FOUS11 KWBC 180752
QPFHSD

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

Valid 12Z Wed Dec 18 2024 - 12Z Sat Dec 21 2024

...Pacific Northwest through the Northern Rockies...
Day 1...

Compact and fast moving but potent shortwave will lift eastward
from the WA coast into the Northern Rockies today through tonight.
This feature will help drive a surface low along the international
border with Canada, pushing a warm front downstream, and then
dragging a cold front from west to east in its wake. The enhanced
ascent in the vicinity of this frontal system will act upon robust
moisture (PW anomalies +2 sigma according to NAEFS) to wring out
some heavy precipitation from the Olympics through the Cascades
and into the Northern Rockies. Snow levels will steadily rise
behind the warm front, reaching as high as 8000 ft,
limiting significant snowfall to just the highest terrain, band WPC
probabilities reach 50-70% for 6+ inches in the northern WA
Cascades and Northern Rockies near Glacier NP. Although snow levels
will crash behind the cold front, this will be coincident with a
rapid drying of the column, so any residual snowfall should be light.


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

A wave of low pressure will develop across the TN VLY and then race
northeast along a cold front, with secondary wave development
likely east of Maine Wednesday night. 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.


...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, with this low diving progressively
southeast through D1 and D2.

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
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