Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
239 PM EDT Thu Apr 13 2023

Valid 00Z Fri Apr 14 2023 - 00Z Mon Apr 17 2023


...Central and Northern Rockies into Northern and Central Plains...
Days 1-2...

Broad longwave trough digging across the inter-mountain West will
sharpen into Saturday as a lead shortwave ejects into Manitoba,
Canada, while a secondary shortwave drops towards Colorado and
then takes on a negative tilt into the Central Plains by Saturday
night. This mid-level evolution will be paired with favorable jet
streak positioning as a lead jet streak pivots into Ontario
leaving favorable RRQ atop the lead shortwave, with a secondary
jet streak pivoting around the base of the amplifying trough to
place LFQ diffluence across the Central Rockies Saturday. At the
surface, a wavering frontal boundary will sag across the High
Plains and into the Central Plains, with the resultant baroclinic
gradient being acted upon by the synoptic ascent aloft leading to
surface lows moving northeastward through Saturday. The first of
these will lift across the Northern Plains Friday, with a second
low developing in the lee of the CO Rockies and then pushing
northeast into the Upper Midwest. While neither of these lows are
expected to be exceptionally strong, both should result in areas
of heavy snow.

The lead low moving into the Northern Plains will continue a swath
of heavy snow from eastern ID through northeast MT D1, where an
axis of deformation and robust moist advection advecting
northwestward overlap. The heaviest snow should be confined to the
terrain above around 4000-5000 ft where the thermal structure is
more supportive to heavy snow, but some moderate accumulations are
possible as far east as the NW corner of ND where a band of fgen
aided by the upper level jet streak position could bring snow
rates of around 1"/hr. WPC probabilities for more than 4 inches of
snow are modest D1, generally as high as 20-30% and focused in the
Absarokas and NW WY ranges, with an additional local maxima in far
northeast MT. The secondary low pressure developing in the lee of
the Rockies Friday into Saturday will help enhance ascent into the
terrain of CO, including the front range, as moist air wraps
cyclonically and upslopes into the terrain behind the associated
cold front. Snow levels here will be around 6000-7000 ft, but will
drop rapidly into Saturday which could allow at least light
accumulations to spread into the I-25 urban corridor and Palmer
Divide. However, WPC probabilities for more than 6 inches both D1
and D2 are above 50% only in the higher terrain of the CO Rockies
and into the Front Range where locally 1-2 feet of snow is
possible. A few inches of snow is possible across the Palmer
Divide and Cheyenne Ridge D2, with less than 1 inch expected in
the Denver metro area.


...Upper Midwest...
Day 3...

Mid-level trough closing off over the Central Plains will take on
an impressive negative tilt as it advects eastward on Sunday. This
will help promote poleward acceleration of the upper jet streak,
leaving increasingly impressive upper diffluence atop the region.
The overlap of these features with a low-level front will cause a
surface low to track northeastward while deepening, potentially
rapidly, expanding a shield of precipitation into the Great Lakes
and Upper Midwest Saturday night into Sunday. While this will
initially be all rain due to a warm column, slow cooling as the
low and front push eastward, combined with dynamic cooling within
an intensifying deformation axis characterized by theta-e lapse
rates of near or below 0C/km will allow for a changeover from rain
to snow. The timing and placement of when this will occur is still
very uncertain, but it appears the threat for accumulating snow
within this band is increasing, even during the day on Sunday.
There is bust potential on both the high end (if snow changes over
more quickly) or the low end (should dynamic cooling be less
intense), but current WPC probabilities are 10-30% for 2 inches
for the Arrowhead of MN down to Duluth, and eastward across the
Bayfields Peninsula. Additional heavy snow is possible into D4,
generally farther to the east, as this band progresses.


The probability for icing over 0.1" is less than 5% Days 1-3.

Weiss