Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
450 PM EST Wed Mar 10 2021

Valid 00Z Thu Mar 11 2021 - 00Z Sun Mar 14 2021

...Upper MS Valley through the upper Great Lakes...
Day 1...

Low pressure will continue to develop as it tracks northeast from
southern Minnesota to Lake Superior by Thursday morning and
produce a swath of moderate to heavy snow from near the SD/ND/MN
border to northeastern MN. Strong low/mid level moisture
transport/convergence allows a corridor of strong lift on the cool
side of the system...which will help dynamically cool the column,
resulting in a transition to a heavy wet snow in west central to
northeast MN.  This snowfall should accumulate 4-8" of snow from
northeastern SD to northeastern MN. the probabilities show about a
30 percent chance of 8 inches of snow in northeast MN northwest of
Lake Superior, as precip may start as mixed (not all snow) along
the shoreline. The event winds down Thu afternoon and the low and
associated ascent move northeast into Canada.

Accumulations should struggle closer to the low track, where the
rain/snow line exists...due to marginal BL temps and a rain/snow
mix or heavy, wet snow with low snow to liquid ratio snowfall in
the MN arrowhead. 

An area of icing is expected in northern WI and into the western
UP of MI where overrunning with sufficient surface cold to
freezing supports a stripe of icing near a tenth of an inch. There
is about a 30-40 percent chance of a quarter inch of ice in
northern WI close to Lake Superior.  The steady rainfall rates may
lead to runoff, so the heavy icing threat seems low.


...Southwestern U.S/Southern Great Basin/Central to Southern
Rockies and Central High Plains....
Days 1-3...

A closed 700 mb low centered over northern CA near coast drifts
east, reforming over Nevada on Thu. Areas of snow will continue to
occur in higher elevations of central/southern CA as Pacific
moisture is adverted inland over terrain of the Sierra Nevada and
southern CA ranges. As the low moves inland Thu., snow coverage
and rates taper as ascent dwindles in CA. Most of the accumulation
in the Sierra Nevada and southern CA ranges will be above 3500
feet.

Moisture will spread eastward across the Great Basin and Desert SW
to the Rockies tonight through Friday. 700 mb convergence maxima
cross the ranges of central to northern NV, and then the UT
Wasatch, resulting in several inches of snow. Day 1 snow
probabilities are moderately high for 6 or more inches over
central/eastern NV ranges, northern AZ and the Wasatch.

On Thu night-Fri, the elongated circulation continues to move
east, with bands of 700 mb coverage and ascent both near the
circulation and also downstream from the circulation as the cold
front moves east from UT into CO and WY. As the warm front moves
across the front range,   warm advection and frontogenesis results
in snow developing over central CO, with marginal temperatures in
valley areas in the I25 corridor
resulting in heavier amounts in the foothills and mountains/front
range of CO to southeast WY.

On Friday night through Saturday, the 700 mb low redevelops in
southeast CO according to the NAM/Canadian global/ECMWF/GFS.  The
leads to both strong low level convergence near the low and also
wrap around strong/moisture advection extending across northeast
CO into southeast WY.
The wraparound jet leads to strong upslope flow in northeast CO to
southeast WY, where 300 mb divergence maxima increase in
magnitude, supporting strong ascent. Consequently, confidence is
increasing in heavy snow developing in these areas, and continuing
beyond this period.
Probabilities are high for 4 inches and moderate for 8 inches on
Day 3 in the front range of CO to southeast WY. 

The slow motion of the low looks likely to result in a major snow
storm for eastern slopes of the northeast CO/southeast WY
mountains,
with snowfall expected to be 2-4 feet when added over a multi-day
period. The primary uncertainty will be the duration of snow on
the Plains as the east movement of the low results in cooling and
transition to snow on the high plains of CO/WY/western NE.


Petersen