Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
225 PM EDT Tue Oct 31 2023

Valid 00Z Wed Nov 01 2023 - 00Z Sat Nov 04 2023

...Great Lakes and interior Northeast
Day 1...

Potent closed 500mb low will advect rapidly eastward across the
Southern Great Lakes this evening while weakening into an open
wave across New England by Wednesday afternoon. 500mb height
anomalies according to the NAEFS ensemble tables are initially as
low as -3 sigma over the Great Lakes, but fall to around -2 over
New England through the filling of this wave. The movement of this
feature will drive downstream height falls, divergence, and PVA to
drive ascent, overlapped with increasing RRQ diffluence within the
tail of a potent jet streak arcing into the Canadian Maritimes.
This will push a wave of low pressure rapidly eastward, with an
accompanying cold front, and then secondary reinforcing trough,
dropping NW to SE behind it. This will result in areas of lake
effect snow and some elevated snow showers into the higher terrain
of Upstate NY tonight into Wednesday.

Lake Effect snow downstream of Lake Erie and Lake Ontario will
likely peak late tonight into early Wednesday before the best
forcing shifts eastward. The NW flow should drive multi-bands
across the shorter fetches of these lakes, but impressive
instability above 2000 J/kg thanks to lake temperatures still
around 12-15C and 850mb temps falling to around -8C will support
steep lapse rates and impressive inversion depths to support
enhanced precipitation. With the lakes being still so warm, some
mixed rain/snow is possible along the immediate lake shores, but
just inland periods of heavy snow are possible, especially near
the Chautauqua Ridge where the WPC snowband tool is picking up on
the potential for 1-2"/hr rates. LES accumulations downwind of
both Lake Erie and Lake Ontario could exceed 4 inches as reflected
by WPC probabilities reaching 40-60% southeast of Lake Erie,
highest along the Chautauqua Ridge, and around 10% into the Tug
Hill Plateau.

Farther downstream, a combination of the impressive synoptic lift
and increasing upslope flow into the higher terrain of the
Adirondacks and Greens could result in a few inches of wet
snowfall accumulating Wednesday aftn/eve. The timing of this
should limit impacts as noted by just small percentages in the
WSSI-P for minor impacts, but WPC probabilities for 4+ inches
reach as high as 20%, highest in the northern Adirondacks.
However, these accumulations should generally be confined to
elevated and grassy surfaces.


...Northern Rockies...
Day 3...

A shortwave on the leading edge of Pacific moisture (atmospheric
river) and ahead of a modest upper jet streak will drive ascent
through jet level diffluence and downstream divergence/height
falls. Increasing moisture noted by IVT exceeding 2 sigma
according to the NAEFS ensemble tables will impinge into the
Northern Rockies Thursday night, but at the same time will be
accompanied by rising snow levels within the Pacifically sourced
airmass. Snow levels are progged to climb above 8000 ft, which
will limit significant snow accumulations to the higher terrain
around Yellowstone National Park, where WPC probabilities for more
than 4 inches reach 40-60%.


Weiss