Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 PM EST Sat Nov 21 2020

Valid 00Z Sun Nov 22 2020 - 00Z Wed Nov 25 2020


...Pacific Northwest/Northern Rockies...
Days 2-3...

A short wavelength flow will persist through early next week with
periods of shortwave ridging and troughing moving across the
Pacific Northwest. After ridging D1, a fast moving trough will dig
across the region Monday accompanied by brief but robust upper
diffluence in the LFQ of a 130kt Pacific jet streak. The
combination of height falls and this upper ventilation will allow
a wave of surface low pressure and associated cold front to move
from the Pacific across the Northwest and into the Northern
Rockies before dissipating. Ascent through these features will
cause moderate to heavy snow along the WA and OR Cascades,
spreading inland to ID and MT. WPC probabilities for 4 inches are
moderate, highest in the WA Cascades. After brief ridging again
late D2 into D3, yet another shortwave and accompanying jet streak
will lift onshore bringing additional moderate snow to the WA
Cascades on D3.


...Central Rockies...
Days 2-3...

Zonally oriented jet streak Monday will begin to intensify and
sharpen northward in response to a longwave trough amplifying and
digging through the Great Basin into the Four Corners. Embedded
weak mid-level impulses moving through the flow on Monday will
create period of moderate to heavy snow, especially in the San
Juans which will be more orthogonal to the flow to drive
significant upslope ascent. WPC probabilities on D2 are high for 8
inches in the San Juans, with locally more than 12" likely. By D3
/Tuesday/ the poleward arcing jet streak will produce a more
robust LFQ diffluent region which will interact with the
sharpening trough to drive low pressure development in the lee of
the Rockies. Ascent in the vicinity of this low will enhance
already robust deep synoptic lift to produce more widespread snow
across much of the CO Rockies. Amounts on Tuesday will be somewhat
muted compared to D2, but WPC probabilities are high for 4 inches
in the San Juans, with lower probabilities stretching into the CO
Rockies, Front Range, and Sangre De Cristos.


...Northeast...
Days 2-3...

A warm front ahead of a wave of low pressure lifting northeast
from Pennsylvania to Maine through Monday night spreading
precipitation across New York and New England. Initially, this
warm front and associated WAA will spawn an fgen band which will
cause a period of snow as it lifts northward. However,
accumulations are likely to be light. Strong WAA following this
front will quickly drive a warm nose northward to cause a p-type
changeover from snow to sleet to freezing rain, and then rain.
Cold high pressure to the north at onset may lock in at the
surface for a long enough duration that up to 0.1" of freezing
rain is possible, and WPC probabilities for this amount are
highest in the Adirondacks. Otherwise, expect the WAA to overwhelm
the column and cause primarily a rain event. Further north as the
low pulls into the Canadian Maritimes, stronger ascent and a
colder column should produce periods of light to moderate snow in
northern New England, especially Maine. WPC probabilities for 4
inches are as high as 20% across the terrain of these areas.


...Upper Midwest/Great Lakes...
Days 1-3...

On Sunday, a wave of low pressure will deepen as it lifts
northeast from Indiana into Pennsylvania. This low will deepen in
response to mid-level shortwave becoming negatively tilted beneath
a coupled jet structure. This deep layer ascent will allow the low
to intensify. As this occurs, a secondary shortwave moving across
MN will interact, causing a band of deformation to lay atop some
enhanced fgen on the NW side of the surface low. High-res guidance
this aftn has converged on a signal for a changeover from rain to
snow as dynamic cooling occurs through this intense mesoscale
ascent. While uncertainty continues into placement and amounts due
to the rain changing to snow scenario, WPC probabilities have
increased for accumulating snow, with a greater than 50% chance
for 2 inches, and a narrow window of 10% for 4 inches, highest in
SE Michigan.

After a break on D2, a larger scale system will develop across the
High Plains on Tuesday and lift northeast in response to the
poleward extending and intensifying jet streak downwind of a
longwave trough advecting eastward. A divergence maxima within the
LFQ will colocate with maximum height falls to intensify the
surface low as it lifts northeast into the Upper Midwest. Robust
WAA downstream of this system will spread moisture northward,
which will initially fall as snow. However, a LLJ of 40-50 kts
will drive strong fgen to cause a p-type changeover to rain across
most locations. However, strong WAA into the cold antecedent
column combined with the potential for some ageostrophic cooling
beneath the jet streak should allow for a burst of moderate to
heavy snow to accumulate several inches in WI and MN. WPC
probabilities for D3 are moderate for 4 inches in northern WI and
the Arrowhead of MN.



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

Weiss