Prognostic Discussion for Monthly Outlook
NWS Climate Prediction Center College Park MD
300 PM EST Wed Nov 30 2022


30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2022

The updated monthly temperature and precipitation outlooks for December 2022 
are based on the latest dynamical model guidance, WPC temperature and 
precipitation forecasts during the first week of the month, the CPC 6-10/8-14 
day temperature and precipitation outlooks, and the likelihood of a negative 
Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) through at least 
mid-December. The Madden-Julian Oscillation (MJO) strengthened during 
mid-November with an eastward propagation to the West Pacific. The MJO 
continues to shift eastward over the Western Hemisphere. The GEFS depicts a 
reemergence of the MJO across the Indian Ocean later in December, while the 
ECMWF weakens it. If the GEFS solution verifies, then a transition to a warmer 
pattern for the northern and eastern contiguous United States (CONUS) may occur 
late in December. However, large uncertainty exists given the MJO weakening 
favored by the ECMWF model and strong high latitude blocking likely through at 
least mid-December. 

Arctic high pressure is forecast to shift southward to the western and 
north-central CONUS at the beginning of December. Ensemble means remain in 
excellent agreement that high latitude blocking develops over the Northern 
Hemisphere early in December, consistent with a negative phase of the AO. In 
addition, a very strong blocking ridge is forecast to shift westward to 
Greenland and the Davis Strait with a west-based negative NAO. Given the 
excellent model agreement and consistency on this high latitude blocking and 
anomalous cold during the first two weeks of the month, below-normal 
temperatures are favored for the north-central CONUS along with parts of the 
West. The largest probabilities (50 to 70 percent, or more) extend from the 
Pacific Northwest east to the northern Great Plains and upper Mississippi 
Valley where the negative temperature anomalies are greatest early in the month 
and is also consistent with La Niña composites. Following a mild start to the 
month across the eastern CONUS, colder temperatures are forecast for these 
areas by the second week of December. Therefore, equal chances (EC) of below, 
near, or above-normal temperatures are forecast for this region. Despite some 
tools leaning warmer, the likelihood of a -NAO through mid-December resulted in 
EC for parts of the Southeast. Although above-normal temperatures are slightly 
favored for the southern tier of the CONUS, forecast confidence is low given 
the strong high latitude blocking and potential for modified Arctic air to 
shift southward at times during December. 

Ensemble means are in good agreement that anomalous 500-hPa ridging persists 
over the north-central Pacific through at least the first two weeks of the 
month. Downstream of this longwave ridge axis, a persistent trough of varying 
amplitude is forecast near or offshore of the West Coast during early to 
mid-December. This trough is likely to enhance onshore flow and lead to a 
relatively wet pattern for much of the West through mid-December. The week 3-4 
model solutions (valid Dec 14 to 27) favor a continuation of this wet pattern 
into the latter half of the month. Therefore, increased probabilities for 
above-normal precipitation are forecast for much of the West. EC is forecast 
for southern California and the desert Southwest where precipitation signals 
are weaker. A dry start to December along with good consistency among recent 
daily CFS model runs supports elevated probabilities for below-normal 
precipitation from eastern New Mexico and the southern high Plains 
northeastward to parts of the Midwest. Northwest flow, associated with cold air 
advection, early in the month is another reason for this dryness across the 
Midwest. Heavy precipitation (1 to 3 inches, or more) during the first week of 
the month along with the expectation for a mean surface front and additional 
precipitation during the remainder of December increases chances for above 
normal precipitation across the Ohio and Tennessee Valleys. A dry start to the 
month and recent daily runs of the CFS model favor below normal precipitation 
across much of the Coastal Plain of the Southeast and Florida. This wet to dry 
dipole from the Ohio River southward to the Gulf Coast is typical during La 
Niña. EC is forecast for the lower Mississippi Valley, Mid-Atlantic, and 
Northeast due to weaker precipitation signals at various time scales throughout 
December. 

Model guidance remains steady as anomalous 500-hPa ridging is likely to persist 
over the north-central Pacific and Aleutians. Above-normal temperatures are 
favored for the Aleutians along with western and northern Mainland Alaska. 
Below-normal temperatures are most likely for the Alaska Panhandle, based on 
colder-than-normal temperatures during early to mid-December and also 
consistent with recent daily CFS model runs. Along with the favored 
below-normal temperatures, elevated probabilities for below-normal 
precipitation are forecast for the Panhandle. Predicted southwest flow and 
excellent agreement among precipitation tools support increased probabilities 
for above-normal precipitation across much of western and northern Mainland 
Alaska. 

 
*********** Previous discussion, released on November 17, is below ***********


The December 2022 temperature and precipitation outlooks are based on the Weeks 
3-4 model solutions (valid for early to mid-December), dynamical model output 
for December, La Niña composites, consideration on the forecasted state of the 
Madden Julian Oscillation (MJO), 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 MJO is forecast to strengthen and propagate eastward to the West Pacific 
during late November. Although the GEFS and ECMWF models are in good agreement 
on this MJO evolution, large ensemble spread exists on the strength of the MJO 
with many GFS ensemble members depicting a higher amplitude MJO event compared 
to the majority of ECMWF ensemble members. Lagged composites, based on phase 5 
of the MJO RMM index, would favor below-normal temperatures overspreading the 
central and eastern contiguous U.S. (CONUS) during early to mid-December. Given 
the expected mild start to the month for most areas east of the Rockies, this 
would translate to a highly variable temperature pattern during December which 
lowers forecast confidence in the monthly temperature outlook for the central 
and eastern CONUS. Therefore, a large coverage of equal chances (EC) of below, 
near, or above-normal temperatures is necessary. Below-normal temperatures are 
favored for the northern Great Plains where the week 3-4 ECMWF model along with 
La Niña composites feature a cold signal. Above-normal temperatures are only 
slightly favored along the East Coast, consistent with the week 3-4 model 
solutions and the North American Multi-Model Ensemble (NMME). The dynamical 
model output along with La Niña composites support increased probabilities for 
above-normal temperatures throughout the southern tier of the CONUS. Although 
there is support for larger probabilities of above-normal temperatures across 
the Southeast, the GFS ensemble mean depicts a negative NAO heading into 
December which limits forecast confidence for this region. The highest forecast 
confidence in the temperature outlook exists across the Southwest where week 
3-4 solutions along with dynamical and statistical tools are in good agreement. 
 

La Niña composites for November-December-January (NDJ) are the major guidance 
relied upon for the December precipitation outlook along with the NMME. 
Above-normal precipitation is most likely across the Pacific Northwest, 
northern to central Rockies, and northern high Plains. Conversely, below-normal 
precipitation is favored for parts of the Southwest, central to southern Great 
Plains, Gulf Coast, and Southeast. Although statistical tools support a drier 
outcome for California, EC is forecast for the state due to a lack of support 
from monthly dynamical models and a robust wet signal evident in the week 3-4 
ECMWF model solution. Also, the ensemble means are in good agreement on an 
amplified 500-hPa trough centered near the West Coast during the final week of 
November. EC is also forecast for the Great Lakes and Ohio Valley where the wet 
signal, associated with La Niña, typically occurs later in the winter season. 

The dynamical and statistical tools are in good agreement and depict elevated 
probabilities for below (above)-normal temperatures for the Alaska Panhandle 
(western Mainland Alaska). Despite the NMME favoring above-normal temperatures 
across the Aleutians, EC was forecast for this region due to negative sea 
surface temperature anomalies nearby and a strong cold signal in the week 3-4 
ECMWF model. The NMME and decadal trends support increased probabilities for 
above-normal precipitation for northwestern Alaska. Although La Niña typically 
favors dryness for the Alaska Panhandle during the winter, EC was forecast due 
to a weak signal among the dynamical model output. 

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 15 2022

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