central and eastern tropical Pacific persist at La Niña levels (0.8 °C below average). Waters are also cool beneath the surface. While the Southern Oscillation Index (SOI) has eased in the last few days, the SOI has been consistent with La Niña for several weeks. Cloudiness patterns across the Pacific region also show a clear La Niña signal.

In order for 2017-18 to be classed as a La Niña year, thresholds need to be exceeded for at least three months. [Editor’s Note:  The NOAA Criteria is more stringent requiring five consecutive overlapping three month periods which is more or less equivalent to a duration of seven months]. Most climate models surveyed by the Bureau suggest this event is likely to last through the southern summer, and decay in the early southern autumn of 2018. International climate models surveyed by the Bureau indicate that some further cooling of the equatorial Pacific sea surface temperatures is expected over summer. Six of the eight models remain within La Niña thresholds during March 2018.

The Bureau's model POAMA suggests that sea surface temperatures in the equatorial Pacific will remain within La Niña thresholds throughout summer and autumn. However, only three out of eight international climate models remain at La Niña levels by the end of autumn (May). In order for 2017-18 to be considered an event, La Niña conditions need to persist for at least three months.

La Niña typically brings above average rainfall to eastern Australia during summer, particularly in northern New South Wales and Queensland. However, with a weak event expected, this typically means less influence upon Australian rainfall. La Niña events can also increase the likelihood of prolonged warm spells for southeast Australia.

Here is the most recent JAMSTEC forecast issued on December 1, 2017

December 1, 2017 JAMSTEC Forecast

And here is the short discussion that was released on November 22, 2017 (we expect an update discussion this week which we will release with our Seasonal Outlook Update on Saturday December 23).

Nov. 22, 2017 Prediction from 1st Nov., 2017

ENSO forecast:

The weak La Niña-like condition will persist until boreal spring of next year. Then the tropical Pacific will return to a normal state by summer.

Indian Ocean forecast:

A normal state in the tropical Indian Ocean will persist until spring of next year. Then we expect a positive Indian Ocean Dipole in summer of 2018. However, there is a large uncertainty in the prediction at present because of the large spread in the prediction plumes of the dipole mode index.

Regional forecast:

On a seasonal scale, most part of the globe will experience a warmer-than-normal condition, while some parts of Africa and Brazil will experience a colder-than-normal condition in boreal winter.

As regards to the seasonally averaged rainfall, a wetter-than-normal condition is predicted for some parts of East Africa, eastern Southern Africa, Philippine, East Australia, and northern Brazil during boreal winter, whereas most parts of Indonesia, West Australia, West Africa, southern Europe, western U.S, eastern China and southern Brazil will experience a drier condition during boreal winter. Those are partly due to the weak La Niña-like condition.

In winter, most parts of Japan will experience warmer- and drier-than-normal conditions.

Indian Ocean IOD (It updates every two weeks)

BOM IOD Forecast.

Indian Ocean Dipole Outlook Discussion Issued December 19

The Indian Ocean Dipole (IOD) is neutral. The weekly index value to 17 December was −0.5 °C. All six of the climate models surveyed by the Bureau indicate that the IOD will remain neutral into autumn 2018.

The influence of the IOD on Australian climate is weak during December to April. This is because the monsoon trough shifts south over the tropical Indian Ocean changing wind patterns, which prevents the IOD pattern from being able to form.

The IOD Forecast is indirectly related to ENSO but in a complex way. It is important to understand how and where the IOD is measured.

IOD Measurement Regions

IOD Positive is the West Area being warmer than the East Area (with of course many adjustments/normalizations). IOD Negative is the East Area being warmer than the West Area. Notice that the Latitudinal extent of the western box is greater than that of the eastern box. This type of index is based on observing how these patterns impact weather and represent the best efforts of meteorological agencies to figure these things out. Global Warming may change the formulas probably slightly over time but it is costly and difficult to redo this sort of work because of long weather cycles.

D. Putting it all Together.

At this time it would seem a La Nina is here for this Winter and Spring with La Nina Conditions already in place. But the situation for next Summer is not yet clear. However, we are getting very close to being able to forecast the end of this La Nina event. But it may only be a temporary end for the Summer and next Fall as it may return for next Winter or Spring. But that is now looking less likely with ENSO Neutral highly likely.

Forecasting Beyond Five Years.

So in terms of long-term forecasting, none of this is very difficult to figure out actually if you are looking at say a five-year or longer forecast.

The research on Ocean Cycles is fairly conclusive and widely available to those who seek it out. I have provided a lot of information on this in prior weeks and all of that information is preserved in Part II of my report in the Section on Low Frequency Cycles 3. Low Frequency Cycles such as PDO, AMO, IOBD, EATS. It includes decade by decade predictions through 2050. Predicting a particular year is far harder.

The odds of a climate shift for the Pacific taking place has significantly increased. It may be in progress. The AMO is pretty much neutral at this point (but more positive i.e. warm than I had expected) so it may need to become a bit more negative for the "McCabe A" pattern to become established. That seems to be slow to happen so I am thinking we need at least a couple more years for that to happen. So our assessment is that the standard time for Climate Shifts in the Pacific are likely to prevail and it most likely will be a gradual process with a speed up in less than five years but more than two years. The next El Nino may be the trigger.

E. Relevant Recent Articles and Reports

Weather in the News

Top Five November Globally

Weather Research in the News

Nothing to report

Global Warming in the News

Nothing to report.

F. Table of Contents for Page II of this Report Which Provides a lot of Background Information on Weather and Climate Science

The links below may take you directly to the set of information that you have selected but in some Internet Browsers it may first take you to the top of Page II where there is a TABLE OF CONTENTS and take a few extra seconds to get you to the specific section selected. If you do not feel like waiting, you can click a second time within the TABLE OF CONTENTS to get to the specific part of the webpage that interests you.

1. Very High Frequency (short-term) Cycles PNA, AO,NAO (but the AO and NAO may also have a low frequency component.)

2. Medium Frequency Cycles such as ENSO and IOD

3. Low Frequency Cycles such as PDO, AMO, IOBD, EATS.

4. Computer Models and Methodologies

5. Reserved for a Future Topic (Possibly Predictable Economic Impacts)

G. Table of Contents of Contents for Page III of this Report - Global Warming Which Some Call Climate Change.

The links below may take you directly to the set of information that you have selected but in some Internet Browsers it may first take you to the top of Page III where there is a TABLE OF CONTENTS and take a few extra seconds to get you to the specific section selected. If you do not feel like waiting, you can click a second time within the TABLE OF CONTENTS to get to the specific part of the webpage that interests you.

1. Introduction

2. Climate Impacts of Global Warming

3. Economic Impacts of Global Warming

4. Reports from Around the World on Impacts of Global Warming

H. Useful Background Information

The current conditions are measured by determining the deviation of actual sea surface temperatures from seasonal norms (adjusted for Global Warming) in certain parts of the Equatorial Pacific. The below diagram shows those areas where measurements are taken.

El Nino Zones

NOAA focuses on a combined area which is all of Region Nino 3 and part of Region Nino 4 and it is called Nino 3.4. They focus on that area as they believe it provides the best correlation with future weather for the U.S. primarily the Continental U.S. not including Alaska which is abbreviated as CONUS. The historical approach of measurement of the impact of the sea surface temperature pattern on the atmosphere is called the Southern Oscillation Index (SOI) which is the difference between the atmospheric pressure at Tahiti as compared to Darwin Australia. It was convenient to do this as weather stations already existed at those two locations and it is easier to have weather stations on land than at sea. It has proven to be quite a good measure. The best information on the SOI is produced by Queensland Australia and that information can be found here. SOI is based on Atmospheric pressure as a surrogate for Convection and Subsidence. Another approach made feasible by the use of satellites is to measure precipitation over the areas of interest and this is called the El Nino - Southern Oscillation (ENSO) Precipitation Index (ESPI). We covered that in a weekly Weather and Climate Report which can be found here. Our conclusion was that ESPI did not differentiate well between La Nina and Neutral. And there is now a newer measure not regularly used called the Multivariate ENSO Index (MEI). More information on MEI can be found here. The jury is still out on MEI and it it is not widely used.

The below diagram shows the usual location of the Indo-Pacific Warm Pool. When the warm water shifts to the east we have an El Nino; to the west a La Nina.

Western Pacific Warm Pool

Click for Source

Interaction between the MJO and ENSO

This Table is a first attempt at trying to relate the MJO to ENSO

El Nino La Nina MJO Active Phase MJO Inactive Phase Relationship of MJO and ENSO Eastern Pacific Easterlies Western Pacific Westerlies MJO Active Phase MJO Inactive Phase

  • Weaker
  • Stronger
  • Part of Decay Process
  • Counteracts Easterlies
  • Enhances Easterlies
  • Stronger
  • May Create or Stimulate the Onset of El Nino via Kelvin Waves
  • Weaker
  • Part of Decay Process
  • Strengthens Westerlies
  • Weakens Westerlies
  • More likely
  • Stimulates
  • Less likely and weak
  • Retards development of a new La Nina
  • Stimulates the Jet Stream
 
  • Less Likely
  • Suppresses
  • More likely but weak
  • Accelerates development of a new La Nina and the Decline of a mature La Nina
 
  • Slows the Jet Stream and can induce a Split Stream especially during a La Nina

 

Table needs more work. Is intended to show the interactions. What is more difficult is determining cause and effect. This is a Work in Progress.

History of ENSO Events as measured by the ONI

December 4, 2017 Revised Historical ONI Readins.

Note: Without fanfare the base climatology was recently changed from ERSST.V4 to ERSST.V5. This is done every five years and is totally proper but it does shuffle the deck re what were and were not ENSO events so it perhaps should have had more press but that is not the fault of NOAA but if they had kept me in the loop I would have covered it. I was not excluded just not included and did not notice it until after the fact. No big  deal but it is important. 

The new SON reading of -0.7 is the first La Nina Reading. These would have to extend through JFM 2018 for this to be recorded as a La Nina. The chances of this are about  50:50. The full history of the ONI readings can be found here. The MEI index readings can be found here.

Click here for a list of Sig Silber's Weather Posts

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