Prognostic Discussion for Monthly Outlook 
NWS Climate Prediction Center College Park MD 
300 PM EST Sat Nov 30 2024 
 
 
30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2024 
 
The updated temperature and precipitation outlooks for December 2024 are based 
on the latest dynamical models , 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 week 3-4 outlook (valid December 14-27). El 
Niño Southern Oscillation (ENSO)-neutral conditions continue with a limited 
area of below-average sea surface temperature anomalies observed across the 
east-central Pacific. During late November, the Madden-Julian Oscillation (MJO) 
contributed to anomalous low-level easterly winds  across the equatorial Central 
Pacific. These enhanced trade winds may lead to a transition towards La Niña. 
The GEFS and ECMWF models indicate that the MJO propagates eastward from the 
Maritime Continent to the West Pacific during early December. However, beyond 
that time, ensemble spread increases on the MJO amplitude. Due to uncertainty 
on how much influence the MJO or the transition to La Niña has on the 
mid-latitude circulation pattern, the updated temperature and precipitation 
outlooks relied mostly upon the dynamical model guidance at varying time scales 
throughout December. 
 
During early December, the GEFS and ECMWF ensemble means are in good agreement 
and remain consistent that a highly amplified 500-hPa pattern persists over 
North America. These ensemble mean solutions depict a ridge over the western 
contiguous U.S. (CONUS) with anomalous troughing centered near the East Coast. 
This amplified longwave pattern is likely to result in above-normal 
temperatures from the West Coast east to the Rockies with below-normal 
temperatures across the eastern third of the CONUS. By mid-December, the GEFS 
indicates a retrogression of the longwave pattern with the amplified ridge 
along the West Coast shifting west to the North Pacific. This would eventually 
lead to a pattern change downstream with a 500-hPa trough (ridge) over the 
western (eastern) CONUS. The updated temperature outlook added a lean towards 
below-normal temperatures across the Mid-Atlantic and Northeast based on the 
anomalous cold through early December. However, probabilities are limited due 
to the predicted longwave pattern change which would result in a warming trend 
during the latter half of the month. For the western CONUS, the likelihood of 
above-normal temperatures through mid-December and the lack of a cold signal 
among the week 3-4 tools support an increased chance of above-normal 
temperatures. The largest probabilities (exceeding 60 percent) are forecast for 
southern California and the Desert Southwest where the magnitude of the 
anomalous warmth is predicted to be the strongest early in the month. Elevated 
above-normal temperature probabilities extend east to the Central and Southern 
Great Plains since model guidance, especially for the 8-14 day period, has 
trended warmer. Equal chances (EC) for below, near, or above-normal 
temperatures are forecast across portions of the Northern Great Plains, 
Mississippi Valley, and Southeast where temperatures are expected to be 
variable throughout the month. Following a cold start to the month across 
Alaska, a rapid flip to above-normal temperatures is forecast around December 4 
due to enhanced onshore flow. A two-category change (favored below to 
above-normal temperatures) from the initial release on November 21 was 
necessary for southeastern Alaska as increased chances for above-normal 
temperatures are forecast to continue through the month. 
 
The highly amplified ridge over western North America is likely to result in a 
dry pattern for much of the lower 48 states through early December. A couple 
exceptions include areas downwind of the Great Lakes with heavy lake-effect 
snow and portions of Texas and the Lower Mississippi Valley as a southern 
stream shortwave trough tracks eastward. Above-normal precipitation is slightly 
favored for those areas which are forecast to have a wet start to the month. In 
addition, the week 3-4 outlook favors above-normal precipitation across eastern 
and southern Texas. Since the first week of the month is forecast to be mostly 
dry and there is no strong wet signal later in the month, the elevated 
below-normal precipitation probabilities for parts of the eastern CONUS are 
maintained from the outlook released on November 21. Drier-than-normal 
conditions through at least mid-December elevate below-normal precipitation 
probabilities across the Southwest, parts of the Great Basin, and California. 
Also, the week 3-4 outlook favors below-normal precipitation for southern 
California and the Desert Southwest. Although the first week of December is 
forecast to be on the drier side across the Northern Rockies and Northern Great 
Plains, the 8-14 day and week 3-4 outlook depict a wetter signal which leads to 
favoring above-normal precipitation across those areas. Despite a dry start to 
the month for the Central Great Plains, Middle Mississippi Valley, and Ohio 
Valley, a pattern change later in December towards a trough upstream could lead 
to an increase in precipitation. Therefore, EC are forecast for these areas. 
Enhanced onshore flow is forecast to develop across Alaska during the first 
week of December and tools throughout the month generally lean towards 
above-normal precipitation for much of the state. 
 
 
***************** Previous discussion, released on November 21, is below 
********************* 
 
 
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. 
 
The next monthly outlook...for Jan ... will be issued on Thu Dec 19 2024 
 
These outlooks are based on departures from the 1991-2020 base period. 
$$