Updated Outlook For December, 2018 – Temperature Change, Disagreement On Precipitation

Written by Sig Silber

At the end of every month, NOAA issues an update to their Early Outlook which was issued on the Third Thursday of the Month. Usually the changes are minor but not so much this month. Not only are there changes but some disagreement between the part of NOAA that creates the monthly forecast and the part that produces the Week 3 – 4 forecast. So we will try to sort that out in our report tonight.

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Although the general theme is a bit warmer and a bit wetter, it is important for readers to look at the maps for the month and the maps that we have in the article for first five days, Days 6 – 10, Days 8 – 14 and Week 3 – 4 as the pattern is expect to change during the month. Other than the full -month map, the other maps will auto-update so this article will remain useful throughout the month.

Now let us address the NOAA Update of the December, 2018 Forecast.

A note about terminology; the deviations from climatology/normal are color coded but also labeled “A” for more than (above) normal and “B” for less than (below) normal. The area designated EC means Equal Chances of being more or less than normal. In the Short Term forecasts, NOAA has switched from using EC to N for Normal. I believe this was a recommendation of a Focus Group. I find those new graphics harder to read especially if printed in black and white. For most purposes, EC and N can be considered to mean about the same thing. “N” implies a bit more certainty that the variation from Normal/Climatology will be small compared to “EC”.

First we will compare the NOAA Early Outlook for December, 2018 with the newly issued update.

Early Outlook Temperature

Updated Temperature Outlook

Early Outlook Precipitation

Updated Precipitation

We cover it later but I will also show the Week 3 – 4 Precipitation forecast here now.

Here I show them side by side.

I have extracted the key paragraph from the two discussions released today and now show them side by side.

And here is the full NOAA discussion released with the December Update. It is worth reading.

30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2018

THE UPDATED TEMPERATURE AND PRECIPITATION OUTLOOKS FOR DECEMBER 2018 ARE BASED ON THE WPC TEMPERATURE AND PRECIPITATION FORECASTS DURING THE FIRST WEEK OF DECEMBER, THE CPC 8-14 DAY AND WEEKS 3-4 TEMPERATURE AND PRECIPITATION OUTLOOKS, AND THE LATEST MONTHLY FORECASTS FROM THE CFS MODEL. ALTHOUGH SEA SURFACE TEMPERATURES REMAIN ABOVE NORMAL THROUGHOUT THE EQUATORIAL PACIFIC, THERE CONTINUES TO BE A LACK OF ENHANCED CONVECTION NEAR THE DATE LINE. THEREFORE, ENSO NEUTRAL CONDITIONS ARE PRESENT. A ROBUST MADDEN-JULIAN OSCILLATION (MJO) CONTINUES WITH ITS ENHANCED PHASE PROPAGATING EAST OVER THE WESTERN HEMISPHERE DURING LATE NOVEMBER. THE MJO IS FORECAST TO SHIFT EAST TO THE INDIAN OCEAN DURING EARLY DECEMBER WHICH WOULD FAVOR A TRANSITION TO A WARMER PATTERN FOR MUCH OF THE CONTINENTAL U.S. LATER IN THE MONTH.

THE GFS AND ECMWF ENSEMBLE MEANS REMAIN CONSISTENT AND IN GOOD AGREEMENT THATAN AMPLIFIED UPPER-LEVEL TROUGH DEVELOPS OVER THE EAST PACIFIC EARLY IN THE MONTH, WHILE A POSITIVE 500-HPA HEIGHT ANOMALY CENTER BECOMES ESTABLISHED ACROSS CENTRAL CANADA. THIS 500-HPA PATTERN FAVORS A TRANSITION TO ABOVE-NORMAL TEMPERATURES FOR MUCH OF THE CONUS BY MID-DECEMBER. THIS TRANSITION FROM A COLD NOVEMBER AND BEGINNING OF DECEMBER TO A WARMER PATTERN IS ALSO CONSISTENT WITH THE PREDICTED MJO FORECAST. THE D+11 TEMPERATURE OUTLOOK FAVORS NEAR TO ABOVE NORMAL TEMPERATURES FOR MUCH OF THE CONUS WITH INCREASED CHANCES OF BELOW NORMAL TEMPERATURES LIMITED TO THE NORTHEAST. THE NORTH ATLANTIC OSCILLATION (NAO) INDEX BECAME MORE NEUTRAL AT THE END OF NOVEMBER AND MOST GFS ENSEMBLE MEMBERS INDICATE A POSITIVE NAO THROUGH EARLY TO MID-DECEMBER. THE POSITIVE NAO SUPPORTS THE TRANSITION TOWARDS A WARMER PATTERN ACROSS THE CENTRAL AND EASTERN CONUS. HOWEVER, PROBABILITIES FOR ABOVE NORMAL TEMPERATURES ARE TEMPERED DUE TO THE COLD START TO THE MONTH.

BELOW-NORMAL TEMPERATURES ARE FAVORED FOR THE GREAT BASIN AND CENTRAL ROCKIES SINCE THESE AREAS ARE FORECAST TO HAVE THE LARGEST NEGATIVE TEMPERATURE ANOMALIES (10 TO 20 DEGREES F) DURING THE FIRST WEEK OF DECEMBER. THE WEEKS 2 TO 4 OUTLOOKS FAVOR NEAR OR BELOW NORMAL TEMPERATURES FOR THESE AREAS. ALSO, AN INCREASE IN SNOW COVER IS EXPECTED DURING THE NEXT WEEK TO TEN DAYS ACROSS THE INTERIOR WEST. A SLIGHT TILT IN THE ODDS FOR BELOW NORMAL TEMPERATURES IS FORECAST ACROSS NORTHERN NEW ENGLAND WHERE BELOW NORMAL TEMPERATURES ARE LIKELY TO PERSIST THROUGH THE FIRST HALF OF THE MONTH.

DURING THE FIRST WEEK OF DECEMBER, THE DETERMINISTIC MODELS CONTINUE TO INDICATE A VIGOROUS SHORTWAVE TROUGH IN THE SOUTHERN STREAM WHICH IS LIKELY TO LEAD TO SURFACE LOW DEVELOPMENT ACROSS THE SOUTHERN GREAT PLAINS OR WESTERN GULF COAST REGION. THIS SHORTWAVE TROUGH IS FORECAST TO BRING LOCALLY HEAVY PRECIPITATION TO SOUTHERN CALIFORNIA AND THE DESERT SOUTHWEST, WHILE WIDESPREAD HEAVY PRECIPITATION (1 TO 3 INCHES, OR MORE) IS EXPECTED TO ACCOMPANY THE SURFACE LOW ACROSS THE LOWER MISSISSIPPI VALLEY AND SOUTHEAST. A WET PATTERN IS LIKELY TO CONTINUE THROUGH MID-DECEMBER ACROSS CALIFORNIA AND THE SOUTHERN TIER OF THE CONUS DUE TO AN AMPLIFIED UPPER-LEVEL TROUGH OFFSHORE OF THE WEST COAST, AN INFLUX OF SUBTROPICAL MOISTURE, AND A WARMING TREND IN TEMPERATURES. BASED ON HIGH CONFIDENCE IN A WET PATTERN THROUGH AT LEAST MID-DECEMBER, ABOVE-NORMAL PRECIPITATION IS FAVORED FOR CALIFORNIA, THE SOUTHWEST, SOUTHERN GREAT PLAINS, LOWER MISSISSIPPI VALLEY, AND SOUTHEAST. THE HIGHEST PROBABILITIES (ABOVE 60 PERCENT) FOR ABOVE NORMAL PRECIPITATION ARE FORECAST ACROSS THE FLORIDA PANHANDLE WHERE THE DETERMINISTIC 0Z ECMWF MODEL CONTINUES TO FORECAST 3 TO 7 INCHES OF RAINFALL DURING THE FIRST WEEK. A SECOND AREA OF HIGH PROBABILITIES ACROSS THE CENTRAL GREAT PLAINS AND WESTERN CORN BELT IS RELATED TO HEAVY PRECIPITATION (MORE THAN 0.5 INCH, LIQUID EQUIVALENT) FORECAST ON DECEMBER 1. THE MOST LIKELY AREA FOR BELOW NORMAL PRECIPITATION EXTENDS FROM THE NORTHERN GREAT PLAINS EAST TO NORTHERN NEW ENGLAND AS A DRY START TO THE MONTH IS LIKELY WITH ANOMALOUS NORTHERLY FLOW AND PRECIPITATION TOOLS THROUGHOUT THE REMAINDER OF THE MONTH HAVE A DRY SIGNAL.

ABOVE NORMAL TEMPERATURES REMAIN FAVORED FOR ALASKA ALTHOUGH PROBABILITIES ARE REDUCED ACROSS EASTERN ALASKA AS 500-HPA HEIGHTS ARE FORECAST TO AVERAGE BELOW NORMAL DURING EARLY DECEMBER. THE HIGHEST ODDS FOR ABOVE-NORMAL TEMPERATURES ACROSS ALASKA ARE FORECAST FOR AREAS OF THE STATE ADJACENT TO THE BERING SEA WHERE SEA SURFACE TEMPERATURE ANOMALIES REMAIN ABOVE NORMAL. THE FAVORED AREAS OF ABOVE NORMAL PRECIPITATION ACROSS THE ALEUTIANS AND WESTERN ALASKA ARE BASED ON MODEL GUIDANCE FEATURING ABOVE NORMAL PRECIPITATION DURING THE FIRST TWO WEEKS AND RECENT DAILY RUNS OF THE CFS MONTHLY FORECAST.

The week 3 – 4 Discussion is also worth reading.

Week 3-4 Forecast Discussion Valid Sat Dec 15 2018-Fri Dec 28 2018

The Week 3-4 Outlook is based on the latest dynamical model guidance from the CFS, ECMWF and JMA operational ensemble models, as well as the Subseasonal Experiment (SubX), an experimental multi-model ensemble (MME) of research and operational models, as well as statistical guidance based on the state of the MJO and ENSO. ENSO-neutral conditions are present over the equatorial Tropical Pacific, with sea surface temperature anomalies in the Nino 3.4 region exceeding the +0.5 C, El Nino threshold, but the tropical atmosphere is not yet responding with a canonical El Nino circulation pattern. The MJO is currently active with some dynamical models forecasting the propagation of anomalously strong convection across the Indian Ocean in the next two weeks, based on the ECMWF ensemble.

Dynamical models are in good agreement on the 500-hPa circulation pattern over the North American region during the Week 3-4 Outlook period, predicting persistence and amplification of dynamical model 500-hPa circulation pattern forecasts for the week 2 period. A trough is forecast over Alaska, extending southward off of the Pacific coast of North America, while a ridge and associated positive 500-hPa height anomalies are predicted to be centered over Canada. There are some variations on the location of the predicted trough and ridge and associated 500-hPa height anomalies between the various model forecasts. The ECMWF ensemble forecast locates the predicted trough to the west of mainland Alaska, while the CFS and JMA ensemble forecasts place the trough axis over mainland Alaska. The SubX MME is consistent with the CFS, ECMWF and JMA average 500-hPa height forecast, placing the trough axis over western Alaska.

Statistical forecasts of temperature based on the current positive SST anomaly over the Nino 3.4 region and the current phase of the MJO, predict below normal temperatures to be likely over much of the western CONUS centered around the Great Basin region and above normal temperatures to be more likely for the north-central CONUS. In contrast, statistical forecasts based on the current phase of the MJO and ENSO-neutral conditions predict likely below normal temperatures for the Pacific Northwest across the Northern Rockies into western areas of the Northern and Central Plains for the week 3-4 period. Temperature forecasts from the ECMWF, CFS and JMA model consensus, as well as the SubX MME, predict likely above normal temperatures along the Pacific coast of the CONUS, and from the Pacific Northwest across the Great Plains into the eastern U.S., more consistent with statistical forecasts that consider El Nino conditions. The probabilities of above normal temperatures for the Pacific Northwest and western areas of the Northern Plains were reduced in the Week 3-4 Outlook, in consideration of the potential impacts of the active MJO combined with current ENSO-neutral conditions. There is greater uncertainty on the temperature outlook for Alaska during the Week 3-4 period, with significant differences in the location of a trough over the region in various model forecasts. A consensus of operational ensemble models and the SubX MME indicate above normal temperatures are likely for the west and south coasts of Alaska, including the Alaska Panhandle, while equal chances of above and below normal temperatures is indicated for the remainder of the state. Above normal temperatures are likely for Hawaii with greater probabilities for the southeast islands, due to persistent above normal sea surface temperatures supported by dynamical model forecasts.

Above normal precipitation is likely for eastern areas of the south coast of Alaska, as well as for the Pacific Northwest, ahead of the predicted trough in a consensus of dynamical ensemble model forecasts. Below normal precipitation is likely for the north-central CONUS from the Great Divide into the Northern Plains and western Great Lakes region, under a predicted ridge in dynamical model forecasts. A consensus of the CFS, ECMWF and JMA ensemble model forecasts, as well as the SubX MME forecast, predict likely below normal precipitation for much of the Northeast CONUS, however equal chances of above and below normal precipitation is indicated, due to the increased chance of above normal precipitation for the region related to the active MJO. Above normal precipitation is likely for the southeast Hawaiian Islands, as predicted by most dynamical models of the SubX MME.

Visual Consistency Testing.

It is useful to see how the month forecast fits with the set of shorter forecasts that we have. These will continue to update in this article.

First Temperature

The new monthly forecast is to the right. Does it appear to be reflective of the four parts of the month for which we have forecasts? →			↑ ← The Week 3 – 4 forecast was updated today so we do not need to update the forecast when the update is issued.

And Precipitation

The new monthly forecast is to the right. Does it appear to be reflective of the four parts of the month for which we have forecasts? →			↑ ← The current Week 3 – 4 forecast was updated today so we do not have to wait for when that update is issued.

Sometimes it is useful to compare the three-month outlook to the forecast for the first of the three months. It shows how much the pattern changes over the three-month period.

One can mentally subtract the First-Month Outlook from the Three-month Outlook and create the Outlook for the last two months in the three-month period.

And with respect to drought. This was issued today.

Looking back on November and this is preliminary data and one day short of a full month.

The purpose of this article was to present the updated Outlook for December, 2018 with the Early Outlook. It is not to critique the updated Outlook for December based on our opinions. We have assessed the extent to which the monthly outlook seems to fit with the other forecasts provided by NOAA. We do not have a December forecast by JAMSTEC as they only provide three-month forecasts. We do have forecasts for December from other meteorological agencies but the objective in this article was simply to present the new forecast. In reality these forecasts are obsolete when issued as the situation changes day by day or six hours by six hours i.e. each new model run. In our other articles including the Drought Article which was published yesterday (November 29, 2018) and our weekly Weather and Climate Report that we publish on Monday nights, we continually examine the current forecasts from NOAA including the Day 1 -5, 6 – 10, 8 – 14 and Week 3 and 4. So we are continually looking at approximately 25 days into the future.

On December 13, 2018, IRI/CBC will issue their ENSO forecast and on December 14 we will present that and critique it. It is an ongoing process and we attempt to provide our readers with the best current assessment of the next approximately 25 days so our analysis is pretty much ongoing.

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