Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
453 PM EDT Sun Apr 12 2020

Valid 00Z Mon Apr 13 2020 - 00Z Thu Apr 16 2020


...Upper MS Valley to the Upper Great Lakes...
Days 1-2 ...
A rapidly deepening surface low is forecast to track from the
lower Ohio valley to the Great Lakes tonight into early Monday,
and then across the border into Canada Monday.  Northwest of the
developing low, strong upper jet forcing in combination with
low-to-mid level frontogenesis will likely support a stripe of
heavy snows across northern WI and the Upper Peninsula (UP) of
Michigan. Several members of the daytime HREF guidance indicated
potential for heavy banded snow, producing snow amounts of 1-3
inches per hour in these areas.  With the bands already in place
in southeast MN, confidence is better than usual for more bands to
develop downstream into WI and the UP of MI.
With the synoptic system, departing Monday, the event turns into a
lake enhanced/lake effect tomorrow into early Tuesday before
ending.  The heaviest amounts are expected over the western U.P.
of Michigan, where 1-2 feet of snow is still expected. Greater
weighting was given to the WRF runs that have handled the bands
that developed northwest of the axis depicted in the global
models, so near the WRF ARW2 and HREF Mean snowfall.

...CO Rockies to High Plains...
Days 1-2 ...
Confluent low level flow and moisture pooling up along the high
Plains, foothills, and front range combine with sporadic upper
divergence maxima in the west-east jet maxima cross CO to produce
periods of light to moderate snow on both Day 1 and Day 2. Several
inches of snow are expected along the Front Range, with lighter
amounts farther east across the High Plains.  Heaviest
accumulations are expected to occur on Sunday, with some lighter
amounts continuing into Monday.  WPC PWPF shows high probabilities
for storm total amounts of 12 inches or more along the Front Range
for Days 1-2. The departure of the 700 mb wave out of CO leads to
drier air aloft moving across the state and the snow tapering.

...Southwest to the Southern High Plains...
Days 1-2 ...
Mid level energy and upper divergence maxima in the difluent flow
aloft move across the southwest, with enhanced moisture and lift
at 700 mb supporting a stripe of organized precipitation,
including high elevation snows, across northern Arizona and New
Mexico tonight and Monday.  The models show a narrow band of heavy
amounts possible from portions of the AZ Mogollon Rim across the
mountains of central NM, including on either side of Albuquerque,
and along the NM Sangre de Cristo Mountains.  WPC PWPF is
indicating that local amounts of 8 inches or more are likely. 

As the 700 mb wave over southeast CO develops, better lift moves
east of the Rockies across the high plains of OK/TX. Cold air in
place over the southern High Plains will support accumulating
snows over northeastern New Mexico and into the Texas and Oklahoma
panhandles and southwestern Kansas late Monday night through
Tuesday morning.  The threat ends as the wave progresses steadily
east into warmer air in the more eastern Plains.  Light ascent
keeps amounts modest western TX/OK.

Northern to Central Rockies Day 3...
The models show a 700 mb wave developing in MT and ID and
progressing south down into WY on Wed. Confluent flow pools
moisture coming down from Canada into northern ID/MT Tue
afternoon, with low level relative humidity increasing to near 90
percent Tue evening.  Embedded areas of 700 mb lift within the
moisture plume progress across ID/western MT and then down into
Wyoming.  Several inches of snow are expected within these area,
with amounts enhanced where flow turns upslope in the ranges of
western to central MT, Bitterroots of ID, and Absarokas of
southern MT/adjacent northwest WY.
The models agree better than average on the timing and amplitude
of the wave, so no particular preference was given.

Eastern West Virginia to the VA Border/western MD Day3...
Multiple models/ensembles, including the ECMWF/UKMET/Canadian
global/GFS, show a region of confluent 70 mb flow crossing
steadily northeast across WV and western MD/western VA Tue night. 
It is cold enough in the mountains of eastern WV that a period of
snow is expected, with several inches likely if these models are
correct.  The wave continues downstream into lower elevations of
western WV to central MD but the models show initial temps too
warm for snow, limiting duration of snow and thus snow amounts. 
The departure of the wave to the east coast Wed afternoon should
cause the event to end.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen