Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 PM EST Sun Nov 19 2023

Valid 00Z Mon Nov 20 2023 - 00Z Thu Nov 23 2023

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

A 500-700mb disturbance tracking through the Four Corner region
this afternoon will accompany and influx of Pacific moisture to
keep periods of snow around in the higher terrain of the
Intermountain this afternoon and through tonight. The heaviest
snowfall accumulations are likely to occur in the Wasatch, Uinta,
San Juan, and Sangre De Cristo mountain ranges. Elevations
>8,000ft will have the best odds of picking up heavier snowfall
amounts. WPC probabilities show moderate-to-high chances (60-80%)
for additional snowfall totals >4" in the Uinta, San Juans, and
Sangre De Cristo. This also includes the Medicine Bow and Laramie
Range in southern Wyoming. WSSI shows Minor Impacts are
anticipated in these ranges through tonight. It is worth noting a
combination of moderate-to-heavy snow and gusty winds are possible
in the Front Range of the Colorado Rockies and along the Palmer
Divide. Latest WSSI does suggest Moderate Impacts are possible
along the Palmer Divide primarily due to Blowing Snow. NAEFS shows
850-700mb winds Monday morning that are above the 90th
climatological percentile. The result is wind gusts that could top
40 mph that would aid in causing hazardous reductions in
visibilities on Monday.

...Northeast...
Day 3...

A complex setup at upper levels, the expectation is a storm system
tracking into the Ohio Valley Tuesday morning will direct moisture
northward towards a sufficiently cold air-mass in the Northeast. A
dome of high pressure over New England will setup a cold-air
damming (CAD) signature that extends as far south as the central
Appalachians. Moisture streaming north within an IVT that is >750
kg/m/s over the southern Appalachians (NAEFS shows these values
are above the 97.5 climatological percentile) will coincide within
strong 290K isentropic ascent and WAA within the 850-700mb layer.
In the central Appalachians and northern Mid-Atlantic, the
air-mass is not cold enough and the storm track is too far west
for snow to be the primary p-type. The expectation is for a brief
period of sleet/freezing rain to occur in the Potomac and Laurel
Highlands, the Allegheny Mountains, and as far north as the
Northern Tier of PA Tuesday morning. Latest WPC probabilities show
low chances (20-40%) of ice accumulations topping 0.1". These
minor accumulations could lead to slick spots on some roads and
sidewalks.

Farther north into the Interior Northeast, particularly from the
Poconos, Catskills, Adirondacks and points north and east, the
air-mass is colder and drier, supporting colder wet bulb temps
that can keep boundary layer temps sub-freezing longer from
Tuesday night into Wednesday. In addition, 850-700mb frontogenesis
will be more pronounced and lead to a heavier burst of snow before
any changeover to sleet/freezing rain. The wildcard in this setup
is if a coastal low is able to form off the southern New England
coast. If it takes longer to form, the strong SW flow at low-mid
levels will likely result in a longer period of wintry mix and a
faster intrusion of dry air aloft. If the coastal low forms
sooner, it could mean less mixing and a warm conveyor belt
containing Atlantic moisture will be ed back into the interior
Northeast. This could mean heavier snowfall totals, particularly
in northern New England. WPC probabilities support a
moderate-to-high risk (60-80%) for snowfall totals >4" from the
White Mountains to Maine's Central Highlands. In fact, there are
some lower odds (20-40%) for >8" in the White and Blue Mountains
(northern NH into western ME), suggesting some guidance in the WPC
WSE is hinting at a heavier snowfall event there. Given the
complex setup, this will be closely monitored as changes in
thermals within the boundary layer and when/if a coastal low can
develop will play critical roles in both totals and impacts for
this event in the Interior Northeast.

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

Mullinax