Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 PM EDT Tue Oct 29 2019

Valid 00Z Wed Oct 30 2019 - 00Z Sat Nov 02 2019

...Central Rockies through the Great Lakes...

Days 1-3...

Shortwave digging through the Four Corners tonight will
sharpen/close off over CO during day 1 producing weak cyclogenesis
dropping into New Mexico. The combination of height falls,
intensifying jet level diffluence, upslope flow as high pressure
ridges southward, and moist isentropic ascent downstream of the
trough axis will produce additional heavy snow in the San Juans
and eastward into the High Plains of CO/KS, with lesser amounts as
far south as the TX Panhandle. The heaviest snow is likely in the
high terrain where WPC probabilities are moderate for 8 inches,
with moderate probabilities for 4 inches spreading eastward into
western KS and the OK Panhandle.

As the upper trough opens and the surface low drops southward, a
brief break in heavy snow is forecast across eastern KS into NW
MO, before another stripe of heavy snow likely develops during D2.
This second swath of snow is forecast from NE MO through northern
MI, and will develop in response to renewed deepening of the upper
trough due to phasing of energy aloft. The GFS remains a
strong/deep outlier and is mostly ignored for this forecast,
although a little weight was included of the GEFS mean, along with
the ECMWF, ECENSmean, and NAM. As energy phases and renewed
surface cyclogenesis occurs, cold air will flood southeastward
turning rain to snow. While some mixing is possible, the
predominant p-type is expected to be rain to snow. This suggests
rates will be extremely important to snowfall amounts, and robust
700mb deformation combined with a stripe of 850-600mb modest fgen
pivoting across NW IL implies heavy snowfall rates despite a
relatively weak signature for CSI. Still, a prolonged period of
precipitation, much of which could be snow, and at least modestly
heavy snow rates at times is likely. WPC probabilities are as high
as 50% for 4" centered in NW IL on D2 and into the northern
portion of the L.P. of Michigan on day 3. Any banding or heavier
snow rates could produce isolated amounts several inches higher
than this, with the best chance for that appearing to be from the
Quad Cities northeast into Southern Wisconsin.


...Texas Panhandle...

Day 1...

Trough digging through the Four Corners will interact with a cold
front to produce warm isentropic ascent atop a cooling low-level
airmass. While the GFS is the most aggressive with freezing rain
potential, low-level thermal profiles suggest a short duration
potential of freezing rain as the surface cools below freezing but
a warm nose persists aloft. Enough ascent during this time may be
enough to squeeze out some light freezing rain accretion, but
confidence is low. The duration should be limited enough to keep
maximum accretions light, and WPC probabilities are 10-20% for 0.1
inches.

Weiss