Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
424 PM EST Sat Mar 13 2021

Valid 00Z Sun Mar 14 2021 - 00Z Wed Mar 17 2021

...Southwest to the Central Rockies and High Plains...
Days 1-2...
A major snow storm is expected across portions of the central
Rockies and the central High Plains tonight through early Monday. 
This includes the I-25 corridor from Cheyenne to Denver. Snow
amounts near 3 feet are expected in the ranges on northern CO and
3-4 feet in the Laramie Mountains of southeast WY.

A deep upper low over the four corners region this evening is
expected to continue moving east northeast tonight to near the
CO/NM border Sun morning , then turning northeast  across eastern
CO tomorrow afternoon.  A well defined upper jet wraps around the
low and moves east to west across northern CO and southern
WY,accompanied by 300 mb divergence maxima.  The anomalously
strong easterly flow tonight support strong upslope flow coming
into the mountains of southeast WY.  The combination of synoptic
lift and orographic lift is expected to result in heavy snowfall
rates tonight and Sunday, with potential for record setting snow
in southeast Wyoming.
For the Day 1 period, WPC PWPF continues to shows high
probabilities of a foot of snow across the Wind River Mountains,
Casper Arch, and then most of southeast WY to the NE border, and
the Fort Collins/Denver corridor. A secondary area of high
probabilities continues over the Uinta Mountains in northern Utah
and southwestern Wyoming.  The primary uncertainty is
precipitation type as precip now rain in northeast CO and adjacent
NE gradually changes over to snow, with several inches expected by
Sunday evening.
Additional 2 to 4 inch amounts occur as moisture continues to wrap
around the circulation in northeast CO tomorrow evening,
continuing under the upper jet in western NE and southwest SD
tomorrow evening before the upper low and jet depart later
tomorrow night.

...Upper MS Valley to the Upper Midwest/Upper Great Lakes/Central
Appalachians...

Day 2...
As the deep low centered over eastern Colorado Sunday evening
begins to lift farther northeast, precipitation will shift
northeast across eastern NE and SD, continuing across southern MN
and Iowa.  Lowering heights results in precip changing over to all
snow.
The confluent low-mid level flow results in 850-700 mb theta-e
advection and convergence maxima, which supports liquid equivalent
amounts of half to one inch. Heavy snow is possible where the
longer duration snow is forecast across southern MN to norther
central IA.

The other uncertainty regard the models gradually tapering QPF and
resultant snow amounts as you go across WI as the 850-700 mb
theta-e advection gradually weakens as Mon progresses.  Models
have been trending cooler and wetter, with higher probabilities
for accumulations of 4-inches or more across southeastern South
Dakota, southern Minnesota and northern Iowa. Probabilities
decrease the further south across NE/IA you go, due to shorter
duration snow that is forecast.

The models have indicated that the mid level warm advection
downstream in northern IL and IN should result in precip when sfc
temps are still at or below freezing.  With warming aloft
occurring first, light freezing rain appears likely across much of
northern IL and IN.  Several models forecast near a tenth of an
inch before the change over to rain.  A low risk of a quarter inch
is shown as a small minority of ensemble members forecast freezing
rain amounts that high. Light icing is possible on Day 3 from
northern IN across southern lower MI, and in the mountains of
western PA to western MD, and straddling the WV/VA border.

...Oregon and California Day 2 and Southern Great Basin Day 3...

The next approaching upper level trough results in the forward jet
maxima arriving in western OR and CA late tomorrow
afternoon/evening.  Increasing moisture and advection and
convergence results in snow developing late tomorrow
afternoon/evening and continuing overnight, starting in the ranges
of southwest OR and northwest CA and spreading inland as Sun night
progresses.

With lowering heights, temperatures aloft cool, so snow levels
fall in OR/northern CA Monday. This will support lower elevation
snows across southeastern Oregon into northwestern California with
significant high elevation snows possible from the Klamath
Mountains and southern Cascades to the Sierra.  The models have
the highest QPF in the form of snow in the northern CA Sierra
Nevada, so the highest probabilities for heavy snow are there,
supported by amounts of a foot in the NAM, GFS, and ECMWF.

As the upper trough/embedded closed low move southeast tomorrow
night to Tue morning, snows taper in CA/OR and develop in the
ranges of southern NV Tue, and continue on to the ranges of
southern UT as the upper low crosses Tue during the day. Several
inches of snow are expected in these areas.

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

Petersen