Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
252 PM EDT Sat Apr 6 2024

Valid 00Z Sun Apr 07 2024 - 00Z Wed Apr 10 2024


...Northern/Central Rockies and High Plains...
Days 1-2...

A strong low pressure system moving slowly across the Central High
Plains will continue to deepen D1 in response to impressive height
falls within a closed mid-level low and accompanying upper level
diffluence within the LFQ of a potent upper level jet streak. The
amplitude of this mid-level low will peak by Sunday morning,
reaching as low as -4 sigma with respect to 700-500mb heights
according to NAEFS, supplying the tremendous ascent into this
robust system. Later on Sunday, the evolution will force the system
to become vertically stacked and begin to occlude, causing it to
finally drift away to the east while subsequently shutting off some
of the moisture advection. Before this occurs, however, moisture
wrapping cyclonically into the system will be impressive as noted
via strong theta-e advection lifting into the TROWAL, supporting PW
anomalies as high as +1 to +2 sigma. This moisture wrapping into
the High Plains/Central Rockies will be wrung out by the deep layer
ascent, especially where upslope flow or deformation can
contribute.

While the column will be marginal for wintry precip outside of
higher terrain features, but as moisture wraps into the higher
elevations of WY and MT, generally above 400 ft should see heavy
snow accumulations D1 as reflected by WPC probabilities for more
than 6 inches above 90% from the Laramies northward through the Big
Horns and Black Hills, and as far NW as the Snowy Range of MT. In
the Big Horns, high probabilities continue on D2, and locally 2+
feet of snow is likely there.

The more challenging aspect of this forecast is for the foothills
and High Plains of central MT through eastern WY and into western
SD/NE. Here, the column thermal structure is marginal, but forcing
is likely to be intense D1. An impressive deformation axis will
pivot SE to NW across the region, into which the moisture and
TROWAL will overlap. At the same time, some enhanced fgen is likely
to develop around the low, producing a region of intense lift
across eastern WY. This will cause strong omega into a deepening
DGZ characterized by SREF probabilities reaching 90% for 50mb of
depth, supporting efficient snow growth and dynamic cooling. At the
same time, extremely strong winds reaching 60-70kts in regional
soundings may fracture dendrites, somewhat limiting SLR and hence
accumulations. Still, think the intense ascent should support
1-2/hr snowfall rates at least through Sunday aftn, and WPC
probabilities have increased to 50-80% for 6+ inches, with locally
more than 12 inches probable near the WY/NE/SD borders before
things wane slowly on D2.

The probability of significant icing of at least 0.25 inches is
less than 10 percent.

Weiss