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

Valid 12Z Mon Mar 30 2020 - 12Z Thu Apr 02 2020


...The West...
Days 1-3...

An expansive upper low will drift southward from the Gulf of
Alaska through mid-week while periodic impulses rotate around the
low and onshore the Northwest. Several impulses will spread inland
as the low drops south. By Wednesday, the core of the upper low is
expected to be centered over Alberta, Canada, with a large trough
and lowered heights extending into the Great Basin. With each of
these vort lobes/impulses, a wave of low pressure and cold front
will race across the Northwest and towards the Northern Plains,
bringing periods of moderate to heavy snow. On Days 1 and 2
(through 12Z Wednesday) the heaviest snow will be confined to the
terrain between 2000 and 4000 ft from the Olympics eastward to the
Northern Rockies and ranges of NW Wyoming, and south into the
Sawtooth of Idaho and Oregon Cascades. WPC probabilities across
these ranges are high for 8 inches both days, with 2-day totals
possibly reaching 30 inches in the highest terrain of the Cascades
and the Tetons.

On Day 3, a final impulse will push eastward and rotate through
the trough towards the High Plains of WY. This is likely to spawn
surface cyclogenesis near the CO/WY border, with intensification
of this low likely late on D3 as it lifts east/northeast into
Nebraska. The upper forcing driving the surface low is likely to
be transient and modest in intensity, so the low may weaken into
more of a WAA pattern Thursday morning, but interaction of this
feature with a cold front/baroclinic zone will lead to some modest
frontogenesis and a swath of moderate to heavy snow. WPC
probabilities for 4 inches are moderate for the High Plains of WY
into the Black Hills and points northeast through central SD on D3.

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

Weiss