Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
459 PM EDT Mon Mar 30 2020

Valid 00Z Tue Mar 31 2020 - 00Z Fri Apr 03 2020


...Northwest...Northern Rockies...and Northern Great Plains...
Days 1-3...

A cold core low drifting south-southeast from near the AK/BC
coastal border this afternoon will reach Vancouver Island Tuesday
before a reinforcing shortwave trough opens this portion of the
low (the rest of the upper low will anchor over Alberta Tuesday
through Thursday) into a positively tilted trough and swings
southeast through the Pacific Northwest through Wednesday then
east to the Dakotas through Thursday. The associated cold front is
over the across the WA/OR Cascades and will shift east ahead of
the upper trough Tuesday and spill onto the central Plains
Wednesday night through Thursday. Snow levels of 3000 to 4000ft
can be expected along the frontal boundary through Tuesday.
Pacific moisture ahead of the trough/cold front will continue to
stream inland on westerly low level flow of 30 to 40kt through
tonight before getting shunted well south as offshore flow veers
more northerly. This makes for ongoing heavy snow over the OR/WA
Cascades/Olympics, Blue Mtns, Salmon/Sawtooth and other northern
Rockies into WY continue through Tuesday (northwestern WY ranges
get into the heavy snow tonight) before rates drop off
significantly Tuesday night. Day 1 WPC snow probabilities are high
for 12 or more inches along the OR Cascades, the Salmon/Sawtooth
in ID up to the MT border and for the WY Absarokas/Tetons/Wind
Rivers with up to 2ft in 24hrs possible for the highest windward
elevations.

As the mid-level trough axis shifts east across OR/ID on Wednesday
the lee side cyclogenesis shifts south from WY to CO where the low
and strong ridging to the north sets up strong frontogenesis and a
snow swath for the northern Plains of southeast MT, eastern WY,
western SD, and much of ND Wednesday through Thursday. There is
uncertainty with the low level track with basically two camps, the
farther west ECMWF/CMC and the farther east NAM/GFS/UKMET.
Preference is given to the ECMWF given ensemble support which form
most of the basis for QPF and winter ptypes for the second half of
Day 2 and Day 3. Day 3 snow probabilities for 6 or more inches are
over these portions of the northern Plains with moderate to high
probabilities for eastern WY and the Black Hills. Will need to
keep an eye on the snow axis northeast of the Black Hills into ND
for potentially heavy snow in the coming days. Farther east in
southerly flow enough cold surface air looks to be present for
some icing in southeast ND and northeast SD. Day 3 ice
probabilities for a tenth or more of an inch are 20 to 40 percent
near the east-central SD/ND border.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson