Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
314 PM EST Sat Feb 22 2020

Valid 00Z Sun Feb 23 2020 - 00Z Wed Feb 26 2020


...Southern Great Basin through Southern/Central High Plains...
Days 1-2...

A southern stream closed 700-500mb low will eject eastward tonight
from the coast of CA to near the TX panhandle on D1, and then
weaken into the Missouri Valley as it becomes absorbed by a larger
northern stream closed low towards the end of D2. Moist advection
on southerly 700mb flow will be pronounced into the Four Corners
region, with PWAT anomalies of more than +3 standard deviations
surging northwards into CO/UT. This will occur in conjunction with
the max mid-level divergence ahead of the closed upper low,
driving robust ascent in the moist column. Although snow levels
will be moderately high, heavy snow is likely in the terrain of
the Mogollon Rim, The Wasatch, San Juans, Sangre De Cristos, and
other Colorado Rockies on D1. WPC probabilities are high for 6
inches across these ranges, with snow amounts likely exceeding 12
inches in parts of the Wasatch and San Juans.

Late Sunday into Monday, the associated surface wave developing in
the lee of the Rockies will shift eastward while the upper support
begins to wane. Thermal profiles into the Southern Plains are
marginal, but moisture advection is pronounced so despite
weakening forcing, precipitation will spread northward in the
vicinity of the progressive surface low. Some light to moderate
snowfall is likely on the N/NW periphery of the precipitation
plume, but WPC probabilities for 2 inches on D2 are less than 20
percent outside of the far SE Plains of Co.


...Pacific Northwest/Northern Rockies/Northern High Plains...
Days 1-2...

500mb trough digging from the Gulf of Alaska will race southeast
onshore Washington State early Sunday and then continue southeast
around a large ridge over the Eastern Pacific ocean. One surface
low will advect onshore beneath this feature Sunday before
dissipating across Alberta, while a second low develops across MT
and drops southeast into the Northern Plains Sunday night through
Tuesday. Associated with this shortwave, a plume of Pacific
moisture beneath a 120kt jet streak will advect inland, while
height falls and LFQ jet level diffluence produce ascent. By the
end of day 2, the upper feature will again close off across the
Northern Plains, shutting off the forcing west of the Continental
Divide.

The heaviest snowfall D1-2 is likely in the Olympics and Cascades
where synoptic ascent will be aided by nearly orthogonal low/mid
level to drive upslope forcing. WPC probabilities on both D1 and
D2 /Saturday night through Monday/ feature a high risk for 12
inches in the Cascades, with only subtly lower probabilities in
the Olympics. Total snowfall through the 2 days could eclipse 3
feet in the highest terrain. On day 2 as the jet streak weakens
and pushes southeast beneath the upper shortwave, forcing shifts
east and WPC probabilities for 6 inches become high in the terrain
of northern ID, the Northern Rockies, and into the ranges of NW WY.


...Upper Midwest...
Day 3...

Phasing of northern and southern stream energy will help
strengthen a surface low moving northeast into the Ohio Valley. At
the same time, dual 700mb troughs will likely interact and stretch
NW into the Upper Midwest maintaining a trough for ascent. While
guidance continues to show a strong dichotomy in latitudinal
placement of the surface low, as well as amount of interaction
between the two mid-level waves, there is increasing likelihood in
a swath of heavy snow from Iowa through northern Illinois, WI, and
southern MN. This will be due to enhanced deformation in response
to the aforementioned 700mb trough, as well as the potential for
TROWAL development as an impressive theta-e ridge lifts
northwestward. The blend for this update included heavy weighting
of the ECMWF and its ensemble mean due to its more consistent
run-to-run evolution, with some GEFS and CMC as well to show the
potential for a more northward solution than the operational ECMWF
currently suggests, which will be dependent on the mid-level
pattern. There is potential for significant snowfall across the
upper midwest, and while placement is still uncertain, WPC
probabilities currently indicate a moderate risk for 4 inches.


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

Weiss