Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
424 AM EDT Mon Apr 26 2021

Valid 12Z Mon Apr 26 2021 - 12Z Thu Apr 29 2021

...Great Basin to the Rockies...
Days 1-2..

A closed 500mb low will drift southward today from Oregon into the
southern Great Basin, and then drift slowly eastward into
Wednesday as it transitions to a full latitude positively tilted
trough. Through this evolution, an upper jet streak will intensify
downstream of the primary trough axis, reaching 130kts Tuesday
night. The combination of this Pacific jet energy and persistent
mid-level confluence/warm advection will produce high mid-level RH
and PW anomalies reaching more than +1 standard deviation above
the climo mean. This moisture will be wrung out as periods of rain
and snow showers on D1, as deep layer ascent maximizes during the
period of greatest height falls, and snow is likely in many of the
ranges of the Great Basin and Central Rockies, generally above
5000 ft. WPC probabilities on D1 are high for 6 inches in the Big
Horns, Wind Rivers, and Absarokas where locally 12 inches of snow
is possible. Lesser but still significant accumulations in excess
of 4 inches are likely in the terrain of NV, UT, and ID.

By D2 as the trough extends down into Mexico and begins its trek
eastward, height falls, mid-level divergence, and LFQ upper
diffluence will combine to drive lee cyclogenesis in the High
Plains of Colorado. As the surface low strengthens Tuesday, it
will produce easterly upslope flow into the Front Range of CO,
while also tapping into more robust low-level moist advection as
850mb flow arcs out of the Gulf of Mexico. This setup will likely
produce heavy snow in the Rockies of CO, with the most intense
snowfall and greatest accumulations in the Front Range where WPC
probabilities are high for 8 inches, and locally more than 12
inches is likely.


...Northern Great Lakes...
Day 1...

WAA associated with a warm front lifting through the Upper Midwest
will drive a band of precipitation northward, aided by
frontogenetical ascent. At the surface, temperatures will be below
freezing, but this WAA will produce a warm nose near 850mb which
is likely to exceed 0C through the aftn. While this precipitation
is expected to begin as snow, and may accumulate a few inches,
especially within the elevated regions, the rapidly advancing warm
nose will cause a p-type transition to freezing rain. It is late
April and ground temperatures are warm, so freezing rain accretion
is expected to be primarily on elevated surfaces. However, several
hours of light freezing rain are likely which may accrete to more
than 0.1". The highest WPC probabilities for freezing rain are
along the Arrowhead of MN where they reach 10% for 0.25", although
this accretion should be confined to the highest terrain, with
lesser amounts elsewhere and into the U.P. of Michigan.


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

Weiss