Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 PM EST Wed Feb 19 2020

Valid 00Z Thu Feb 20 2020 - 00Z Sun Feb 23 2020


...Central Plains and Rockies...
Day 1...

Low pressure developing in response to a departing jet streak in
the Lee of the Rockies will spread snowfall into the High Plains
of Colorado and Kansas, with low-level easterly flow supporting
upslope enhancement into the Sangre De Cristos. Most of this snow
should be light as the low-level surface feature drops quickly
southward and the jet streak escapes eastward, bringing an end to
the strong synoptic forcing. WPC probabilities for 4 inches are
low in the SE Plains of CO. Into the terrain, however, the upslope
enhancement will persist as the low drops southward, and WPC
probabilities feature isolated high risk for 6 inches in the
highest terrain.


...Great Lakes...
Day 1...
Strong cold advection behind a cold front will spread NW flow
within the CAA across the Great Lakes tonight into Thursday. While
the duration of this favorable CAA to produce LES will be limited,
it will also be robust, with effective fetch from lake Superior
and Huron aiding with the snowfall downwind of Lake Ontario. A
period of heavy snowfall is likely SE of Lake Ontario, and WPC
probabilities are high for 4 inches, and may locally exceed 8".


...Southern Appalachians and Mid Atlantic...
Days 1-2...
Guidance coming into much better agreement this afternoon in the
evolution of a positively tilted upper trough and surface low
affecting the Southern Appalachians and into NC/VA. Outside of the
NAM, there is good consensus that mid-level WAA ahead of the upper
trough will spread moisture northeast across the area atop a
surface cold front which will sag into South Carolina and off the
coast. At the same time, jet streak energy will merge and
intensify to 170+kts as it exits near New England, leaving the
diffluent RRQ atop the Carolinas, supporting ventilation to lower
pressure and spawn cyclogenesis along the baroclinic zone
offshore. The combination of all these features will produce
widespread precipitation across the area, with cold air advection
slowly transitioning p-type from rain to snow. The low-level
thermal profile will be marginal, and antecedent conditions are
unfavorable to snow accumulations, however, robust forcing,
especially within an intensifying mid-level frontogenetic band
pivoting across NC/VA will overcome these marginal conditions to
produce a period of heavy snow. The most likely locations for
heavy snowfall exceeding 4 inches will be in the terrain of NC/TN,
with a secondary maximum possible in northeast NC where the
longest duration of precipitation falling into the colder air will
occur. Additionally, guidance indicates the potential for a band
of snow to rotate eastward as the low pulls away Friday morning,
which could also enhance snowfall locally. WPC probabilities for 4
inches are around 20% in the mountains, and 30-40% in northeast
NC/far southeast VA.


...Southern Great Basin...
Day 3...

An upper low and associated surface wave will approach the
Southern California coast Friday before moving onshore. Moisture
will spread inland on southerly warm/moist advection ahead of this
feature, but at the same time will drive snow levels to 6000 ft or
higher. Moisture becomes significant as evidenced by PWAT
anomalies reaching +2 to +3 standard deviations above the climo
mean, but modest forcing and the high snow levels confine WPC
probabilities for 6 inches to moderate/high in the Sierra and
southern Wasatch ranges.

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

Weiss