Prognostic Discussion for Monthly Outlook 
NWS Climate Prediction Center College Park MD 
830 AM EST Thu Nov 21 2024 
 
 
30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2024 
 
The December 2024 Temperature and Precipitation Outlooks are based on: the 
Weeks 3-4 model guidance, the North American Multi-Model Ensemble (NMME), and 
International Multi-Model Ensemble (IMME), the consolidation (combination of 
statistical and dynamical tools), consideration of potential Madden Julian 
Oscillation (MJO) influences, and decadal trends . Although El Niño Southern 
Oscillation (ENSO)-neutral conditions continue, below-average sea surface 
temperature anomalies are observed across the east-central Pacific. La Niña is 
favored to develop by the end of December and La Niña composites were a factor, 
especially in the precipitation outlook. 
 
During late November, a major pattern change is forecast as an amplified 
500-hPa ridge over Alaska results in surface high pressure with anomalous cold 
shifting south from Canada into the central and eastern contiguous U.S. 
(CONUS). By the beginning of December, the GEFS and ECMWF ensemble means are in 
good agreement and consistent that below-normal temperatures extend from the 
Great Plains to the East Coast. The latest week 3-4 GEFS (valid December 5-18) 
favors below-normal temperatures continuing across the Great Lakes and 
Northeast. Lagged MJO composites would favor a flip to above-normal 
temperatures across the central and eastern CONUS by mid-December. Due to an 
expected variable temperature pattern during December, equal chances (EC) of 
below, near, or above-normal temperatures are forecast across the Northern 
Great Plains, Midwest, and Northeast. The week 3-4 models, NMME, consolidation, 
and decadal trends  support increased above-normal temperature probabilities 
across the Southeast, Lower Mississippi Valley, Southern Great Plains, and 
Southwest. The largest above-normal temperature probabilities (more than 50 
percent) are forecast for the Rio Grande Valley and southern New Mexico where 
the strongest warm signal exists in the consolidation tool. EC is forecast for 
the Pacific Northwest, Northern Rockies, and much of California due to a weak 
signal in the NMME. 
 
The NMME, consolidation, and any influence from La Niña favor below-normal 
precipitation across the Southeast, Gulf Coast States, and Southeast. This 
favored dryness extends northward along the East Coast to the Mid-Atlantic and 
southern New England based on the NMME and daily CFS model runs. However, there 
is only a slight lean towards below-normal precipitation for portions of the 
eastern CONUS since an amplified 500-hPa trough over eastern North America 
early in the month would favor multiple low pressure systems tracking either 
along or offshore of the East Coast. Below-normal precipitation probabilities 
are also lower across the Florida Peninsula as an eastward propagating MJO over 
the Western Hemisphere could eventually lead to a more active southern stream 
with enhanced precipitation. In addition, the daily CFS model runs have less 
support for below-normal precipitation for that part of the Southeast. Week 3-4 
model output, most inputs to the NMME, and La Niña composites support elevated 
above-normal precipitation probabilities for the Pacific Northwest and Northern 
Rockies. A large spatial extent of EC is forecast for the remainder of the 
CONUS due to a weak model signal and limited skill at this time lead for a 
monthly precipitation outlook. 
 
The increased chances of above (below)-normal temperatures forecast for the 
North Slope (southeastern Alaska) are supported by the NMME and consolidation 
tool. Lagged MJO composites would also favor below-normal temperatures across 
southeastern Alaska during mid-December. The favored wetness across western and 
northern Mainland Alaska is based on the NMME and also consistent with decadal 
trends. 
 
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. 
 
An updated monthly outlook... for Dec will be issued on Sat November 30 2024 
 
These outlooks are based on departures from the 1991-2020 base period. 
$$