Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
245 PM EST Tue Nov 07 2023

Valid 00Z Wed Nov 08 2023 - 00Z Sat Nov 11 2023

...Northern & Central Rockies...
Days 1-2...

Periods of mountain snow will continue through Tuesday night in
parts of the Bitterroots, the Lewis Range, and as far east as the
Big Snowy Mountains of central MT. WPC probabilities depict
moderate chances (40-60%) for snowfall totals >4" tonight and into
early Wednesday morning. Snow will taper off by midday as high
pressure builds in from the west. Meanwhile, a positively tilted
upper trough axis over the Intermountain West and the right
entrance region of a 150kt 250mb jet streak over the Central
Plains will foster strong synoptic-scale ascent at the mid-upper
levels of the atmosphere over the Central and Southern Rockies
tonight and into Wednesday. A cold front surging south through the
High Plains on Wednesday will result in strengthening 850mb N-NE
flow, providing some modest upslope ascent into the Front Range of
the Rockies of CO/NM. In addition, there are also modest amounts
of 700mb moisture flux overhead on Wednesday and lingering into
Wednesday night. WPC probabilities show moderate chances (40-60%)
of snowfall accumulations topping 6", but note that some of the
tallest peaks (>10,000ft) along the Front Range have an outside
chance (10-30%) for >12" of snowfall.

...Northern Plains, Northern Great Lakes, Northeast...
Days 1-3...

Several upper level disturbances embedded within an upper trough
over south-central Canada on Wednesday will deepen and become a
closed low over southern Ontario late Tuesday night. A warm front
as 850mb will track east across the Upper MS Valley tonight and
into the northern Great Lakes early Wednesday morning. Meanwhile,
a sufficiently cold air-mass anchored by high pressure over
Ontario lays the ground work for a classic overrunning setup
featuring sub-freezing surface temperatures and a burgeoning warm
nose of >0C temps within the 850-750mb layer. This will result in
an icy wintry mix near the WI/MI border where WPC probabilities
sport moderate chances (30-50%) of ice accumulations >0.1". Ice
accumulations will be light enough to only warrant a general
Winter Weather Area on the WSSI, but some patchy ice on trees, car
tops, and untreated surfaces cannot be ruled out tonight and into
Wednesday morning.

A similar setup looks to transpire in the Northeast Wednesday
night with cold high pressure to the north and a new developing
low pressure center tracking from the Lower Great Lakes to the
Southern Tier of NY. Before high pressure builds in Wednesday
morning, strong 850mb CAA on the western flank of a strengthening
cyclone north of Nova Scotia provides an unusually cold and
exceptionally dry air-mass to the Northeast. To the north and east
of the approaching warm front, an expanding warm nose of >0C temps
will result in a combination of freezing rain and sleet from the
Catskills and Adirondacks to the Berkshires, Green, and White
Mountains. WPC probabilities generally show low-moderate
probabilities (30-60% chance) for ice accumulations >0.1" with the
Adirondacks on the higher end of that range. It is worth noting
that recent colder trends in CAMs means there is a scenario for
slightly less icing and slightly higher snow/sleet accumulations
in these ranges. Latest WSSI does include some Minor Impacts in
portions of these ranges with Ice Accumulation being the primary
driver. Minor impacts suggest some hazardous driving conditions
are possible due to icy conditions on some roads. There remain
some differences in CAMs regarding the strengthen and depth of the
surface-low level sub-freezing temps, but most guidance supports
some light icing (<0.1") in portions of the Catskills, the Upper
Hudson Valley, and as far east as the Merrimack Valley and
Worcester Hills. Farther north, snow will be the primary
precipitation type but WPC probabilities suggest low chances (up
to 5% at most) for snowfall accumulations >4" in far northern
Maine.

...WA Cascades...

The next frontal system to approach the Pacific Northwest will
direct a slug of Pacific moisture at the Cascade Range. This is
depicted by an influx of 850-700mb moisture that initially arrives
within S-SW flow, but in wake of the frontal passage, the mean
wind direction will switch to out of the west. This should lead to
additional snowfall thanks to enhanced upslope flow. WPC
probabilities show low-moderate chances (30-50%) for >6" of
snowfall in the WA Cascades with the bulk of the snow arriving
Thursday night and concluding by Friday morning. The latest
Probabilistic WSSI (WSSI-P) shows low chances (20-30%) for Minor
Impacts in the Cascades north of I-90. Some measurable snow is
also possible in the Olympics, but WPC probabilities show low
chances (10-20%) for totals >4" for its heights peaks.

Mullinax