Written by Sig Silber
At the end of every month, NOAA issues an update to their Early Outlook for the following month, in this case, December, which was issued on the Third Thursday of the Month. Today, nine days later, NOAA has issued the updated Outlook for December. It is somewhat but not dramatically different than the Early Outlook nine days ago. In general, it is slightly warmer and quite a bit wetter than the Early Outlook. The dry precipitation forecast for Southern Texas and Florida is somewhat questionable in our minds, especially for South Texas.
Some housekeeping: On November 22, 2019 we published Part I of our analysis of the NOAA and JAMSTEC Three- to Four- Season Outlook and that can be accessed here. In that report, the December Early Outlook was issued. This article presents the NOAA update of their Early Outlook for December.
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Now let us address the NOAA Update of the December, 2019 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. 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 2019 with the newly issued update.
Early Outlook Temperature
Updated Temperature Outlook
Early Outlook Precipitation
Updated Precipitation
November 21, 2019 Forecast for December | November 30, 2019 Forecast for December | |
Temperature | ||
Precipitation |
Here is the discussion released today:
30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2019
The updated temperature and precipitation outlooks for December 2019 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 equatorial sea surface temperatures (SSTs) are above average across much of the Pacific Ocean, anomalous convection and winds continue to reflect ENSO neutral conditions. The Madden-Julian Oscillation (MJO) constructively interfered with the ongoing positive Indian Ocean Dipole (IOD) during late November. Dynamical model forecasts are in reasonably good agreement that the MJO resumes its eastward propagation across the Indian Ocean during the next two weeks. This predicted MJO evolution would favor above normal temperatures across the central and eastern U.S. during December.
A rapid pattern change is likely to occur over the middle to high latitudes of the Northern Hemisphere at the beginning of December. Positive 500-hPa height anomalies at the higher latitudes are forecast to be replaced by negative 500-hPa height anomalies. The Arctic Oscillation (AO) index is forecast to become strongly positive at the beginning of the month. A positive AO index typically corresponds to above normal temperatures across the north-central U.S. Many GFS ensemble members indicate that the AO index trends back to neutral or even a negative phase by mid-December which slightly lowers forecast confidence in the temperature outlook across the north-central U.S. Despite the good agreement in recent model guidance, probabilities for above normal temperatures were not increased too much across the north-central U.S. given the poor consistency among daily CFS model runs during the final ten days of November. Multiple early season winter storms resulted in widespread, heavy snow from the Sierra Nevada Mountains east to the Colorado Rockies during late November. Anomalous snow cover and depth for this time of year may are likely to result in a cooling effect on surface temperatures across the Great Basin, at least through early December. Therefore, the updated temperature outlook calls for equal chances of below, near, or above normal temperatures across the Great Basin. The highest probabilities (above 60 percent) for above normal temperatures are forecast across the central and southern Great Plains where maximum temperatures are forecast to average 5 to 10 degrees F above normal during the first week of December with above normal temperatures likely persisting into the latter half of the month.
The updated precipitation outlook expanded the favored area of above normal precipitation to include the entire West Coast. The greater chances for above normal precipitation throughout the West are mostly due to a wet start to the month along with an amplified 500-hPa trough extending from the Aleutians southeastward to near the West Coast during the second week of December. The highest probabilities (above 70 percent) for above normal precipitation are forecast from San Francisco to the Sierra Nevada Mountain range where 7-day precipitation amounts are forecast to be near the upper monthly tercile. Heavy precipitation (2 to 7 inches, liquid equivalent) is likely across northern and central California during the first few days of the month as a low pressure system interacts with tropical moisture originating from Hawaii.
The revised precipitation outlook also increased the coverage for above normal precipitation to include areas from the Midwest eastward to parts of the Northeast. This expanded coverage is consistent with the upper-level trough upstream, a northward shift in the mid-latitude storm track, and likelihood of above normal temperatures for much of the central and eastern U.S. Also, heavy precipitation at the beginning of December supports elevated odds for above normal precipitation from the northern mid-Atlantic north to southern New England. Based on a dry first week of the month coupled with a consistent dry signal among recent CFS model runs, elevated odds for below normal precipitation are forecast for parts of central and southern Texas along with much of Florida.
Little to no change was needed for the updated Alaska temperature outlook with the highest confidence remaining that above normal temperatures prevail along most coastal areas of mainland Alaska and the Aleutians. Based on excellent model agreement that an amplified trough becomes established over the North Pacific with enhanced onshore flow, above normal precipitation is most likely across the Kenai Peninsula.
Visual Consistency Testing.
It is useful to see how the full-month forecast fits with the set of partial-month forecasts that we have. These will continue to update in this article. It is important to remember that with the partial-month forecasts, we generally have about 25 days of the subsequent month to look at when we do this at the end of the month as we are doing now. It depends on what day of the week the month ends. So this month, which ended the day after the Week 3 – 4 forecast was issued, we only are missing the last four days of the month.
First Temperature
And Precipitation
So we have attempted to review the NOAA forecast of a dry anomaly in South Texas and another one in Florida.
I have added today’s short-range forecast to the discussion.
…Heavy snow for the Upper Mississippi Valley/Upper Great Lakes and the Northeast with rain/freezing rain over parts of the Great Lakes into Northeast…
…Heavy snow for the Sierra Nevada…
….There is a slight risk of severe thunderstorms over parts of the Lower Mississippi Valley/Tennessee Valley…
A storm over the Middle Missouri Valley will move eastward to off the Northeast Coast by Monday evening. The system will produce heavy snow over parts of the Upper Mississippi Valley/Upper Great Lakes on Saturday evening into Sunday evening. In addition, rain/freezing rain and sleet will develop over parts of the Northern Lower Peninsula of Michigan overnight Saturday into Sunday morning. As the storm moves eastward, rain/freezing rain will develop over parts of the Northern Mid-Atlantic, late on Saturday night, that will move into parts of Southern New England on Sunday evening into Monday. On Sunday afternoon into Monday evening, heavy snow will develop over parts of the Northeast. Difficult to dangerous driving conditions will be associated with the winter weather over the Upper Midwest into the Northeast.
Additionally, the storm will produce showers and thunderstorms over parts of the Tennessee and Lower/Middle Mississippi Valleys overnight Saturday into Sunday morning with some becoming severe. The showers and thunderstorm will move into parts of the Southeast and Southern Mid-Atlantic on Sunday moving off the Southeast/Southern Mid-Atlantic Coast by Sunday night. Rain will also develop over parts of the Middle Mississippi Valley into parts of the Great Lakes/Ohio Valley overnight Saturday into Sunday morning. The rain will turn over to snow overnight Sunday. Rain will develop over parts of the Mid-Atlantic on Sunday, ending overnight Sunday for the Southern Mid-Atlantic. The rain will continue over parts of the Northern Mid-Atlantic and changing over to snow with some rain near the coast on Sunday evening into Monday. Furthermore, along with the original low pressure system, another surface low will develop on Sunday in the Mid-Atlantic and move as a nor’easter Sunday night and into Monday along the Northern Mid-Atlantic Coast and Southern New England Coast. Travel impacts can be expected With this storm on this busy travel weekend.
Upper-level energy over the Eastern Pacific and associated plume of moisture entering California overnight Saturday and persisting through Monday. Heavy rain and heavy higher elevation snow will develop over parts of California on Saturday evening and expand into parts of the Pacific Northwest overnight Saturday. Some flooding is possible in parts of Northern California, especially for areas with burn scars. Meanwhile, the Sierra Nevada could see snow measured in feet. Lighter snow should spread into the Intermountain West Sunday.
What is normal for Texas in December?
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 also issued on November 30, 2019.
Here is the short discussion issued with the new drought forecast.
Latest Monthly Assessment – During the past month, widespread above normal rainfall and below normal temperatures across much of the eastern CONUS helped to improve or remove short-term drought over this region. Drought conditions persisted across parts of the Four Corners states following a lackluster Monsoon, although beneficial precipitation arrived during late November. Short-term drought continues to plague the central and southern Great Plains.
Early in December, widespread precipitation is forecast across the mid-Atlantic and Northeast which is likely to remain drought-free. For the Southeast, one class drought improvement or drought removal for South Carolina and northern Georgia is anticipated due to forecast above normal precipitation during early to mid-December and a favorable time for soil moisture recharge. Drought conditions in southern Georgia and the Florida Panhandle are likely to persist and drought development is anticipated in the currently abnormally dry areas in the southern Florida Peninsula due to forecast below normal precipitation and above normal temperatures for December. Based on a drier climatology, and precipitation and temperature outlooks for the next month, drought persistence is more likely across the central and southern Great Plains, while the monthly precipitation outlook supports drought development in parts of Texas. Recent widespread precipitation over the southwestern CONUS, a favorable time of year for soil moisture recharge, and a continuation of a wet pattern through at least early December support drought improvement and removal across much of the Southwest. Based on forecast widespread above normal precipitation for the next two weeks, a wet climatology, early season snowpack, and reservoir conditions, drought development is not anticipated in California through the end of December. A wet climatology and forecast precipitation favor drought improvement or removal across the Alaska Panhandle and Hawaii.
Looking back on November to relate the forecast for December to the actuals in November.
First October Temperature (29 out of 30 days).
And then October Precipitation (29 out of 30 days).
We then show the new forecast and the prior month actuals (less one day) side by side.
Prior Month (usually missing one day) | Forecast for current Month | |
Temperature | ||
Precipitation |
Conclusion
The purpose of this article was to present the updated Outlook for December. 2019 and compare it with the Early Outlook. It is not to critique the updated Outlook for December based on our opinions but we point out possible inconsistencies if we find them. 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 but we do have a prior December/January/February forecast from them this month.
In reality, all 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, 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 12, 2019, IRI/CBC will issue their ENSO forecast and 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 (except when we cover the Four-Season Analysis or medium-frequency cycles such as ENSO), so our analysis is pretty much ongoing. We also publish a “LIVE All WEEK” Weather Article which updates in real-time to reflect NOAA Weather Headlines. All of our reports including past versions of these reports can be located via the Directory/Archive which can be accessed here.