Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
315 PM EST Mon Feb 14 2022

Valid 00Z Tue Feb 15 2022 - 00Z Fri Feb 18 2022


...Pacific Northwest through the Rockies...
Days 1-3...

A potent closed low dropping out of the Gulf of Alaska will move
into the Pacific Northwest Tuesday morning /early D1/ and then
maintain amplitude as it shifts southward along the Pacific Coast
into the southern Great Basin Wednesday morning. This closed low
will then open as it shifts eastward across the Four Corners and
then into the Central Plains D3, but immediately in its wake a
secondary shortwave will race out of Alberta to elongate the
mid-level trough across the Central and Southern Rockies D2 into
D3. This evolution will lead to an amplified, but positively
tilted, mid and upper level trough, through which jet streaks will
intensify to produce a coupled jet structure to intensify ascent
across much of the Intermountain West. This combined with at least
modest upslope enhancement behind a surface cold front, especially
into the Central/Southern Rockies, should produce heavy snowfall,
which will be only limited by modest available column moisture.

On D1, widespread precipitation is likely from the OR Cascades
southward through the Sierra and then through the Great Basin and
into the Northern Rockies. With snow levels generally 5000-6000 ft
during the period of most robust ascent aided by WAA, the heaviest
snow should be confined above these levels, and WPC probabilities
for 6+ inches are above 30% only in the Sierra and higher terrain
of the Absaroka range. By D2, the evolution of the mid-level
features will produce the most robust ascent across Wyoming, with
heavy snowfall likely encompassing much of the state except the
valleys below 4000 ft. WPC probabilities for more than 6 inches
are above 40% in many of the ranges, with locally up to 12"
possible in the Wind River range. By D3 /Wednesday night and
Thursday/ the heaviest snow is expected to shift into the Central
and Southern Rockies including the Front Range, Sangre De Cristos,
and San Juans, where WPC probabilities feature a moderate risk for
6+ inches of snowfall, with local amounts to 10" likely. In the
lower terrain, D3 probabilities for 2 inches reach 50% across the
western High Plains of WY and CO, including the I-25 corridor and
as far south as the White Mountains of AZ.


...Central Plains to Great Lakes...
Day 3...

A closed mid-level low ejecting out of the Four Corners will
gradually open into a negatively tilted trough across the
Central/Southern Plains Thursday before lifting northeast towards
the Great Lakes. As this wave shifts eastward, it will shed a
vorticity lobe back into the Desert Southwest, while a piece of
northern stream energy drops out of Saskatchewan, and this latter
shortwave may interact or phase with the leading piece of the
closed low across the Upper Midwest Thursday night. The net result
of this complex evolution is a longwave, positively tilted, trough
draped from the Great Lakes through the Southwest, with a
pronounced subtropical jet streak arcing across the Southern tier
interacting with a departing jet streak into Canada. Both of these
jets are likely to strengthen, with increased coupling leading to
surface low development followed by at least modest deepening as
the surface low moves from the High Plains of NM into the
Mid-Mississippi Valley. This surface low is likely to move
northeast along a surface cold front, and ahead of this front,
warm and moist advection will become impressive noted by 850mb
winds of 30-50 kts driving PWs to more than +2 standard deviations
above the climo mean. This anomalous moisture acted upon by robust
synoptic ascent should produce a swath of heavy precipitation,
with rain, snow, sleet, and freezing rain all likely.

While guidance continues to feature quite a bit of latitudinal
spread, mostly in response to different intensities and positions
of the upper jet streaks, the guidance in general has trended a
bit stronger and further NW, towards the GFS/NAM camp. This
implies a heavy swath of snowfall from far northern Oklahoma
through eastern KS, northern MO, and then northeast into the Great
Lakes. WPC probabilities for more than 4 inches of snow is above
50% across much of this area, including Kansas City, MO and
Chicago, IL. Within this area of heavy snow, some accumulations of
greater than 8 inches appear likely as mid-level fgen and
deformation overlap within a deepening DGZ to suggest banded
structures, and the WSE 90th% is over 12 inches within this swath.

South of this heavy snow axis, a corridor of sleet and freezing
rain is likely as the warm nose persists even as CAA behind the
front drops surface temperatures below freezing. As with most
mixed p-type events, there remains uncertainty into the depth of
the cold nose and strength of the warm nose, so the demarcation
between IP and ZR is uncertain. However, some models are
predicting more sleet than freezing rain due to the intensity of
the CAA, which would limit freezing rain accretions. Still, WPC
probabilities for 0.1" of freezing rain are as high as 40% in a
small area north of Saint Louis, MO, with a broad swath of more
than 10% extending from southeast KS through the southern L.P of
Michigan, including Detroit.


Weiss