Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
347 PM EDT Fri Oct 25 2019

Valid 00Z Sat Oct 26 2019 - 00Z Tue Oct 29 2019


Days 1-3...

...Rockies and Western High Plains...
Mid-level trough will sharpen and amplify while digging down from
Alberta, Canada through the Mountain West into early next week.
The evolution of this system continues to show considerable model
spread, but the current forecast favors a heavy weight of the
ECMWF/ECENS/NAM for their slower and more consistent results. Some
weight on the GEFS and UKMET is also included.

As the trough digs southward and sharpens, two shortwaves will
rotate through the base. Additionally, jet streaks will intensify
both upstream and downstream of the trough axis, placing a
favorable RRQ and subtly coupled jet structure across CO/WY during
the weekend. This enhanced upper ventilation combined with height
falls and PVA will drive surface cyclogenesis as well, with this
low dropping slowly southward into Monday. At the same time, a
cold front diving southward will leave gusty NE low-level winds in
its wake, providing enhanced ascent through a considerable upslope
component.

The setup overall becomes very favorable for a heavy snow event
across the Rockies and High Plains all 3 days. D1 into D2, large
scale synoptic ascent will produce snow in the mountains from the
Northern Rockies near Glacier NP, southward into the Big Horn and
Wind River ranges. The heaviest snow will be confined to the
highest terrain, and WPC probabilities are high for 4 inches in
the Absarokas, Big Horns, and Northern Rockies.

More significant snow is likely D2-D3 from SE WY through much of
eastern CO and into the Panhandle of NE. This heavier snow is in
response to prolonged isentropic moist ascent, combined with
robust synoptic ascent, and intense mesoscale forcing where
700-500mb fgen collocates with a deep DGZ and -EPV to produce
intense snow rates over a prolonged temporal duration. Although
the heaviest snow will again be in the terrain where low-level
upslope flow drives enhancement, strong dynamics will push heavy
snow eastward into the High Plains of WY/CO/NE as well. WPC
probabilities are moderate for 12 inches in the highest terrain of
SE WY and N/Central CO, with high probabilities for 4 inches
extending into the High Plains, including the Pine Ridge of NE
where a local maxima may occur.

Late in the forecast period the mid-level trough begins to eject
eastward, but there remains uncertainty into the speed at which
this occurs. Some light snow is likely to spread northeast into
KS?NE during this period, but attm probabilities are low for 2
inches of accumulation.

For Days 1-3, the probability of significant icing is less than 10
percent.

Weiss