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

Valid 12Z Wed Mar 10 2021 - 12Z Sat Mar 13 2021

...North-central High Plains through northern Minnesota...
Day 1...

Broad low pressure currently over the central High Plains and
pushing downstream of a rather positively tilted trough will
continue to develop as it tracks northeast to Lake Superior by
Thursday morning and produce a swath of moderate to heavy snow
from central WY, along the western Neb/SD border, northeast from
there across SD and and across north-central and northeastern MN.
00Z guidance generally shifted north a bit with the track with
another slight uptick in precip/snow potential. Mild temperatures
ahead of the system, as well as marginal cold on the immediate
north side will help limit snow accumulations. However, the good
dynamics of this system and elongated orientation in the direction
of motion should allow some heavy snow to occur. The trough
develops into a closed low tonight over northern MN, strong
low/mid level moisture transport/convergence, and even some weak
instability within the warm sector of the low 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, particularly tonight over northeast SD and MN.
Accumulations should struggle closer to the low track and near the
rain/snow line...where marginal BL temps and maximum lift below
the DGZ suggests more of a rain/snow mix or less efficient
snowfall. However, a bit further north there should be a narrower
band of moderate to heavy snowfall...where lift is maximized
within the DGZ and BL temps cool to just below freezing. This
snowfall will be more efficient, and should see enough QPF, for a
swath of 4-8" of snow from eastern SD to interior (inland from the
lake) northeastern MN.

An areas of icing is expected in northern WI and into the western
UP of MI where overrunning with sufficient surface cold makes for
a 20 to 45% chance for a tenth inch of ice and around a 10% chance
for a quarter inch. This area is at the north end of the
instability potential and plenty of convergence will allow some
pretty heavy rainfall rates, so with temperatures above freezing
falling to the wet bulb of just below freezing during the
precipitation, the icing threat seems low.


...Western U.S....
Days 1-3...

A closed low centered along the far northern CA coast this morning
will track south roughly along the CA coast through Thursday
before turning inland over southern CA. This low then drifts east,
reaching the Four Corners by Saturday morning. Areas of heavy snow
will continue to occur ahead in CA of this low as Pacific moisture
is sent inland over terrain of the Sierra Nevada and southern CA
ranges. This cold core low will allow snowfall to be showery and
quite heavy at times...with hourly rates of 1-2" expected to
continue through at least this morning. Most of the accumulation
in the Sierra Nevada and southern CA ranges will be above 3500
feet, although some lighter accumulations down to 3000 feet are
expected in the Sierra Nevada.

Moisture will spread eastward across the Great Basin and Desert SW
to the Rockies today into Friday. Since this moisture outpaces the
low center, snow levels will be higher inland, generally 4000 to
5000ft. Day 1.5 snow probabilities are moderately high for 6 or
more inches over central/eastern NV ranges, northern AZ and the
Wasatch and south through the higher terrain of southern UT. 
While troughing east of the southern Rockies redevelops Thursday,
allowing upslope precip to begin over central/eastern CO, marginal
temperatures (snow levels around 5500ft) are expected to keep the
mainly light snow rates from accumulating much in the higher
terrain. However, by Friday night with the low center over
northern AZ and lee-side cyclogenesis beginning over NM bringing
increasing moisture and lift, rates will begin to increase with
snow levels lowering to around 4500ft. The slow motion of the low
looks to maintain a major snow storm for eastern slopes of the
southern Rockies of NM/CO and into WY through the weekend with
high potential for very impactful snowfall and associated wind.
The main model uncertainty seems to be with the quantity of
precipitation with the GFS v15 (current operational version) much
heavier than all other global models. Though it should be noted
that all major global models still have quite a winter storm with
plenty of snow.

Jackson