Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
321 PM EST Fri Feb 21 2020

Valid 00Z Sat Feb 22 2020 - 00Z Tue Feb 25 2020


...Southern Great Basin through Southern Plains..
Days 1-3...

An upper low will lift onshore Southern California Saturday and
then continue progressively to the east while slowly weakening
over the Missouri Valley on Monday. On days 1-2, the core of the
subtropical jet streak racing across the southern tier of the
country will enhance ascent through upper level diffluence in the
southern Great Basin and into the central Rockies. This synoptic
lift will wring out moisture which will be enhanced within the
column via robust mid-level moist advection on tropically-sourced
southerly flow ahead of the mid-level impulse. PWATs become quite
anomalous, forecast by the GFS to reach +3-+4 standard deviations
above the climo mean. Although snow levels will initially be quite
high, 7000-9000 ft, they will crash as the upper low moves
eastward. Still, the heaviest snow through D2 will be confined
above 7000 ft. On Saturday, WPC probabilities for 6 inches are
moderate in the Sierra and southern Wasatch. On Sunday /D2/ snow
becomes more significant and widespread, with WPC probabilities
indicating a high risk for 6 inches across most of the terrain of
the Four Corners, with snow accumulations potentially reaching 18"
in some of the higher terrain of the Wasatch and San Juans.

The forecast snow becomes more challenging D3 as less cyclogenesis
occurs in CO and then sinks E/SE into the Southern Plains. There
remains considerably latitudinal spread with the position of this
feature, and the column in the vicinity of this low is marginally
cold enough for any wintry precipitation. Favoring a GEFS/ECMWF
blend solution, most of the precipitation should be rain except in
the CAA behind the low across eastern CO and NW KS where a few
inches of snowfall are possible. Further northeast, a stripe of
light snow or sleet/freezing rain is possible from central KS
northeast into northern IL.


...Pacific Northwest...
Days 2-3...

Surface low pressure will move onshore British Columbia Saturday
night/Sunday beneath a potent upper shortwave digging southwest
from the Gulf of Alaska. This feature will come onshore
accompanied by the LFQ of a Pacific Jet streak, and mid-level
moisture convergence pushing PWATs to +0.5 standard deviations
above the climo mean. As these features move into Washington
State, precipitation will spread west to east, impinging
orthogonally into the Olympics and Cascades on D2, and then diving
further SE into the Northern and Central Rockies on D3. The
longest duration of intense forcing and best moisture will occur
in the Washington Cascades where WPC probabilities for 12 inches
are moderate on D2 and D3, and total snowfall could exceed 3 feet.
Heavy snow will spread into the ranges of western MT, ID, and NW
WY during Monday /D3/ and WPC probabilities are high for 6 inches
with isolated amounts over 12 inches possible.

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

Weiss