Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
455 PM EST Mon Nov 25 2019

Valid 00Z Tue Nov 26 2019 - 00Z Fri Nov 29 2019

...Central Plains to Great Lakes...
Days 1-2...
Rapidly deepening low pressure will move northeastward from the
lee of the Rockies in CO, into the U.P. of Michigan. At the same
time, a cold front will dig southward through the Plains and bank
against the mountains, providing enhanced upslope flow into the
terrain. As this low deepens and lifts northeast, a robust 250mb
jet streak will develop across the Southern Plains, reaching above
170kts, and this enhanced LFQ diffluence will combine with height
falls to produce intense synoptic scale ascent. Additionally, as
the surface low deepens and lifts northeast, bands of strong
850-600mb frontogenesis are likely to develop from Nebraska into
South Dakota, Iowa, and Minnesota, with intense deformation likely
across MN as the storm develops a comma head.

Guidance is in very good agreement in the placement of the surface
low, and a subtle NW track shift has been noted this aftn. This
suggests he heaviest snow will occur in a stripe from the Plains
of NE Colorado and SE Wyoming, into NE, eastern SD, NW IA, central
MN, northern WI, and into the U.P. of MI. In this swath, the best
overlap of moisture and dynamics will occur, and WPC probabilities
are high for 6 or more inches. The heaviest snowfall is likely in
the Plains of CO/WY where isolated amounts of well over 12 inches
are possible. Additional lollipops of heavier snow are possible
across central NE where an fgen band may produce snowfall rates to
2"/hr, and additionally in Minnesota/Northern WI where a deepening
DGZ may overlap with intense deformation to again produce snowfall
rates of 1-2"/hr.

...Maine...
Day 3...
The low pressure lifting from the Great Lakes will shift eastward
and redevelop near Upstate New York before moving off the coast of
Maine. Warm advection with the leading warm front will produce
ascent into New England Wednesday night. Much of the area will
have a column that is too warm for snowfall. However, across parts
of Northern Maine, the ascent will occur in a sub-0C column, and
periods of moderate snow are likely. WPC probabilities are high
for 4 inches in the far northern portions of the state.


...Mountain West...
Days 1-3...
An extremely intense surface low pressure will shift onshore the
Oregon coast late Tuesday night before slowing and weakening
across the Great Basin through Thursday. This low will be
accompanied by an anomalously strong upper trough, and intense
Pacific Jet arcing onshore, with repeated pieces of jet energy and
vort lobes rotating around the main trough through the period. On
day 1 /Monday night and Tuesday/ the heaviest snowfall associated
with this system is across the Oregon Cascades and into the
Siskiyous/Shastas of California. In this region, intense ascent
through the synoptics and upslope flow on orthogonal 700mb winds
will drive heavy snowfall, and WPC probabilities are high for 6
inches.

The most significant snowfall will begin on D2 /Tuesday night/ and
persist through the remainder of the forecast period. Significant
moisture advection on increasing SSW winds around the upper low
will reach as high as +1.5 standard deviations with respect to
PWAT. This moisture will be wrung out through the aforementioned
synoptics as well as intense upslope on S/SW 850-700mb flow.
Guidance indicates two periods of the most intense omega, late D1
into D2 across the Sierra, and a subsequent round across the
Southern Coastal Ranges of CA and into AZ/NV/UT D3. WPC
probabilities for 8 inches are above 90% in the Shastas,
Siskiyous, and Sierra of California, and accumulations of over 2
ft are likely in the Sierra. By D3, a surge of moisture and
rapidly lowering snow levels (becoming as low as 1000 ft in
central CA, and as low as 3000 ft in southern CA) will bring heavy
snow from the San Bernadinos northeast through the San Juans and
Uintas, where snowfall will likely again exceed 12 inches. With
the lowering snow levels, lighter accumulations are possible even
onto the desert floors of NV/UT where WPC probabilities show
moderate risks for 1" of snowfall.


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

Weiss