Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
306 AM EST Mon Nov 20 2023

Valid 12Z Mon Nov 20 2023 - 12Z Thu Nov 23 2023

...Central Rockies...
Day 1...

Though the upper low has passed out of the region, combination of
trailing vort max around the main circulation and incoming
anticyclonically wave-breaking jet will maintain and enhance
lingering snowfall over eastern CO into northern NM today before
tapering off tonight. Favored areas will be around the Palmer
Divide and the Raton Mesa on generally northerly flow. WPC
probabilities for an additional 4 inches of snow are low-moderate
(10-70%).


...Northeast...
Day 2-3...

Miller-B type evolution over the Mid-Atlantic into the Northeast
Tue-Thu as the upper low over the Plains weakens and lifts
northeastward in a positively-tilted orientation. The main surface
low will track through the Ohio Valley into the eastern Great
Lakes before weakening/dissipating as its triple point low over
the Mid-Atlantic takes over near the DelMarVa early Wed and tracks
northeastward over southeast Mass into the Gulf of Maine by early
Thursday. Cold surface high over northern New England on Tuesday
will bring in the cold air, but its movement due east will mean a
return flow from the south as the precipitation shield advances
northward and eastward, favoring a changeover from snow to a
mixture to rain in areas as far north as central New England,
which may hold on long enough before the coastal low starts to
wrap in more northerly to northeasterly flow. Farther west, mild
air will surge northward west of the Appalachians and keep nearly
all precipitation as rain. In between, from the central
Appalachians northward to the North Country of NYS,
snow/sleet/freezing rain are likely at precipitation onset before
a changeover from south to north at least through PA into the
Southern Tier and Southeastern NY as well as into New England
(again, depending on the retreat of colder air and strength of
incoming warm air in relation to the surface low track).

From south to north, freezing rain may be the bigger impact over
far western MD into the Highlands over central PA early Tuesday,
with sleet mixing in as well. WPC probabilities for at least 0.25"
ice are low -- around 10% -- over western MD across into PA, but
are around 30-50% for at least 0.10" in a north-south axis from
eastern WV northward into north central PA. Some icing potential
will stretch farther northeastward into NY as well. However, there
the cold air will be deeper and snow is favored more especially
from the eastern Adirondacks across the Green and White Mountains
into northern Maine, which will have the longest residence time
with sub-freezing temperatures and may see the highest
precipitation totals. WPC probabilities of at least 4 inches in
the day 2-3 time range are moderate to high (40-80%) especially
over northern NH and far interior Maine (central Highlands).
Impacts may be greatest due to snow load with snow-to-liquid
ratios fairly low (8-10:1) due to marginal thermal profiles.


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

Mid-level height falls will dip into Montana on day 3, with a
surface cold front dropping southward into Wyoming late Wed into
early Thu. Passage of the RRQ of the west-to-east upper jet will
promote broader lift while the slower moving part of the front
just east of the Divide will provide for a region of surface
convergence. Precipitable water values will briefly rise to above
normal (about +1 sigma) over western Montana as much colder air
eventually moves into/through the region, supporting widespread
snow in the terrain through the end of day 3. Snow will continue
into day 4, but through 12Z Thursday WPC probabilities of at least
4 inches of snow are low (10-30%) around Glacier NP.


Fracasso