Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
241 AM EDT Thu Apr 06 2023

Valid 12Z Thu Apr 06 2023 - 12Z Sun Apr 09 2023

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

An active pattern of troughs from the Gulf of Alaska through the
Pacific Northwest coast continues into next week. The next trough
axis arrives at the PacNW coast late Thursday night with pre-cold
frontal precip from enhanced Pacific moisture and ascent bringing
intensifying mountain snows to the Cascades to northern Sierra
Nevada tonight/Friday with snow levels dropping from 6000ft to
5000ft.

Diffluence within the left exit region of an approaching Pacific
jet streak will additionally enhance ascent, while also driving
more pronounced moisture onshore adding to the impressive moisture
plume driven by the mid-level evolution. CW3E probabilities from
the GEFS and ECENS suggest the accompanying atmospheric river (AR)
will exceed 500 kg/m-s today, resulting in an expanding area of
precipitation along and ahead of a surface cold front, first along
the coast, but eventually spreading east of the Cascades and into
the Northern Rockies by Friday morning. WPC probabilities for 6"+
for Day 1.5 are moderate to high in the higher terrain of the
Olympics, Cascades, Shasta/Trinity region, northern Sierra Nevada
and Blue Mtns of OR. Locally more than 12 inches is likely above
about 7000ft.

A rather weak reinforcing trough pushes into western WA Saturday
with moderate snow chances above the 4000 to 5000ft snow level.
Day 3 snow probabilities for 6"+ are moderate and limited to the
higher Olympics and northern WA Cascades.


...Upper Midwest...
Days 2/3...

A narrow band of heavy snow looks to develop Friday night into
Saturday in response to some increased 850-700mb fgen overlapped
with the favorable left exit of a strengthening but departing jet
streak ahead of a warm front. It is likely that at least a narrow
corridor of moderate to heavy snow will develop over north-central
WI and extend east across Lake Michigan into the northern L.P. of
MI. With the event generally expected to occur at night and enough
dynamics/ascent from the fgen there should be a localized
favorable thermal profile for accumulating snow. Non-NAM guidance
is on board for a stripe of 2-4" with local 6" possible over these
areas. However, the stripe is narrow enough and placement variance
among guidance sources varies enough that a more broad-brushed
approach to the snowfall forecast is acceptable at this time.


...Northern Maine...
Day 1...

A wintry mix over northern Maine comes to an end quickly this
morning with perhaps a few more hundredths inch of icing after 12Z.


The probability for icing over 0.1" is less than 5% Days 1-3.


Jackson