Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
412 AM EDT Wed Oct 20 2021

Valid 12Z Wed Oct 20 2021 - 12Z Fri Oct 22 2021

...Pair of Pacific storms to bring heavy rain and mountain snow to
Northern California and the Pacific Northwest...

...Periods of rain and severe storms possible from the Midwest to the
northern Mid-Atlantic; severe storms also possible in the Southern Plains
on Thursday...

...Abnormally warm temperatures return in the East, cooler in the Midwest
and along the West Coast...

The first in a parade of Pacific storm systems to slam the West Coast over
the next 5-7 days is striking the Pacific Northwest this morning. Rain
will be the dominant precipitation type in the valleys and coastal areas,
while snow falls in the highest elevations of the Cascades and Shastas.
Precipitation should diminish as the frontal system weakens late Wednesday
into early Thursday morning, but a second and more intense Pacific low
approaches on Thursday. The storm will direct a moisture-rich atmospheric
river at Northern California and the Pacific Northwest on Thursday,
pumping ample amounts of precipitation into these regions. This is a
welcome sight to much of the drought-stricken and wildfire-riddled
Northwest, particularly in southwest Oregon and Northern California where
the heaviest amounts are expected. However, rainfall rates could reach
excessive levels at times, making flash flooding, rapid run off, and
debris flows a concern in burn scar areas. As a result, a Marginal Risk
for Excessive Rainfall is in place for portions of Northern California and
far southwest Oregon for both Thursday and Friday. Only the highest
elevations can expect measurable snowfall due to high snow levels aloft.

In the Heartland, a strengthening area of low pressure tracking into the
Midwest today is still causing snow to fall in southwest South Dakota.
Snow should taper off soon as the storm tracks east this morning. This
area of low pressure is set to produce periods of rain in the Upper
Mississippi Valley. Thunderstorms are also in the forecast with some
storms potentially becoming severe as far east as the Great Lakes and as
far south as the mid-Mississippi Valley. A Marginal Risk for severe
weather has been issued today for northern Iowa and southern Minnesota. As
the wave of low pressure pushes east into the Great Lakes Thursday
morning, so will its swath of showers and thunderstorms. Thursday's most
at risk area for severe weather is in the eastern Great Lakes and western
Pennsylvania, but thunderstorms could even develop in the northern
Mid-Atlantic. Severe weather is also possible along a warm front emanating
out of a new disturbance forming in the Southern Plains. Another Marginal
Risk is in place for southern Oklahoma where severe storms may develop
Thursday evening.

The East witnesses the return of seasonally warmer temperatures after
experiencing more autumn-like conditions the last few days. Widespread 70s
are anticipated from the Mississippi Valley to the Mid-Atlantic coast
while the South Central U.S. sees highs soar into the 80s. By Thursday,
even parts of the Mid-Atlantic can expect highs that approach 80 degrees
but considerably warmer conditions will be common up and down the East
Coast and along the Gulf Coast. In contrast, the persistent influence of
onshore flow from the onslaught of Pacific storm systems will generally
keep the West Coast on the cooler than normal side. In the Northern
Plains, a cold front is ushering in another shot of cooler temperatures
across the region today. This cold front makes its way east into the
Midwest and Great Lakes on Thursday, leading to a similar cooling trend
there. The cold front eventually reaches the East Coast by Friday,
allowing for a cool, crisp air-mass to return across the Northeast for the
upcoming weekend.

Mullinax

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