Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
306 PM EST Wed Dec 18 2024

Valid 00Z Thu Dec 19 2024 - 00Z Sun Dec 22 2024


...Mid-Atlantic and Northeast...
Days 1-3...

No big updates for the system moving through the Mid-Atlantic and
Northeast. A fast-moving low will bring a general light snowfall
from northwest PA into New England through tonight as moisture from
the Gulf interacts with a potent shortwave moving across the
Northeast. Due to its fast movement, any periods of heavy snow in
this area will be brief.

A wave of low pressure will develop across the TN Valley and then
race northeast along a cold front, with secondary wave development
likely east of Maine tonight. Moisture along and ahead of this
system will increase, with PWs surging towards the 90th
climatological percentile, highest east, as WAA intensifies along a
northward advancing warm front. The antecedent airmass is
marginally conducive for wintry precipitation, and without any
strong cooling from the north, locations along and east of this low
track will likely remain all rain, with just a small temporal
window for changeover back to snow as the low pulls away Thursday.

The exception will be from the Laurel Highlands through the
Adirondacks, and across much of northern New England where, despite
still a marginal atmospheric column, the precipitation should fall
primarily as snow, except in the lower valleys. A heavy and wet
(low SLR) snow is likely, which when combined with the fast
motion of this wave will keep snowfall amounts modest, and WPC
probabilities D1 above 30% for more than 4 inches of snow are
confined to the Adirondacks, Greens, Whites, and mountains of
north/central ME.

As the low moves away Thursday, some modest upslope/lake-effect
snow may occur, but in general precip intensity and coverage will
wane during Thursday. This sets the stage for an interesting
development on Friday. A shortwave digging from the Midwest and
embedded within the larger trough will sharpen and may become
negatively tilted over the Mid- Atlantic Friday, while a surface
low develops well offshore and downstream of this larger scale
trough. The guidance has trended a bit deeper with this shortwave,
leading to greater interaction with the larger low pressure
offshore. A lot of uncertainty remains, but if these systems can
interact, the overlap of moisture and some more intense ascent
due to deformation/height falls, could result in widespread, at
least light, snow across much of the Mid-Atlantic and Northeast.
Current WPC probabilities for more than 4 inches of snow are modest
at just 10-30% across much of Upstate NY, the Poconos, and eastern
MA/ME, but this event will need to be monitored with recent model
runs for the potential for heavier snowfall in some areas.

For the day Saturday, lingering lake-effect will persist on all of
the Lakes as Arctic high pressure builds over all the lakes, and
overhead for the upper Lakes. This very dry polar air mass will
greatly diminish the lake-effect, but due to a still very unstable
air mass between 850 temps between -15C and -20C and lake temps
around +5C, it's unlikely the lake-effect will shut off completely
until more significant WAA begins. Rather, expect light but
extremely high SLR snow in this air mass for areas mostly adjacent
to the lakes where the winds die off, while the lake-effect
continues southeastward and inland due to continued northwesterly
flow off the lower lakes.


...Northern Plains to the Midwest...
Days 1-2...

A shortwave amplifying over the Canadian Rockies will dig southeast
on Wednesday moving into the Northern Plains by Thursday morning
and then continue through the Upper Midwest and then reach the
Mid-Atlantic early on Friday. As this shortwave digs and amplifies,
it will be accompanied by a sharpening jet streak to drive deep
layer ascent through overlapping height falls, PVA, and diffluence.
This synoptic lift impinging on a low-level baroclinic gradient
will result in cyclogenesis in the form of a clipper low, which
dives southeast through D1 and D2 into the Ohio Valley.

Downstream, moisture will begin to enhance through the region as
impressive 280-285K isentropic lift drives rich theta-e northward,
with weak TROWAL development possible on Thursday. This overlap of
moisture and ascent produces an expanding swath of snow, with the
attendant WAA surging north to deepen the DGZ while concurrently
producing a corridor of impressive fgen. The column will be
extremely cold, so the presence of a deepening DGZ with strong fgen
should cause fluffy and above-climo SLRs which will accumulate
rapidly as reflected by the WPC prototype snowband tool suggesting
1"/hr rates moving across ND and into MN. The progressive nature
of the low will somewhat minimize the potential for significant
snowfall amounts, and there remains considerable latitudinal spread
by D2, but WPC probabilities suggest a moderate risk (50-70%) in a
stripe from northern ND into eastern WI for more than 6 inches of
snow.

Weiss/Wegman