Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 AM EDT Tue Apr 30 2019

Valid 12Z Tue Apr 30 2019 - 12Z Fri May 03 2019

...Great Basin, Central Rockies, and Central High Plains...
Days 1-2...

Southern stream shortwave moving across the Four Corners this
morning will shear out to the northeast to be quickly replaced by
a reinforcing northern stream shortwave digging into the Central
Rockies late tonight and Wednesday. Between these two systems a
brief respite from precipitation is possible, but in general snow
will persist from this morning through Wednesday afternoon.

With the trough remaining positively tilted, deep layer SW flow
will advect significant moisture into the region from the Pacific,
enhanced by jet streaks embedded within the flow. Deep layer
ascent through height falls and diffluence within the right
entrance region of a jet streak this morning, left exit late
tonight, will combine with periods of intense 700-600mb
frontogenesis as a frontal boundary lingers around the terrain.
These features together will produce heavy snow, with snow levels
ranging from around 2000 ft in southern Montana, to as high as
7000 ft in Colorado. As the 850-700mb low tracks near the CO/WY
border, enhanced deformation and upslope flow will focus the
heaviest snowfall in the Wasatch, San Juans, and Colorado Rockies,
where WPC probabilities on day 1 are high for 8 inches, with more
than 12 inches possible in the highest terrain. In the ranges to
the north where forcing is slightly less robust, WPC probabilities
are high for 6 inches in the Wind River, Absaroka, and Big Horn
Ranges.

Further to the north and east into the High Plains of WY/NE/SD,
diffluence within the right entrance region of a slowly departing
jet streak this afternoon, and again within the left exit region
of the subsequent jet max tonight, will produce moderate to heavy
snow. Here, forcing is weaker, of shorter duration, and low level
thermal profiles are more marginal, but a few inches of snowfall
is likely. The highest probability for 4 inches or more is
centered in the high terrain of the Black Hills.

By day 2, the trough axis will swing east of the High Plains with
broad ridging developing in its wake. This will bring a slow end
to the snowfall, and WPC probabilities for 4 inches are modest
across many of the same areas, with most of this snow occurring
early on Wednesday.


The probability for icing a tenth inch or more is less than 10
percent all 3 days.


Weiss