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Official 90-day Outlooks are issued once each month near mid-month at 8:30am Eastern Time. Please consult the schedule of 30 & 90-day outlooks for exact release dates.

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HOME> Outlook Maps>Seasonal Forecast Discussion
 
Prognostic Discussion for Monthly Outlook 
NWS Climate Prediction Center College Park MD
300 PM EST Tue Nov 30 2021


30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2021

The updated December 2021 temperature and precipitation outlooks are based on
the Weather Prediction Center’s forecasts for the first week of the month, the
Climate Prediction Center’s Week-2 outlooks, Week 3-4 model guidance, and daily
CFS model runs initialized during late November. La Niña composites and decadal
trends continue to play a role in the December outlooks. Dynamical models
depict a better defined and eastward propagation of the Madden-Julian
Oscillation (MJO) over the Pacific during the next two weeks. This MJO
evolution would typically lead to an increased chance of anomalously cold
temperatures overspreading the CONUS by week 3. However, large uncertainty
remains on how much, if any, the MJO influences the mid-latitude circulation
pattern over North America as the MJO destructively interferes with the
well-established La Niña. The week 3-4 (Dec 15-28) model solutions depict
elevated probabilities for below normal temperatures across the Pacific
Northwest, northern Rockies, northern Plains, and Upper Mississippi Valley.
Above normal temperatures are generally favored across these areas during the
first two week of December as the Arctic Oscillation is forecast to be in a
positive phase with anomalous cold remaining north of the U.S.-Canada border.
Given the potential for this anomalous cold to shift south during the latter
half of December, equal chances (EC) of below, near, or above normal
temperatures are forecast across much of the northwestern and north-central
CONUS. EC is also forecast for most of the Northeast where a variable
temperature pattern is expected throughout the month. Probabilities of below
normal temperatures are slightly elevated across northwest Oregon and western
Washington since week-2 tools have recently trended colder there and week 3-4
model solutions favor below normal temperatures. Elsewhere across the lower 48,
temperature tools feature increased chances of above normal temperatures
throughout December.

A relatively dry start to the month is likely due to fast, progressive flow
prevailing, but by week-2 model guidance depicts very weak precipitation
signals. Week 3-4 precipitation tools are generally consistent with La Niña
composites. Based on the latest precipitation tools, no major revisions were
required for the updated December precipitation outlook. The coverage for
favored dryness across southern California and the Southwest was reduced since
model solutions are depicting a developing longwave trough over the western
CONUS by mid-December. Probabilities for below normal precipitation were
increased across the Southeast given the good model agreement and consistency
at all time scales.

The anomalously cold pattern across much of Alaska is likely to persist into
December with a continuation of negative 500-hPa height anomalies and remain
for much of the month as even week 3-4 tools are depicting increased chances of
below normal temperatures. The larger probabilities of below normal
temperatures were not extended east to include the Alaska Panhandle due to
positive sea surface temperature anomalies offshore. Precipitation tools depict
weak signals across Alaska, but the favored dryness across southern Mainland
Alaska is consistent with the temperature outlook.


----------The initial discussion released on Nov 18 follows----------


The December 2021 temperature and precipitation outlooks are based on the
predicted evolving pattern from Week-2, Weeks 3-4 model consensus, dynamical
model output for December, La Niña composites, and decadal trends .
Below-average sea surface temperatures continue across the central and eastern
equatorial Pacific, while suppressed convection was observed near and west of
the Date Line this past month. The oceanic and atmospheric observations reflect
La Niña conditions which are likely to persist through the winter. The
Madden-Julian Oscillation is forecast to remain weak during the next two weeks
and is unlikely to influence the mid-latitude circulation at this time.

The highest forecast confidence in the December temperature outlook exists
across the Southwest and southern Great Plains where above temperatures are
most likely, based on good agreement among inputs to the North American
Multi-Model Ensemble (NMME) along with support from La Niña composites and
decadal trends . Probabilities for above normal temperatures decrease farther to
the north across the Great Basin and to the east along the Gulf Coast due to
weaker signals among dynamical models . During late November, a large positive
500-hPa height anomaly center is likely to become centered over Greenland and
the northern Atlantic as the North Atlantic Oscillation (NAO) becomes negative.
The ECMWF ensemble mean depicts this anomalous ridging persisting into the
beginning of December and a majority of GFS ensemble members feature a
continuation of a negative NAO heading into December.  The week 3-4 model
consensus for the 500-hPa height pattern, using the GEFS, CFS, ECMWF, Canadian,
and JMA models initialized on Nov 15, depicts this negative phase of the NAO
ending sometime during early December. Due to uncertainty on when the anomalous
ridging downstream over the north Atlantic weakens and the associated negative
NAO ends, a large area of equal chances (EC) of below, near, or above normal
temperatures is necessary for the Ohio Valley, Great Lakes, Northeast, and
Mid-Atlantic. Also, inputs to the NMME vary largely for these areas. The EC
forecast for the northern Great Plains, northern Rockies, and Pacific Northwest
is related to the lack of a signal in the calibrated NMME and conflicting
signals between model output and wintertime La Niña composites.

Dynamical models and La Niña composites favor below (above) normal
precipitation across the southern tier of the CONUS (Pacific Northwest and
northern Rockies). The slightly elevated probabilities for above normal
precipitation forecast for the Great Lakes is consistent with La Niña
composites but also given the above normal Great Lakes temperatures, any cold
air advection would likely lead to enhanced lake-effect precipitation. Although
there is some support among dynamical models for a larger area of elevated
probabilities of above normal precipitation across the north-central CONUS,
large uncertainty at this time lead results in an EC forecast. The northern
Great Plains and upper Mississippi Valley will be reevaluated for the updated
monthly outlook (to be issued on Nov 30) when storm track predictability
improves. Due to anomalous ridging predicted by the ECMWF and Canadian ensemble
means heading into the beginning of December, the favored area of wetness for
the Pacific Northwest is slightly reduced in coverage compared to what La Niña
composites would typically support. This region, along with northern
California, will be reassessed on Nov 30 when the longwave pattern over the
northeast Pacific and western North America is better known.

Negative sea surface temperature anomalies and the calibrated NMME support
increased probabilities of below normal temperatures across southern Mainland
Alaska and the Aleutians. The favored area of dryness forecast for
south-central Alaska is consistent with the predicted colder-than-normal
temperatures. Elsewhere, across Alaska, precipitation tools feature little to
no signal.

FORECASTER: Brad Pugh

The climatic normals are based on conditions between 1991 and 2020, following
the World Meteorological Organization convention of using the most recent 3
complete decades as the climate reference period.  The probability anomalies
for temperature and precipitation based on these new normals better represent
shorter term climatic anomalies than the forecasts based on older normals.

The next monthly outlook...for Jan ... will be issued on Thu Dec 16 2021

These outlooks are based on departures from the 1991-2020 base period.
$$

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