Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
314 AM EST Thu Jan 21 2021

Valid 12Z Thu Jan 21 2021 - 12Z Sun Jan 24 2021

...Great Lakes...
Days 1-3...

The heaviest lake effect snow on Day 1 is forecast to be in the
Tug Hill of NY, as an initial burst of snow showers from a
combination of warm advection and upslope flow is followed by
winds veering towards the west tonight, supporting long cross-lake
trajectories that maximize moisture fluxes and support snow
developing in windward terrain.
The low level westerly flow persists into the first few hours of
Day 2 Fri., with the primary band likely continuing.
WPC probabilities are high both on D1 for 4/8 inches, with local
totals exceeding 12" possible combined over the two days.

In the upper lakes, the primary lake effect occurs on Day 1 as a
secondary front progresses across the Upper Peninsula of MI and
then northern lower MI tonight. Low level frontal convergence is
followed by the frontal passage and then northwest flow that
provides lee shore convergence and lift along the south shore of
Lake Superior and in the Traverse Bay portion of northwest lower
MI and adjacent terrain.

The cold front driving through the lower lakes then forces more
veering, winds turning northwest, and the band relocation further
south Fri night.
The northwest flow crossing Lake Erie spawns lee shore convergence
areas with several inches of snow at the end of Day 1 through Day
2.

On Day 3 Saturday, low-mid level warm advection in advance of a
synoptic scale wave provides ascent and moisture return, with snow
developing in the western UP of Michigan.
Probabilities are low for heavy snow given the short duration,as
snow is confined to late Sat night.


...Central Rockies to Great Basin and Southwest...
Days 2-3...

A long duration snow is expected over the mountains of western WY
as an upper level west-east jet persists with embedded divergence
maxima aloft.  The column moistens and low-mid level ascent
resulting from convergence maxima continues tonight into Friday.
Up to a foot is possible in the Tetons from this event.

Moisture streams northeastward from the Pacific Ocean along a 300
mb jet maxima. This is likely to produce widespread snow from the
Sierra eastward across the ranges of NV, northern UT and western
CO on Day 2. Vorticity lobes swinging through the flow may enhance
ascent and snowfall at times, and there is likely to be a 36 hour
window of off-and-on snow across the region. WPC probabilities for
6" are high D3 across many of the ranges of Utah including the
Wasatch and Uintas, as well as into the CO Rockies and San Juans.
Local snowfall may exceed 12 inches in the highest peaks.

The models have indicated potential for a 700 mb low to close off
in the southwest Saturday. The 300 mb jet moves across AZ and NM,
with embedded difluent flow supporting ascent in windward terrain
of the Mogollon Rim of AZ, and then the San Juan Mountains of
northern NM and southwest CO.  Locally heavy snow is expected in
windward terrain of these ranges on Day 3. WPC probabilities are
low to moderate for 8 inches Saturday/Saturday night. 


...Northern Plains to Upper Midwest...
Day 3...

The models show an 850-700 mb shortwave moving east across the
northern Plains towards the Upper MS Valley. Confluent flow
produces 700 mb theta-e advection convergence.  The moisture
return combined with warm advection leads t snow developing. 
About a quarter to four tenths of an inch of liquid equivalent QPF
is forecast across portions of eastern SD, southern MN, northern
Iowa, and western Wisconsin. An area of 4-7 inches of snow is
forecast across this region.

While the forcing for ascent is likely to be transient from west
to east, it could be intense, and forecast profiles indicate a
deepening saturated dendritic growth zone to promote snow growth
and high snow to liquid ratios. A period of moderate to at times
heavy snow is becoming more likely Friday night and Saturday.  WPC
probabilities for 4" are as high as 60% across eastern SD into
southern MN .

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

Petersen