Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
209 PM EST Thu Dec 21 2023

Valid 00Z Fri Dec 22 2023 - 00Z Sun Dec 24 2023

...Heavy rainfall and flash flash flooding will continue to impact areas
of southern California through Friday, with the threat gradually expanding
into the Southwest...

...Unsettled weather will advance across the Pacific Northwest and the
Intermountain West going through Saturday with lower-elevation rains and
higher-elevation snowfall...

...Unusually mild temperatures across the Plains and Midwest will persist
into the early part of the Christmas holiday weekend...

An upper-level low offshore of southern California will gradually begin to
accelerate off to the east tonight and Friday, and will move into the
Southwest U.S. by Friday night. Areas of heavy rainfall associated with
the storm system will continue across especially the coastal ranges of
southern California this evening and into the overnight period, with
locally significant flash flooding possible. Urban flash flooding will
also be possible including areas of the Los Angeles basin. Debris flows
and mudslides will be possible near burn scar areas and areas of steep
terrain. Locally several inches of additional rain is possible by Friday
morning, and the Weather Prediction Center has maintained a Moderate Risk
of excessive rainfall for these areas.

On Friday, the heavy to excessive rainfall threat will still impact some
areas of southern California, including the Peninsular Range, but the
rains will advance east into areas of central and southern Arizona as well
going through Friday night and early Saturday. There is a Slight Risk of
excessive rainfall depicted across these areas where there may be some
isolated to scattered areas of flash flooding. Some of the local slot
canyons and burn scars closer to areas of terrain, and also the dry
washes/arroyos over the deserts, will be vulnerable to seeing these runoff
impacts. However, generally much of the rainfall across these areas should
be beneficial.

Meanwhile, a cold front will advance inland across the Pacific Northwest
and Intermountain West going through Friday and Saturday, and this will
bring low-elevation rains and high-elevation snowfall to the region. The
heaviest snowfall will be over the Washington Cascades, and then
downstream over the northern and central Rockies including the
Bitterroots, Tetons, and Colorado High County. The San Juan mountains of
southwest Colorado will see impacts relating to the storm system
traversing the Southwest, and multi-day snowfall amounts of 1 to 2 feet
will be possible here going through Saturday.

Areas farther east across the central and eastern U.S. will be seeing
generally very mild temperatures through the remainder of the week and
into the start of the Christmas holiday weekend, with areas of the Plains
and Midwest in particular seeing temperatures that will be as much as 15
to 25 degrees above normal. These relatively warm temperatures will be the
result of increasing southerly flow with time and the return of moisture
from the Gulf of Mexico, and some record warm overnight low temperatures
in particular may be set. Expect these warmer temperatures to begin
overspreading the Ohio Valley and Mid-South, with at least above normal
temperatures reaching the East Coast.

Orrison


Graphics available at
https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php