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

Valid 00Z Fri Apr 07 2023 - 00Z Mon Apr 10 2023

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

A modest atmospheric river (AR) with IVT of 250-500 kg/m-s will
continue to move onshore tonight, spreading precipitation across
the Pacific Northwest, with spillover occurring as far east as the
Northern Rockies Friday night. The primary driver of this AR will
be warm/moist advection on backing mid-level flow downstream of a
trough that will pivot into WA Friday morning and then weaken as
it shifts into MT Friday night, aided by weak LFQ diffluence as a
Pacific jet streak approaches the coast. Snow levels during the
heaviest precipitation will rise to 4000-6000 ft, with snow
occurring above these levels from the Olympics through the
Northern Rockies and extending as far south as the Northern
Sierra. A cold front pushing eastward during Friday will cause a
lowering of snow levels into D2, but this will also be accompanied
by a drier column and a brief break to precipitation. A secondary
shortwave with stronger associated jet energy will lift into
British Columbia D3 /Sunday/, driving another modest AR onshore,
but this one is likely to be displaced north of the first one on
D1. This will again spread heavy precipitation onshore, but
generally confined to WA state and into Canada, with snow levels
surging to as high as 7000 ft on D3. WPC probabilities for more
than 6 inches are most widespread on D1 when they reach 50-80% in
the highest terrain from the Olympics and WA Cascades southward
through the Shasta/Trinity region of northern CA and into the
northern Sierra. A significant decrease in coverage occurs on D2,
before moderate probabilities ramp up again D3 in the Olympics and
WA Cascades.


...Upper Midwest...
Day 2...

Challenging forecast Friday night into Saturday as a band of rain
and snow develops near Wisconsin and advects northeast before
dissipating. The guidance is in good agreement that this band will
develop north of a warm front, but placement among the various
guidance is still quite different with some fluctuations SW to NE
in the greatest snowfall footprint. While placement of this band
is uncertain, there is a strong likelihood that it 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. An upper jet streak positioned
southeast of the area may try to arc poleward enough such that
some brief LFQ diffluence will aid in ascent, but if this jet
stays more zonally oriented it could actually inhibit ascent
through subsidence, so that will aid in uncertainty as well. This
event looks to occur overnight Friday night, so insolation and
April heating likely won't be a problem, but the column is still
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 well below climo SLR. However, the general
progressive nature of the band to the northeast should still limit
the duration of any heavy snow, so the ceiling on snow
accumulations should be modest, likely around 4-5" as shown by the
NBM 90th% percentile. At this time, WPC probabilities for more
than 4 inches feature a small footprint across northern WI where
they reach 10-20%. The PWPF may be somewhat inhibited by
uncertainty in thermal structure and 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.


Weiss