Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
305 AM EST Tue Dec 21 2021

Valid 12Z Tue Dec 21 2021 - 12Z Thu Dec 23 2021

...Low pressure system expected to bring locally heavy rain/thunderstorms
across Florida and the Southeast coast today...

...Another low pressure will spread snow across the northern tier states
today and then a quick round of snow for Maine into tonight...

...Unsettled weather spreads farther south from the Pacific Northwest into
California as well as farther inland across the northern Rockies...

A low pressure system moving through the Gulf of Mexico is forecast to
make landfall and traverse the Florida Peninsula this afternoon.  This
system has been ingesting plenty of moisture from the Gulf water and is
forecast to bring a quick round of moderate to heavy rain across northern
Florida today.  To the south of the low track, thunderstorms ahead of a
cold front could become severe this morning.  Some of the heavy rain and
thunderstorms may result in instances of flash flooding.  In addition, a
period of gusty winds can be expected as the low center moves over the
Florida Peninsula this afternoon.  The heavy rain and severe weather
threat will quickly diminish this evening for all of Florida as the low
moves out into the Atlantic.  On the other hand, some of the heavier
rainfall will brush the Southeast U.S. coastal areas tonight into
Wednesday morning as the low turns northeastward and tracks along the Gulf
Stream.  The low pressure system should track far enough offshore
Wednesday night to allow fine to return to the East Coast.  Colder air
will then overspread the entire eastern U.S. into Thursday behind a
reinforcing cold front and as a high pressure system takes over.

Meanwhile, another clipper type low pressure system is expected to bring a
quick round light to moderate snow across the northern tier states today. 
The warmer waters of upper Great Lakes should help deliver somewhat higher
snowfall amounts on the lee side of the lakes.  By later today, some snow
or wintry mix is expected to develop in Maine ahead of this low and should
continue into tonight.  By Thursday morning, the wintry weather in Maine
should come to an end as with cold blustery winds rush in from the west.

Elsewhere, the aforementioned upper-level shortwave is currently found
across the Northwest and responsible for light-to-moderate snow extending
into the Northern Rockies tonight. This area of light snow will race
across the Northern Plains and into the Upper Midwest by Tuesday morning,
where a few inches of accumulating snow is possible. Parts of the Upper
Great Lakes will also enjoy some festive snowfall on Tuesday and Tuesday
night. For the immediate western U.S. coastline, continuous onshore flow
will allow for unsettled weather on Tuesday until the next system
approaches on Wednesday. Heavy snow is possible across the Cascade
Mountains and Sierra Nevada, as well as periods of moderate-to-heavy rain
along the northern California and Oregon coast.

For much of the central U.S., dry and mild weather is in store through
midweek as below average and bitterly cold temperatures remain confined to
the Northern Plains.

Kong/Snell


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