Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
315 AM EST Tue Feb 15 2022

Valid 12Z Tue Feb 15 2022 - 12Z Fri Feb 18 2022


...Pacific Northwest through the Rockies...
Days 1-2...

An impressive, compact closed low currently over the western U.S.
will continue to drop south/southeast toward the Four Corners
region late tonight while immediately in its wake a secondary
shortwave will race out of Alberta to elongate the mid-level
trough across the Central and Southern Rockies today into
tomorrow. 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.

For D1, the focus for greatest snowfall will be across the Sierra
and then through the Great Basin and into the Northern Rockies.
The latest WPC probabilities for 6+ inches are slight to moderate
for the Sierra. By D2, the phasing of the two aforementioned
shortwave energies will lead to greater snowfall widespread
potential for the Central and Southern Rockies including the Front
Range, Sangre De Cristoas, and San Juans where the latest WPC
probabilities are moderate for 6 inches or more. Some of the
highest peaks of the CO Rockies could top 10-12" over the period.


...Central Plains to Great Lakes...
Day 2-3...

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. A northern piece of energy dropping out of
Saskatchewan will track through the northern Plains into the Upper
Mississippi River Valley and the potential phasing of these two
features will help set the stage for a winter storm. Over the last
few model cycles, the guidance has trended toward a stronger,
sharper, and more negatively tilted shortwave while a slower
northern stream shortwave allows for the southern stream to lift a
bit to the northwest compared to the previous cycle. Ahead of the
southern stream shortwave, moisture flow increases with PW
anomalies reaching +1.5 to +2.0 and combined with the robust
forcing for ascent, widespread precipitation with a variety of
winter weather types is expected to develop. The strengthening
upper jet streak coupling will deepen a surface low that develops
along the frontal boundary that then lifts northeast through the
Mid-MS to Ohio Valleys.

Forecast spread has lessened some this cycle but there remains
some latitudinal spread that leads to larger precipitation type
concerns but in general a swath of potentially heavy snow is
increasing likely northwest of the low track from portions of KS
to MI. The latest WPC probabilities for 4 inches are moderate to
high from northern OK through the Great Lakes while the greatest
signal for 8 inches or more lies across eastern KS toward Chicago
and into central Michigan.

South of this heavy snow axis, a corridor of sleet and freezing
rain is likely. As with most mixed p-type events, there remains
uncertainty into the depth of the cold nose and strength of the
warm nose. WPC probabilities for 0.1" of freezing rain are as high
as 40 percent across central/northern Missouri with a broad swath
of more than 10 percent extending from southeast KS through the
southern L.P of Michigan, including Detroit.


Taylor