693
FXUS07 KWBC 211331
PMD30D
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 Nio Southern
Oscillation (ENSO)-neutral conditions continue, below-average sea surface
temperature anomalies are observed across the east-central Pacific. La Nia is
favored to develop by the end of December and La Nia 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 Nia 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 Nia 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.
$$