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

Valid 00Z Sat Apr 08 2023 - 00Z Tue Apr 11 2023

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

Zonal mid-level flow angling into the Pacific Northwest will
gradually back to be more SWly as a sharpening shortwave dives
southward from the Gulf of Alaska. Increasingly confluent flow
within this wind shift will drive an AR into WA state and British
Columbia, with the peak IVT reaching above 5000 kg/m-s according
to CW3E probabilities late D2. At the same time, this backing low
and mid level flow will drive pronounced WAA into the region as
the most intense height falls and PVA shift north of the US/Canada
border, pushing snow levels as high as 6000-7000 ft late Sunday,
with only modest reduction into Monday to around 4000-5000 ft.
With the most intense precipitation and lowest snow levels
expected to remain across British Columbia, impactful snow is
expected to feature a minimal footprint this period, reflected by
WPC probabilities for more than 6 inches reaching only above 30%
in the higher terrain of the Olympics and WA Cascades each day,
but 3-day snow totals may exceed 2-3 feet in the highest terrain.


...Upper Midwest...
Day 1...

The guidance has become much better clustered, fortunately, with
the position of a frontogenetically forced band of precipitation
that is expected to develop this evening and then pivot E-NE
through Saturday morning from WI into the L.P. of MI. This band
will develop ahead of a northward advancing warm front, and will
strengthen in response to strongly sloped 850-700mb fgen to drive
intense UVVs into the DGZ as noted by a cross-hair signature in
regional forecast soundings, and the guidance has a bit more
along-band motion of any snow showers than was depicted in
previous days. This should result in a narrow axis of heavy QPF,
falling as snow in many areas, with heavy rates greater than 1"/hr
likely at times noted by the WPC prototype snowband tool. However,
a marginal thermal structure and a less favorable upper jet
position (LRQ subsidence) with dry air positioned to the north of
this axis combined with sub-climo SLR should limit the snowfall
accumulations. Despite this event occurring generally overnight,
it will likely require strong dynamic cooling within the most
intense snow rates to accumulate efficiently, and as such the
corridor of high accumulation probabilities is narrow. Still some
bust potential on the high end as some -EPV could result in
convective snowfall with higher accumulations, and WPC
probabilities have narrowed to show a 30-50% chance for 4 inches
in parts of NE Wisconsin.


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


Weiss