Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 AM EST Thu Jan 23 2020

Valid 12Z Thu Jan 23 2020 - 12Z Sun Jan 26 2020

...Pacific Northwest...
Days 1-3...

Snow levels will continue to rise into Saturday before crashing
for the weekend, especially east of the Cascades. Persistent and
confluent mid-level flow from the Pacific will maintain a moisture
stream into the region, with embedded mid-level impulses and
periodic jet streaks producing ascent within this moist column.
With the warm snow levels and the jet angled well north into
Washington State, the heavy snow should be confined through the
forecast period to the highest terrain of the WA/OR Cascades and
Northern Rockies, as well as isolated mountains of WY, Northern
CA, and eastern OR. The heaviest snow is likely in the Cascades on
D1 when WPC probabilities are high for 12 inches, highest in WA.
Probabilities fall off for heavy snow on D2 and D3, but are still
high for 6 inches in the Cascades/Olympics of WA and Northern
Rockies on D2, and again in the WA Cascades on D3. Snow totals in
the Cascades may reach 4 feet during the next 3 days.

Snow melt and heavy rain are the greater threats in the Pacific
Northwest, so please see the excessive rainfall discussions
(QPFERD) for further information.


...Great Plains/Midwest/Great Lakes/Mid-Atlantic/Northeast...
Days 1-3...

Complex interaction of multiple upper lows will lead to a slow
moving surface low beneath modest upper dynamics to produce a long
period of light to moderate snow extending from The Dakotas
southward into the Missouri Valley, and eastward into the Great
Lakes. The primary closed low will gradually shift east from
Kansas this morning into Ohio Saturday, while other vorticity
impulses rotate to the NW and SE of this primary feature. The
lagging shortwave will help to persist snow through the Upper
Midwest, while the feature to the SE will eventually lead to
secondary surface low development across the Mid-Atlantic on
Saturday. It is this secondary feature that will become
predominant and drive stronger dynamics into the Mid-Atlantic and
New England by day 3.

Forcing D1-D2 is generally modest in the vicinity of the slow
moving upper low. WAA east of the primary low will spread moisture
northward and then westward, with some mesoscale enhanced ascent
likely in the immediate vicinity of the surface feature as well as
where some Lake Enhanced snow will occur in the U.P. The highest
WPC probabilities for 4 inches on D1 are in Missouri and the U.P
for these reasons. By D2, WAA increases as the LLJ rises towards
50kts, and again two maxima of snow are possible. The highest
probabilities for 4 inches exist across parts of MO once again,
with additional moderate to heavy snow likely in WI as a weak WCB
wraps around the low.

On D3, the secondary low will intensify as it moves across the
Mid-Atlantic, with moisture on increasing WAA spreading northward.
Ascent will increase through height falls, the WAA, and better LFQ
jet diffluence to produce a swath of snow and mixed precipitation.
The heaviest snow is likely in the terrain of Northern New England
and Upstate New York where WPC probabilities are moderate for 4
inches. With the WAA overwhelming the low-level cold air, freezing
rain is likely in the transition zone as well, and WPC
probabilities are 10-20% for 0.1" in parts of PA and NY.

Weiss