Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 AM EDT Thu Apr 27 2023

Valid 12Z Thu Apr 27 2023 - 12Z Sun Apr 30 2023

...Central Rockies...
Days 1-2...

A broad trough across the eastern half of the country will become
reinforced by a potent shortwave digging out of Alberta Thursday
morning, with this impulse dropping nearly due south across the
Rockies, reaching the Four Corners Friday morning. The amplitude
of this wave is progged to maximize late Thursday night into
Friday as it begins to swing southeast into the Southern Plains
around the base of the longwave trough while a secondary, weaker,
impulse lifts across the Upper Midwest. The interaction of these
two features will help drive a cold front rapidly southward across
the Plains and Rockies, with a less progressive shift eastward
behind a wave of low pressure beneath the northern impulse. At the
same time, modest upper diffluence in the diffluent portion of a
southward sinking upper jet will combine with the primary
shortwave to drive low pressure development in the Southern High
Plains, and the overlap of ascent through height falls, upper
diffluence, convergence along the front, and increasing upslope
flow on easterly flow north of the developing low pressure will
result in widespread precipitation, especially in the Central
Rockies. As the cold front drops southward, snow levels should
drop rapidly from around 8000 ft to 4000 ft, and this will cause a
p-type changeover to snow above these elevations, with a lot of
the precipitation occurring in the post-frontal and upslope
regime. However, moisture advecting into the system is modest, and
the system overall is progressive, so any heavy snow should be
confined to the higher elevations of the Front Range and Central
Rockies. The heaviest snow is expected in the higher terrain from
the Big Horns southward through the Wind Rivers, Laramies, into
much of the CO Rockies including the Front Range, and extending
into the Sangre de Cristos. WPC probabilities for more than 4
inches are generally 50-80% across this terrain, with locally more
than 8 inches likely in a few areas. Lighter snows adding up to a
few inches are possible along the Palmer Divide and Raton Mesa,
but the I-25 urban corridor should be spared any significant
accumulations.


...Great Lakes...
Day 3...

A shortwave digging out of Manitoba/Saskatchewan within large
scale troughing across the eastern half of the CONUS will amplify
into a closed low near WI Saturday evening. While there remains
some spread in the placement and timing of this feature, the
guidance is in good agreement that this closed low will pivot
slowly across the region before departing to the northeast late in
the forecast period. The overlap of these height falls with a
modest, but increasingly coupled, jet structure will result in
surface low development near the U.P. of MI, and as features
become vertically stacked during Saturday, this low may
stall/retrograde before ejecting eastward on D4. Overall forcing
appears modest for precipitation, but spokes of vorticity rotating
around the closed low combining with waves of mid-level
deformation will provide periods of enhanced ascent. This should
result in precipitation pivoting around the low, and despite a
marginal thermal structure, some light to moderate snowfall is
likely, especially W/NW of the surface low, and south of Lake
Superior where increasingly northerly flow will provide some lake
enhanced snowfall. Still, only modest accumulations are currently
expected, and WPC probabilities for more than 4 inches peak at
20-30% across some of the higher elevations of the western U.P. of
Michigan.

The probability of significant icing greater than 0.25" is less
than 10 percent.


Weiss