Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EST Mon Dec 20 2021

Valid 12Z Mon Dec 20 2021 - 12Z Wed Dec 22 2021

...Unsettled weather lingers over the Pacific Northwest and the northern
Rockies as the next wave of moisture reaches the West Coast...

...Low pressure system in the Gulf of Mexico to deliver heavy showers and
thunderstorms to Florida and the Southeast coast...

...Dry and mild throughout much of the nation's midsection but cold with
occasional light snow across the northern tier...

More active weather is expected across the Pacific Northwest in a battle
zone between a large cut-off upper low in the northeast Pacific and arctic
air over western Canada.  A frontal system just off the coast will be
responsible for periods of coastal/valley rain and heavy mountain snow. 
Snow totals through tonight are likely to exceed a foot in parts of the
Cascades and Bitterroots.  A brief building bubble of high pressure over
southern British Columbia gives the Northwest a break in the action early
on Tuesday before the next push of Pacific moisture arrives Tuesday
evening into Wednesday with more lower elevation rain and accumulating
mountains snow, this time extending as far south as central California.
Meanwhile, a quick moving wave of low pressure in Montana will produce
light snow accumulations across the northern Plains late Monday into early
Tuesday.  This system then reaches the upper Great Lakes by Tuesday night
where several inches of snow are possible.

To the south, an area of low pressure is forming along a frontal boundary
in the Gulf of Mexico this morning.  Spotty showers will track along the
immediate central Gulf Coast on Monday as the storm traverses the central
Gulf of Mexico. By early Tuesday morning the storm will have strengthened
and a threat for heavy rain and thunderstorms, some of which could be
severe, is expected across most of the Florida Peninsula.  Some
thunderstorms could produce damaging wind gusts, tornadoes, and
waterspouts.  Over an inch of rainfall is forecast across northern
Florida, southern Georgia, and far southern South Carolina through Tuesday
evening before the low tracks off the Southeast U.S. coast Tuesday night.

Sandwiched between the active weather patterns in the Northwest and the
Southeast is upper level ridging that extends from the Great Basin to the
Mid-Atlantic coast. The result is high pressure dominating the pattern in
the central Rockies, the Nation's Heartland, and the Northeast. Mostly dry
conditions and abnormally mild temperatures for late December are forecast
in these regions the first half of the week, although chilly morning lows
are still likely in the Northeast Monday morning. In fact, mostly dry
conditions persist even as a more spring-like air-mass arrives late week
for most of the Plains and Mississippi Valley.

Mullinax/Kong


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