On November 17, NOAA released their Seasonal Outlook. NOAA is all in on La Nina determining our weather but the dynamic forecast models are not in sync with the statistical models that are based on historical La Nina's . The wild card is the Arctic Oscillation (AO) which could make the East Coast and Europe feel like Siberia (Alaska is unusually warm right now).
Last week, JAMSTEC, which is a Japanese Research Institute, issued their Outlook based on their November 1 forecast for ENSO. We compare the JAMSTEC temperature and precipitation forecast maps which are for the World with the NOAA temperature and precipitation forecast maps that cover only CONUS and Alaska. We do this primarily for educational purposes. Remember: these are forecasts not guarantees. But the differences between the NOAA and JAMSTEC forecasts are significant and mostly related to the differing perspectives on the strength and duration of this Cool ENSO Event
Some Housekeeping Issues
This report will be posted Saturday November 19 and will be referred to in the Weekly Report for reference until the Saturday after the next NOAA Update on December 15. The Regular Weekly Report will be published on November 21. If you are reading this Update Report and wish to transfer to the Current Weather and Climate Report, Click here for the list of Weather Posts. That link takes you to the archive of all weather articles written by Sig Silber so you can if the new Weekly Weather Report has been published go there or back to an earlier report but please keep in mind that the graphics in earlier reports in some case auto-update and the text may no longer apply to the graphics shown. 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.
NOAA Updated Seasonal Outlook
NOAA issued their updated Seasonal Outlook on the third Thursday of the month i.e. November 17, 2016 as is their normal schedule. Let's take a look.
We have nothing to compare these maps with as NOAA does not in their Update provide a forecast for two months out separate from their three-month forecast. Thus I do not have December maps from the October 20 NOAA Report to compare against. And November is not over so we can not really compare the December forecast against November actual. It is probably best to just try to understand what NOAA is trying to convey about December which can be summarized as warmer than climatology in the extreme Northeast and the Western half of CONUS from Mid-Montana southeast through Mississippi. For precipitation, we have a dry southeast, a wet Northern Great Lakes, and a wet Montana/Wyoming/Northeast Idaho area.
Now we consider the three-month Outlook.
Notice that the three-month periods are abbreviated e.g. December/January/February is shown as DJF. You will see such abbreviations often in this report.
Prior Temperature Outlook for DJF 2016
New Temperature Outlook for DJF 2016
It is almost identical to the prior forecast with the Maine warm anomaly extended further south to now cover all of New England. The area designated EC means Equal Chances of being more or less than normal.
Prior Precipitation Outlook for DJF 2016
New Precipitation Outlook for DJF 2016
It is pretty much the same but with the probabilities of dry anomalies reduced for parts of the Southern Tier mostly Texas and Southern Louisiana.
Now let us focus on the long-term situation and compare the new set of maps with the maps issued on September 15, 2016.
Prior 14 Month Temperature Outlook: DJF 2016 - NDJ 2017/2018
New 14 Month Temperature Outlook: JFM 2017 - DJF 2017/2018
To compare maps from one release to another one needs to remember that the new release drops one three-month period and adds a later one. So to make the comparisons one has to shift the new maps to the right one position and that makes the map on the right drop down to become the left-most map in the next level. I do not have a computer software tool for doing that for you so you have to do it mentally. When I do the comparison, I print the two sets of maps and put them side by side and number the same three-month maps 1, 2, 3,.....,11 in both sets of maps to make it easier for me to easily compare the same three-month period in the new with the previous forecast. One uses the same procedure to compare the precipitation maps. Based on this procedure, I conclude that:
The changes are minor and mainly an extension of the warm anomaly into the Northeast starting with JFM 2017 and in one period AMJ 2017 northward expansion of the warm anomaly and higher probabilities more prominently in the western half of CONUS. The small EC Area in the Dakotas disappears in JAS 2017
New 14 Month Precipitation Outlook: JFM 2017 - DJF 2017/2018
A change in the extension of the wet Northeast Tier anomaly to the Pacific Coast in MAM 2017. If I recall correctly, NOAA has been going back and forth on this feature. A more interesting change is the elimination of the dry Southwest anomaly in AMJ 2017. A new Northern California dry anomaly shows up in NDJ 2017/2018 a year from now.
If you want larger versions of each map (temperature and precipitation) you can find them here. And each of those maps can be clicked on to further enlarge them.
Sometimes it is useful to compare the present month outlook to the three-month outlook
One can mentally subtract the December Outlook from the three-month Outlook and create the Outlook for the last two months in the three-month period namely January and February 2017. When I do that, I deduce that:
With respect to temperature, The three-month forecast has a large cool area across the Northern Tier that is not in the December Map. This suggests that January and February may have to have higher percentages for the cool anomaly in order for the three-month average to work out. The same logic but in the opposite direction applies to the Southeast where January and February will need to have higher probabilities for being warm. The opposite reasoning applies to the Northwest which is shown as warm in December but for the three-month average to work out as EC, January and February may need to be cool. The precipitation relates mainly to the Southern Tier west of Louisiana. The dry precipitation probabilities for January and February will need to be a bit higher than shown in the three-month average to make that average work out since that area is shown as EC In December. .
The whole idea is that a three-month average set of probabilities for anomalies is the sum of that information for three months. If you have the first month differing in places from the three-month average you can calculate what a map of the second and third month would look like for the sum of the two maps ( November and December/January) to equal the published three-month map.
Below are excerpts (significantly reorganized and with a lot of the redundancy removed) from the Discussion released by NOAA on November 17, 2016. Headings that are "Initial Cap" only rather than all caps were added by the Author of this Update Report for clarity. Also we have organized the sequence of the sections of NOAA Discussion to first present the Atmospheric Conditions and then the Current Month, the Three or Four-month period, and finally the remainder of the 15 Month Forecast. We think that sequence with the three- to four-month period broken out separately, makes the discussion more useful for more readers.
CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS
IN THE TROPICAL PACIFIC, BOTH OCEANIC AND ATMOSPHERIC OBSERVATIONAL INDICATORS (ON AVERAGE) INDICATE LA NINA CONDITIONS, THOUGH THE TRADE WINDS REMAIN NEAR NORMAL ON AVERAGE. [Editor's Note: Normally a reason not to conclude that La Nina conditions exist]
SEA SURFACE TEMPERATURES (SSTS) ALONG THE EQUATOR IN THE PACIFIC BASIN REMAIN BELOW AVERAGE FROM 180W TO 100W, WHILE SSTS ARE ABOVE AVERAGE ACROSS THE FAR WESTERN PACIFIC AND IN A HORSESHOE PATTERN NORTH AND SOUTH OF THE AREA OF COOLER THAN NORMAL SSTS ALONG THE EQUATOR. IN THE NINO-3.4 REGION, THE MOST RECENT WEEKLY ANOMALY VALUE IS -0.7 DEGREES C WHILE THE MOST RECENT THREE-MONTH AVERAGE NINO-3.4 SST ANOMALY IS -0.7 DEGREES C FOR THE ASO SEASON. A RESERVOIR OF ANOMALOUSLY COOL SUBSURFACE WATERS CONTINUES AT DEPTH AND EXTENDS TO MORE THAN 150 METERS BELOW THE SURFACE, WITH THE LARGEST NEGATIVE ANOMALIES OF 2-4 DEGREES C BELOW AVERAGE FROM 100-150 METERS CENTERED NEAR 140W.
OVER THE PAST SEVERAL WEEKS, ENHANCED CONVECTION HAS BEEN CENTERED OVER THE MARITIME CONTINENT, WITH SUPPRESSED CONVECTION EXTENDING EASTWARD FROM 160E ACROSS MUCH OF THE EQUATORIAL PACIFIC. RECENTLY, HOWEVER, SUBSEASONAL VARIABILITY HAS DISRUPTED THIS PATTERN AND LED TO DRYING ACROSS PARTS OF THE MARITIME CONTINENT AND EASTERN INDIAN OCEAN. TRADE WINDS OVER THE PAST MONTH HAVE BEEN NEAR AVERAGE.
SSTS IN THE NORTHEAST PACIFIC AND ALONG THE COAST OF ALASKA REMAIN ABOVE AVERAGE, ALTHOUGH THE ATMOSPHERIC CIRCULATION HAS FORCED A DRAMATIC COOLING OVER THE NORTH-CENTRAL PACIFIC BASIN. POSITIVE SST ANOMALIES PERSIST IN THE WESTERN ATLANTIC NEAR THE U.S. EAST COAST, AND THE GULF OF MEXICO.
PROGNOSTIC DISCUSSION OF SST FORECASTS
THE CPC SST CONSOLIDATION FORECAST, WHICH INCLUDES THREE STATISTICAL FORECASTS ALONG WITH THE CFS, PREDICTS A WEAK LA NINA DURING DJF 2016-2017 WITH ENSO-NEUTRAL CONDITIONS FAVORED THEREAFTER. BY WINTER 2017-2018, THE CONSOLIDATION FAVORS LA NINA OVER EL NINO. THE NORTH AMERICAN MULTI MODEL ENSEMBLE (NMME) ENSEMBLE MEAN FORECAST FOR THE NINO-3.4 SST ANOMALY IS SLIGHTLY WARMER THAN LAST MONTH, WITH ENSO-NEUTRAL FAVORED EARLY IN OUTLOOK PERIOD. BASED ON THE LATEST OBSERVATIONAL AND MODEL FORECAST INDICATORS, THE OFFICIAL CPC/IRI ENSO OUTLOOK FAVORS LA NINA CONDITIONS AT 55% DURING DJF 2016-2017, THE SAME PROBABILITY THAT WAS ISSUED LAST MONTH. [Editor’s Note: Then why was the ENSO Status changed?]
PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS
THE SUBSEQUENT TEMPERATURE AND PRECIPITATION OUTLOOKS CONSIDERED LA NINA REGRESSIONS, STATISTICAL TOOLS, SUCH AS REGRESSIONS BASED ON THE CPC CONSOLIDATED SST FORECAST, A CONSTRUCTED ANALOG BASED ON GLOBAL SST ANOMALY PATTERNS, AND CANONICAL CORRELATION ANALYSIS THAT USES THE EVOLUTION OF SST AND SEA-LEVEL PRESSURE AS PREDICTORS. ALSO CONSIDERED WERE THE CALIBRATED PROBABILISTIC FORECASTS FROM THE NMME, GUIDANCE FROM PARTICIPANT MODELS OF THE INTERNATIONAL MULTI-MODEL ENSEMBLE (IMME) AND THE CPC CONSOLIDATION (CPC CON) FORECAST. OTHER FACTORS INCLUDED POTENTIAL INFLUENCE FROM MORE LOCAL, COASTAL SSTS.
30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2016
THE DECEMBER 2016 TEMPERATURE AND PRECIPITATION OUTLOOKS ARE BASED ON DYNAMICAL MODEL GUIDANCE, STATISTICAL TOOLS, AND THE TYPICAL INFLUENCE FROM LA NINA. DURING THE PAST COUPLE OF MONTHS, LA NINA CONDITIONS DEVELOPED AS OBSERVED WITH NEGATIVE SEA SURFACE TEMPERATURES ACROSS THE EASTERN AND CENTRAL EQUATORIAL PACIFIC OCEAN ALONG WITH SUPPRESSED (ENHANCED) CONVECTION ACROSS THE CENTRAL TROPICAL PACIFIC (INDONESIA). CLIMATE SIGNALS TYPICALLY ASSOCIATED WITH LA NINA WERE CONSIDERED IN MAKING THE DECEMBER 2016 OUTLOOK, GIVEN THESE CURRENT ATMOSPHERIC AND OCEANIC STATES.
ALTHOUGH THE MJO STRENGTHENED DURING EARLY NOVEMBER, ITS EFFECTS ON THE EXTRATROPICS ARE UNCLEAR DUE TO INTERFERENCE WITH THE LOW FREQUENCY BASE STATE. THE DYNAMICAL MODELS INDICATE A DECREASE IN THE AMPLITUDE OF THE MJO SIGNAL DURING THE NEXT WEEK. THEREFORE, THE MJO DID NOT FACTOR INTO THE DECEMBER OUTLOOK. ITS EVOLUTION WILL BE CLOSELY MONITORED FOR THE REVISED OUTLOOK AT THE END OF NOVEMBER.
DESPITE EXCELLENT AGREEMENT AMONG THE DYNAMICAL MODELS THAT FAVOR ABOVE-NORMAL TEMPERATURES THROUGHOUT THE FORECAST DOMAIN, FORECAST CONFIDENCE IN THE TEMPERATURE OUTLOOK IS LOWER THAN WOULD BE EXPECTED. THE ARCTIC OSCILLATION (AO) INDEX HAD ONE ITS MOST NEGATIVE VALUES FOR OCTOBER. A NEGATIVE AO INDEX IS ASSOCIATED WITH ABOVE (BELOW) NORMAL 500-HPA HEIGHTS AT THE HIGHER (MIDDLE) LATITUDES OF THE NORTHERN HEMISPHERE. THIS PATTERN RESULTED IN BELOW-NORMAL TEMPERATURES ACROSS PARTS OF EURASIA. HOWEVER, ABOVE-NORMAL TEMPERATURES WERE OBSERVED ACROSS MOST OF THE LOWER 48 DURING OCTOBER. AFTER A BRIEF PERIOD WITH A POSITIVE AO INDEX, THE GFS MODEL REMAINS CONSISTENT THAT THE AO INDEX BECOMES SHARPLY NEGATIVE ONCE AGAIN DURING LATE NOVEMBER. BELOW-NORMAL TEMPERATURES ARE TYPICALLY ASSOCIATED WITH A NEGATIVE AO INDEX FROM THE NORTHERN GREAT PLAINS SOUTHEAST TO THE SOUTHERN ATLANTIC COASTAL STATES DURING DECEMBER. GIVEN THE LARGE DISAGREEMENT BETWEEN TEMPERATURE COMPOSITES OF A NEGATIVE AO INDEX (BELOW NORMAL) AND DYNAMICAL MODEL GUIDANCE (ABOVE NORMAL), EQUAL CHANCES FOR BELOW, NEAR, OR ABOVE-NORMAL TEMPERATURES ARE FORECAST FROM THE NORTHERN GREAT PLAINS EXTENDING INTO THE SOUTHEAST. ELSEWHERE, ABOVE-NORMAL TEMPERATURES ARE FAVORED ACROSS NEW ENGLAND AND THE WESTERN CONUS ALTHOUGH PROBABILITIES ARE TEMPERED DUE TO THE BLOCKY 500-HPA PATTERN OVER THE NORTHERN HEMISPHERE EXPECTED AT THE BEGINNING OF DECEMBER.
THE PRECIPITATION OUTLOOK FOR DECEMBER IS BASED PRIMARILY ON THE CALIBRATED NMME AND IS CONSISTENT WITH LA NINA AND THE DJF PRECIPITATION OUTLOOK. HOWEVER, THE FAVORED AREA FOR BELOW-MEDIAN PRECIPITATION OVER THE SOUTHEAST DOES NOT EXTEND INTO THE SOUTHWESTERN CONUS SINCE THE LATEST RUNS OF THE CFS INDICATE AN UPPER-LEVEL TROUGH OVER THIS REGION DURING EARLY DECEMBER. A SLIGHT TILT IN THE ODDS FOR ABOVE-MEDIAN PRECIPITATION DOWNWIND OF THE GREAT LAKES IS BASED ON ELEVATED CHANCES OF LAKE-ENHANCED PRECIPITATION WITH THE ABOVE-NORMAL LAKE TEMPERATURES. THE PROBABILITIES FOR BELOW (ABOVE)-MEDIAN PRECIPITATION FORECAST ACROSS THE SOUTHEASTERN (NORTHWESTERN) CONUS ARE MODEST DUE TO LIMITED SKILL INHERENT IN A MONTHLY OUTLOOK AT THIS TIME LEAD. THIS PRECIPITATION OUTLOOK WILL LIKELY BE REVISED AT THE END OF NOVEMBER.
THE HIGHEST ODDS FOR ABOVE-NORMAL TEMPERATURES THROUGHOUT THE FORECAST DOMAIN ARE FORECAST ACROSS THE ALEUTIANS WHERE SEA SURFACE TEMPERATURE ANOMALIES ARE AVERAGING AS MUCH AS 1.5 DEGREES C ABOVE NORMAL. EQUAL CHANCES FOR BELOW, NEAR, OR ABOVE-NORMAL TEMPERATURES FORECAST ACROSS INTERIOR EASTERN ALASKA IS DUE TO DIFFERENCES AMONG DYNAMICAL MODEL GUIDANCE AND STATISTICAL TOOLS BASED ON LA NINA. THE DYNAMICAL MODELS OFFER WEAK PRECIPITATION SIGNALS ACROSS ALASKA WITH THE CALIBRATED NMME INDICATING INCREASED CHANCES FOR ABOVE-MEDIAN PRECIPITATION ACROSS THE ALEUTIANS AND WESTERN ALASKA.
Three Months DJF 2016/2017
THE DJF 2016-2017 TEMPERATURE OUTLOOK INDICATES INCREASED PROBABILITIES OF ABOVE-NORMAL TEMPERATURES FOR MUCH OF THE SOUTHERN U.S., NEW ENGLAND, AND ALASKA. A SLIGHT TENDENCY TOWARD BELOW-NORMAL TEMPERATURES IS FORECAST OVER PARTS OF THE UPPER MIDWEST AND NORTHERN PLAINS. THIS OUTLOOK IS INFORMED MOSTLY BY OBJECTIVE EMPIRICAL GUIDANCE THAT INCORPORATES ENSO, TRENDS, AND OBSERVED SST EVOLUTION. CALIBRATED DYNAMICAL MODEL GUIDANCE IS ALSO CONSIDERED.
THE DJF 2016-2017 PRECIPITATION OUTLOOK INDICATES INCREASED PROBABILITIES OF ABOVE-MEDIAN PRECIPITATION FOR PARTS OF THE NORTHERN TIER OF THE U.S., CENTERED ON THE NORTHERN ROCKIES AND GREAT LAKES. BELOW-MEDIAN PRECIPITATION IS FAVORED FOR MUCH OF THE SOUTHERN TIER OF THE CONTIGUOUS U.S. WITH THE GREATEST PROBABILITIES ALONG PARTS OF THE GULF COAST. THE OUTLOOK IS BASED ON A NUMBER OF FACTORS INCLUDING LA NINA CONSIDERATIONS AND SHORT TERM CLIMATE MODEL OUTPUT.
PROGNOSTIC DISCUSSION OF OUTLOOKS - DJF 2016 TO DJF 2017 [Editor's Note: Usually I either move the part of this discussion that relates to the first three or four months into the above discussion of the next three months but did not do so this time because I concluded it fits better here even if a bit redundant]
THE TEMPERATURE OUTLOOKS FOR DJF AND JFM REMAIN HIGHLY UNCERTAIN. STATISTICAL GUIDANCE USING SST-BASED PREDICTORS AND TRENDS HINT TOWARD A COLDER SOLUTION ACROSS THE NORTHERN CONUS, WHILE DYNAMICAL MODEL GUIDANCE IS NEARLY UNANIMOUS IN DEPICTING A WARMER-THAN-NORMAL SOLUTION OVER MOST OF NORTH AMERICA. HOWEVER, HINDCAST EVALUATION REVEALS THAT THE MODEL GUIDANCE HAS LIMITED FORECAST SKILL FOR THESE SEASONS, AND CALIBRATED GUIDANCE REVEALS SOME SIMILARITIES IN STRUCTURE TO THE STATISTICAL GUIDANCE. THE OFFICIAL OUTLOOK FAVORS THE STATISTICAL GUIDANCE, DEPICTING A TILT TOWARD BELOW-NORMAL TEMPERATURES OVER THE NORTHERN PLAINS AND UPPER MIDWEST. LA NINA REGRESSIONS WOULD TEND TO FAVOR A COLDER SOLUTION OVER THE NORTHWEST, BUT TRENDS AND BOTH STATISTICAL AND DYNAMICAL GUIDANCE ARE NOT SUPPORTIVE FOR DJF. BY JFM PROBABILITIES FAVORING BELOW-NORMAL TEMPERATURES ARE INTRODUCED.
IT IS POSSIBLE THAT LOW-FREQUENCY EXTRATROPICAL VARIABILITY AND SUBSEASONAL TROPICAL VARIABILITY WILL BE PARTICULARLY IMPORTANT IN DETERMINING THE FINAL DISTRIBUTION OF TEMPERATURE ANOMALIES OVER NORTH AMERICA. THE ARCTIC OSCILLATION HAS BEEN IN A FAIRLY PERSISTENT NEGATIVE PHASE SINCE OCTOBER 1, AND IS FORECAST TO REMAIN GENERALLY NEGATIVE INTO DECEMBER. WHETHER THIS IS A HARBINGER OF THINGS TO COME THROUGHOUT THE WINTER SEASON IS NOT WELL KNOWN, BUT THERE IS SOME EVIDENCE TO SUGGEST AN NEGATIVE AO IS MORE LIKELY THIS WINTER. THIS IN PART LEADS TO VERY LOW PROBABILITIES ACROSS THE EAST COAST, WHERE LA NINA AND TRENDS WOULD TEND TO FAVOR A SLIGHTLY WARMER OUTCOME. MJO ACTIVITY COULD ALSO PLAY AN IMPORTANT ROLE. ONE OR TWO STRONG CONVECTIVE EVENTS OVER THE INDIAN OCEAN COULD FORCE A ROSSBY WAVE RESPONSE THAT WOULD TILT THE WINTER TOWARD THE WARMER OUTCOME INDICATED BY DYNAMICAL MODELS. HOWEVER, IF ENHANCED CONVECTION IS CENTERED FARTHER EAST OVER THE MARITIME CONTINENT AND FAR WESTERN PACIFIC, THE COLDER OUTCOME INDICATED BY STATISTICAL GUIDANCE IS MORE LIKELY ACROSS THE NORTHERN CONUS.
HEADING INTO LATE WINTER AND EARLY SPRING, BELOW-NORMAL TEMPERATURES ARE EXPANDED AND SHIFTED WESTWARD TO INCLUDE NORTHERN AREAS OF THE ROCKIES AND PACIFIC NORTHWEST. BY MAM AND AMJ, THE STATISTICAL AND DYNAMICAL TOOLS COME INTO BETTER AGREEMENT, DEPICTING PROBABILITIES FAVORING ABOVE-NORMAL TEMPERATURES OVER THE SOUTHERN, CENTRAL, AND EASTERN CONUS. ABOVE NORMAL TEMPERATURES REMAIN FAVORED FROM DJF THROUGH AMJ ACROSS THE SOUTHERN TIER OF THE CONTIGUOUS U.S. WITH THE GREATEST PROBABILITIES OVER THE SOUTHWEST.
THE OUTLOOKS FROM MJJ 2017 AND BEYOND PRIMARILY REFLECT INPUT FROM TRENDS, THE CCA, AND THE CONSTRUCTED ANALOG BASED ON GLOBAL SST AND FAVOR ABOVE-NORMAL TEMPERATURES FOR MUCH OF THE CONUS AND ALASKA. TRENDS IN SEA ICE EXTENT INCREASE PROBABILITIES FOR ABOVE-NORMAL TEMPERATURES FOR THE NORTH SLOPE OF ALASKA DURING SON AND OND 2017.
BY WINTER 2017-2018, THE OUTLOOK IS MOSTLY INFLUENCED BY TRENDS DUE TO THE UNCERTAINTY REGARDING ENSO. WHILE LA NINA EVENTS OFTEN OCCUR BACK-TO-BACK, SUCH AN OUTCOME IS NOT FAVORED AT THIS TIME.
THE DJF 2016-2017 PRECIPITATION OUTLOOK WAS PRIMARILY BASED ON CONSIDERATIONS OF LA NINA CONDITIONS (REGRESSIONS ANCHORED TO THE CPC SST CONSOLIDATION WITH TRENDS) AND RELATIVELY CONSISTENT SIGNALS IN CALIBRATED DYNAMICAL MODEL GUIDANCE FROM THE NMME. ABOVE-MEDIAN PRECIPITATION IS FAVORED FOR NORTHWEST ALASKA AND PORTIONS OF THE PACIFIC NORTHWEST, NORTHERN ROCKIES, NORTHERN GREAT PLAINS AND GREAT LAKES. BELOW MEDIAN PRECIPITATION IS FAVORED ACROSS THE SOUTHERN TIER OF THE CONTIGUOUS U.S. WITH THE GREATEST PROBABILITIES ALONG THE GULF COAST. IN ADDITION, BELOW-MEDIAN PRECIPITATION IS FAVORED DURING DJF 2016-2017 FOR COASTAL SOUTH-CENTRAL ALASKA SUPPORTED BY LA NINA CONSIDERATIONS AND DYNAMICAL MODEL GUIDANCE.
THE PRECIPITATION OUTLOOK REMAINS SIMILAR, TO THE FIRST ORDER, THROUGH FMA 2017 WITH A FAVORED ACTIVE NORTHERN STREAM AND WETTER THAN AVERAGE CONDITIONS MOST LIKELY IN THE AFOREMENTIONED AREAS. COVERAGE FOR ABOVE-MEDIAN PRECIPITATION IS SLIGHTLY INCREASED IN DJF 2016-17 AND JFM 2017 IN SOME AREAS AS ANY LA NINA INFLUENCE LIKELY MAY BE MORE PRONOUNCED IN THESE SEASONS AND SIGNALS FROM THE DYNAMICAL MODEL GUIDANCE ARE SLIGHTLY STRONGER. BELOW-MEDIAN PRECIPITATION REMAINS FAVORED FOR THE SOUTHERN TIER DURING THESE SEASONS WITH THE LOWEST COVERAGE AND PROBABILITIES INDICATED IN FMA 2017.
POSITIVE PRECIPITATION TRENDS CONTRIBUTE TO THE OUTLOOK ACROSS SOME AREAS OF THE NORTHERN PLAINS AND GREAT LAKES BEGINNING IN FMA 2017 AND THIS CONTINUES THROUGH MJJ 2017. A POSITIVE TREND SIGNAL IS ALSO EVIDENT AND DEPICTED ACROSS THE NORTHEAST CONUS FROM JJA 2017 THROUGH SON 2017. DRIER THAN AVERAGE CONDITIONS ARE FAVORED FOR MAM AND AMJ 2017 ACROSS THE SOUTHWEST BASED ON NEGATIVE HISTORICAL PRECIPITATION TRENDS.
FOR NDJ AND DJF 2017-2018, BELOW-MEDIAN PRECIPITATION IS SLIGHTLY FAVORED OVER PARTS OF THE FAR SOUTHEASTERN CONUS AND THE WEST COAST CENTERED ON NORTHERN CALIFORNIA. THESE SIGNALS ARE EVIDENT IN THE VARIOUS STATISTICAL FORECAST TOOLS AND IS HEAVILY INFLUENCED BY TRENDS.
You will notice that the Arctic Oscillation (AO) is mentioned a lot in the above discussion so I am now presenting information on the AO and it is relative the NAO
Arctic Oscillation (AO)
The Arctic Oscillation (AO) is a climate index of the state of the atmospheric circulation over the Arctic. It consists of a positive phase, featuring below average geopotential heights , which are also referred to as negative geopotential height anomalies , and a negative phase in which the opposite is true. In the negative phase, the polar low pressure system (also known as the polar vortex) over the Arctic is weaker, which results in weaker upper level winds (the westerlies). The result of the weaker westerlies is that cold, Arctic air is able to push farther south into the U.S., while the storm track also remains farther south. The opposite is true when the AO is positive: the polar circulation is stronger which forces cold air and storms to remain farther north. The Arctic Oscillation often shares phase with the North Atlantic Oscillation (NAO) (discussed below), and its phases directly correlate with the phases of the NAO concerning implications on weather across the U.S. and Europe.
Perhaps the following graphics are helpful. It would be nice to show both phases side by side but the type would be too small to easily read. So I am showing each separately and then a side by side of smaller images.
First the Warm Phase (AO+ also called AO Positive) .
Then the Cool Phase (AO- also called AO Negative)
Here they are side by side for comparison. The warm phase is on the left, the cool phase on the right. .
There are two parts to this discussion: the atmospheric factors that cause the two phases and the impacts on weather which is mainly the location and strength of the jet stream and both are shown in these graphics. All of this is courtesy of the State Climate Office of North Carolina
And now the forecast
The above are not long-term forecasts and there is concern that we may be heading into a severe AO Negative Condition.
The AO is related to the NAO so we might was well discuss that as well.
North Atlantic Oscillation (NAO)
The North Atlantic Oscillation (NAO) consists of two pressure centers in the North Atlantic: one is an area of low pressure typically located near Iceland, and the other an area of high pressure over the Azores (an island chain located in the eastern Atlantic Ocean). It is important to note that these two locations are most commonly used to measure the NAO, but studies have found that the pressure centers move around on a seasonal basis, and other locations have also been used for measuring this index. Fluctuations in the strength of these features significantly alters the alignment of the jet stream, especially over the eastern U.S., and ultimately affects temperature and precipitation distributions in this area. It is also important to note that the AO and NAO are two separate indices that are ultimately describing the same phenomenon of varying pressure gradients in the northern latitudes and the resultant effects on temperature and storm tracks across the continent.
During a positive NAO there is a strengthening of the Icelandic low and Azores high. This strengthening results in an increased pressure gradient over the North Atlantic, which cause the westerlies to increase in strength. The increased westerlies allow cold air to drain off the North American continent rather than letting it build up and move south.
Above average geopotential heights are observed over the eastern U.S., which correlates to above average temperatures
The eastern U.S. often sees a wetter pattern with stronger storms during the winter season in this phase due to increased upper level winds
A negative NAO indicates weakening of both the Icelandic low and Azores high, which decreases the pressure gradient across the North Atlantic. This decreased pressure gradient results in a slackening of the westerlies. The decrease in the westerlies allows cold air to build up over Canada, and this combined with below average heights (troughing) over the eastern U.S. gives the cold air a greater chance to move south and affect the eastern United States.
Below average geopotential heights are often observed over the eastern U.S. during the negative phase of the NAO, which correlates to below average temperatures
The eastern U.S. typically receives colder, drier air masses during the winter season in this phase
The key here is the location of the controlling Low and High and its intensity. Given that a Low spins counter-clockwise and a High clockwise, can see how NAO + sends Gulf Stream influenced warm air to Europe at higher latitudes than when we have NAO -.
Forecasting the Evolution of ENSO
Here is the latest NOAA forecast of Nino 3.4 temperature anomalies. This graphic does auto-update daily but changes slowly. You can see the "blue" newer model runs and the "red" older model runs. The Oceanic Niño Index (ONI) which is the three-month rolling average of the Nino 3.4 values is NOAA's primary indicator for monitoring El Niño and La Niña. The secondary indicator is the Southern Oscillation Index (SOI) which compares the air pressure in Tahiti with the air pressure in Darwin Australia. It is a complicated formula and is intended to assess the response of the atmosphere to the changes in the pattern of warm and cool anomalies measured by the Nino 3.4 Index. It confirms rather than predicts the phases of ENSO.
At this point in time, the forecast by the NOAA model is that Nino 3.4 will continue at about the current level (deviation in degrees centigrade from average conditions in the Nino 3.4 Measurement Area in a particular part of the Eastern Pacific along the Equator) through about January and then increase i.e. become less La Nina-ish. The current values being less than -0.5 are borderline La Nina under NOAA criteria but not Australian BOM criteria. NOAA clams the atmosphere has recently confirmed that borderline La Conditions apply but the SOI does not support that conclusion. The ultimate recording of this as a La Nina event depends on the duration requirements being met and that may or may not occur. It will be close. In reality, fully meeting the criteria or just barely missing may have little impact on the weather that occurs. .
Re The SOI
The Daily Value will not update. Normally La Nina Conditions are confirmed by SOI 30 day values that are greater than or equal to 7.0. As you can see that was the case in September but not October or the last 30 days which have actually been on the El Nino side of 0C anomaly. So any claims that the atmosphere is showing La Nina is questionable at the very least.
The below is first the early-month report from CPC/IRI which I call the reading of the tea leaves.in that it is based on a combination of model results and a survey of the views of meteorologists. Recently the early month analysis has been more favorable for a la Nina than the later in the month model-based analysis.
And now we have the more recent fully model based version .
There is not a lot of difference but the preference for La Nina conditions in NDJ is a bit less than a week earlier when the report was based mainly on a survey of meteorologist. When I see DJF I interpret that as J i.e. January as the J is the middle month in the DJF three month period. It is not a read clear indication that we are having a La Nina. .
Here is the recently released JAMSTEC Nino 3.4 Forecast.
Based on the Nino 3.4 projection, JAMSTEC is saying "no" to La Nina...NOAA says "Maybe to likely but if so marginal". It is basically the same forecast but with different ways to present it.
Here is the Nino 3.4 report from the Australian BOM. It will update automatically on Tuesday morning or late Monday night. If it changes in a way that is significant, I will update this report to include a comment on the change.
This is basically the same forecast as other agencies but with different threshold for considering a SST to be either La Nina or El Nino and slightly higher values than NOAA through October so BOM does not show anything other than ENSO Neutral.
Now the Comparison of the NOAA and JAMSTEC Forecasts. I am only discussing the differences for CONUS since NOAA does not cover the World in this set of forecasts. But the JAMSTEC World forecast is here for you to see:.
For JAMSTEC, all of CONUS and Alaska is warm. NOAA shows a cool anomaly across North Central CONUS and below that a lot of EC with Alaska having a warm coast and cool interior. .
And then I extract an image for Europe on the left and CONUS on the right.
For many purposes 55N latitude is the dividing line in Europe in regards to the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) Impacts. You see that here with Scandinavia wet and southern Europe dry. It is suggestive of AO+. The NAO is highly correlated with the AO so it is also suggestive of NAO +. For CONUS it is a La Nina pattern but different than shown below for NOAA ad it is not a North/South divide.
Notice the different pattern between NOAA and JAMSTEC. NOAA is classical La Nina. They may have it correct but they also may not.
Here is one more forecast but it is not exactly the same months.
This is the precipitation forecast from Queensland Australia.
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 that the falling SOI triggers a lot of wet in Europe. For CONUS it is a different pattern than either NOAA or JAMSTEC.
Again JAMSTEC is warm for all of CONUS and Alaska except for a small cool area on the lee side of the Northern Rockies. NOAA has a larger part of the Northern Tier being EC. There is a lot of similarity here.
JAMSTEC has a lot of wet with dry only for the Middle Atlantic States and Alaska being mainly EC. NOAA has a north/south La Nina divide but impacting only the West.
Strange! JAMSTEC finally goes all in with a mostly cool CONUS except warm for the West up through Canada and to Alaska. NOAA is all warm including Alaska with a small EC area for the Dakotas.
Now, JAMSTEC is basically wet for all of CONUS with mostly EC for Alaska and NOAA is mostly EC with a wet New England and dry Northwest. NOAA is forecasting a normal Southwest Monsoon. JAMSTEC is forecasting a fairly wet Monsoon.
Most of the differences can be explained by the differing forecasts for ENSO with NOAA being far more all in on the La Nina scenario than JAMSTEC. It results in very different forecasts, especially beyond three months.
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