Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
254 AM EST Thu Dec 23 2021

Valid 12Z Thu Dec 23 2021 - 12Z Sat Dec 25 2021

...An active pattern will bring significant mountain snow and heavy lower
elevation rain to the western U.S. into the weekend...

...Snow showers expected across the Great Lakes and Northeast Thursday
before switching to a rain/snow mix on Friday...

...Persistent warm and dry conditions over the central U.S. will generate
fire weather concerns over the Central/Southern High Plains on Thursday...

Throughout the short-range period much of the activity over the CONUS will
be focused over the western U.S. as multiple features impact the region
simultaneously. One such feature is a moderately strong atmospheric river,
which is transporting anomalously high amounts of atmospheric moisture
onshore by way of southern California. Along the low lying coastal regions
this anomalous moisture will translate into heavy rainfall over the next
couple days, with some locations, including the terrain adjacent to the
Los Angeles and San Diego metro areas, forecast to receive 1 to 3+ inches
of rain in 24 hours. Given the potential for flash flooding as well as
runoff over nearby burn scarred and hydrologically sensitive areas, the
Weather Prediction Center has issued a Slight Risk of Excessive Rainfall
for portions of southwestern California through Saturday morning. Further
north, a cold front forecast to move onshore this morning and progress
slowly through the Intermountain West over the next couple of days will
interact with the moisture supplied by the atmospheric river to produce
heavy snowfall over the high elevation terrain from the Cascades to the
Rockies. Over the Sierra Nevada there is potential for 5+ feet of snow to
accumulate through Christmas morning, while more moderate totals of around
1 to 4 feet are expected across the rest of the western mountain ranges.
In preparation for the snow and potential for hazardous travel conditions,
a menagerie of Winter Storm Watches and Warnings as well as Winter Weather
Advisories have been put into effect throughout the western U.S.

Meanwhile, a tight pressure gradient is forecast to develop on the
lee-side of the Rockies later today, generating strong winds with gusts up
to and exceeding 65 mph, particularly in the Southern High Plains. Given
the potential for these winds to be damaging, blow down trees, cause power
outages, and make travel hazardous, High Wind Watches and Warnings have
been put into for portions of the Central/Southern Rockies and the High
Plains. In addition to the aforementioned hazards, strong winds can also
contribute to the spread of wildfires where very dry fuels are present and
low relative humidity values are expected. This combination of conditions
is forecast to occur over the Central/Southern High Plains today,
prompting the issuance of an Elevated Risk of Fire Weather by the the
Storm Prediction Center.

To the east, a quasi-stationary boundary extending through the Great Lakes
will be the focus for the development of light snow from the Upper Midwest
to the Northeast through Friday morning. As a warm front approaches from
the southwest a rain/snow mix will emerge, allowing for freezing rain to
form and a thin layer of ice to accumulate over portions of the Upper
Midwest, New York, and southern New England.

To end the work week a warm, dry pattern will set up over the central
U.S., allowing daily high temperatures across the Plains and Mississippi
Valley to raise significantly. The greatest departures from normal will
likely be felt over the Southern Plains, where temperature anomalies of 25
to 30 degrees above normal are expected on Friday, translating to highs in
the 70s and 80s. Conversely, temperatures along the East and West coasts
are forecast to be near or moderately below normal.


Zavadoff


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