Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EST Fri Dec 22 2023

Valid 12Z Fri Dec 22 2023 - 12Z Sun Dec 24 2023

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

...Next round of unsettled weather will reach the Pacific Northwest today
then quickly move through the Intermountain West going into Saturday...

...Snow is forecast to overspread the central Rockies Saturday night
before expanding into the northern Plains by the morning of Christmas
Eve...

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

A relatively slow-moving low pressure system off the southern California
coast will begin to gradually accelerate off to the east and move onshore
into the Southwest U.S. by tonight.  Deep southerly flow ahead of the low
center will continue to feed moisture-laden air from the subtropical
Pacific Ocean toward southern California today.  Heavy rainfall will most
likely be found once again along the wind-facing coastal ranges of
southern California where prolific rainfall amounts had already been
reported in the past 24 hours, with additional significant flash flooding
expected.  Urban flash flooding will also be possible including areas of
the Los Angeles basin early this morning.  Debris flows and mudslides will
remain likely near burn scar areas and areas of steep terrain.  The
Weather Prediction Center has maintained a Moderate Risk of excessive
rainfall for these areas early this morning.

As the low pressure system moves onshore later today, the heavy to
excessive rainfall threat will begin to spread east into areas of central
and southern Arizona as well by Friday night and into early Saturday.  A
Slight Risk of excessive rainfall is 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 today and into Saturday, bringing the next round of
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.

By Saturday night into Sunday morning, the moisture associated with the
cold front is forecast to head toward the central Rockies and will begin
to interact with the moisture lifting northeastward from the southwestern
U.S. system.  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.  A low pressure system is forecast to consolidate and
strengthen over the central High Plains early on Sunday with snow
expanding northeastward into the northern Plains under strong and gusty
northerly winds.

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.

Kong/Orrison


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