Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
358 AM EDT Mon Apr 29 2019

Valid 12Z Mon Apr 29 2019 - 12Z Thu May 02 2019

...Arrowhead of Minnesota...
Day 1...

Potent closed upper low and associated surface reflection will
push eastward from North Dakota to Lake Superior while dissipating
through day 1. As this feature weakens and moves quickly to the
east, it will be accompanied by enhanced RH and residual synoptic
ascent through the LFQ of an upper jet streak. As the low moves
towards the Lake, E/SE low-level flow will upslope into the Iron
Ranges of the Arrowhead of Minnesota, which combined with WAA will
produce moderate to heavy snow at times during the day. Despite
marginal low-level thermal profiles and late April sun angle, a
long enough duration of enhanced snowfall will create modest
accumulations today. WPC probabilities are high for 4 inches, and
continue to feature a slight risk for 8 inches where snowfall
rates can overcome the otherwise unfavorable profile. Other than
this narrow strip of high probabilities, other portions of
northern Minnesota may see 1-2" of snowfall accumulation.


...Northern and Central Rockies onto the Central High Plains...
Days 1-3...

Two pronged snow event remains progged for the Intermountain West
and into the High Plains. The first of these will begin today as a
closed low off Baja California opens and ejects northeastward
towards the Southern Rockies tonight. Deep SW flow ahead of this
feature will transport copious moisture noted by PWAT anomalies of
+2.5 standard deviations above the climo mean. As column moisture
increases, deep layer ascent will be driven by diffluence within
the RRQ of a jet streak centered over the Dakotas, and increasing
700-600mb frontogenesis as a front banks against the Front Range.
Easterly flow behind this boundary and ahead of a surface low will
serve to both enhance the frontogenesis, but also to drive
orographic enhancement, focused into the San Juans and just west
of the Front Range in Colorado. Above 6000 ft, WPC probabilities
in these areas are high for 12 inches through tonight. Moderate
probabilities for 12 inches also exist in the Wasatch and Uinta of
Utah, although forcing is slightly less robust this far west.
Further east into the High Plains of WY/CO/NE, diffluence within
the jet streak will spread snow northeastward, and WPC
probabilities have increased to show a high risk for 4 inches into
the Black Hills of SD, and as far east as the Panhandle of
Nebraska.

Additionally on day 1, a secondary shortwave digging from British
Columbia will produce snowfall further north into the Northern
Rockies and ranges from the Bitterroots through the Wind Rivers
and Big Horns where WPC probabilities indicate a high chance for 6
inches or more with lowering snow levels.

After a brief respite Tuesday morning, yet one final shortwave
diving out of the Pac NW will be accompanied by another potent
Pacific Jet streak to redevelop snow in much of the same areas hit
hard on Monday. Snow levels fall through Tuesday into Wednesday,
becoming around 4000 ft in Colorado and down to the valleys in
MT/ID/northern WY. Moisture is not as robust with this second
system, but ascent remains strong. Another round of heavy snow is
likely, with WPC probabilities indicating a moderate risk for an
additional 12 inches in the Laramies of WY and the Rockies of CO.
Lower snow levels and continued jet dynamics will push snowfall
well northeast into the High Plains again on day 2, with WPC
probabilities high for 4 inches in the High Plains of WY, the
Panhandle of NE, and into the western portions of SD. By day 3 the
best lift will eject eastward as the shortwave and its associated
jet energy lift away. This will leave only light snowfall with
minor accumulations from Colorado through the Northern Plains on
Wednesday.


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


Weiss