Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EDT Fri Apr 07 2023

Valid 12Z Fri Apr 07 2023 - 12Z Mon Apr 10 2023

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

An upper trough crosses the PacNW coast early this morning with
post-cold frontal conditions cooling and snow levels dropping from
around 5000ft to 4000ft today as precip rates decrease. Day 1 snow
probabilities for additional 6"+ after 12Z today are moderate for
the higher Cascades to the northern Sierra Nevada and Wallowa Mtns
of eastern OR. Expect a lull in precip tonight before a zonal
Pacific jet reaches western WA Saturday, producing a brief round
of moderate precip over western WA with snow levels generally
3000-4000ft. Day 2 snow probabilities are moderate for 6"+ over
the higher Olympics and northern WA Cascades.

The next low to eject from the Gulf of Alaska is Saturday night
which backs the Wly jet to to SWly with an atmospheric river
arriving across the PacNW Sunday. Heavy precip rates can be
expected in a fairly narrow (latitudinally), but snow levels will
be 7000-8000ft, limiting snowfall to only the highest Cascades.


...Upper Midwest...
Days 1/2...

A band of snow and some rain will develop north of a warm front
Friday night over northern WI and shift east into Saturday.
Placement of snowfall among 00Z guidance is better than previous
though the focus really is more in north-central/northeastern WI
and much less in MI. This snow band will develop in response to
strongly sloped 850-700mb fgen which will drive impressive omega
directly into the DGZ noted by a cross-hair signature in regional
soundings and at night with snowfall generally between 03Z and 15Z
Saturday. An upper jet streak positioned southeast of the area
looks to keep northern WI/MI in the left entrance region - where
subsidence occurs, which contributes to magnitude uncertainty. The
thermal column is marginally favorable for snow, so it will likely
require some dynamic cooling to produce any significant
accumulations. Nearly all the guidance suggests near 0C/km theta-e
lapse rates around 06Z Saturday invof this band, with the NAM and
RGEM indicating some collocated -EPV* indicating the potential for
some convective precipitation. If convective rates can develop, it
will likely dynamically cool the column sufficiently for rapid
accumulation despite what should be generally below a 10:1 SLR.
Day 1.5 WPC probabilities for more than 4 inches are now 10-30%
over northeastern WI between 90W and the Green Bay (the water
feature, not the city). WPC PWPF are inherently inhibited by a
lack of dynamic cooling inherent to the PWPF, so a few isolated
amounts of more than 4 inches are likely. However, most of the
snow areas should be generally 2-4" or less.


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


Jackson