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QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
122 AM EST Thu Feb 20 2025

Valid 12Z Thu Feb 20 2025 - 12Z Sun Feb 23 2025


...Central Appalachians to New England... Day 1...

A major winter storm which brought significant snow to the
Tidewater area of Virginia and surrounding portions of VA/NC will
continue to eject to the E/NE today while deepening. Snow will end
quickly this morning over VA, but as the low deepens and tracks
well southeast of the 40N/70W benchmark, snow will overspread
portions of coastal southern New England, especially Cape Cod and
the Islands. The system is progressive, but moisture spreading
north ahead of it will maintain periods of snow in far southeastern
MA, with some additional ocean enhanced snowfall likely across the
Cape Thursday night. Total additional snowfall is expected to be
modest, but may exceed 4" especially across the Outer Cape and
Nantucket as reflected by WPC probabilities of 30-50%.

Behind this potent surface low, an anomalous upper low will race
from the Ohio Valley to off the Mid-Atlantic coast by this evening,
bringing an arc of 500mb heights below the 2.5th climatological
percentile according to NAEFS. Steep lapse rates beneath this upper
low combined with moistening of the low-to-mid levels of the
column will result in a pivoting axis of moderate snowfall, which
may accumulate significantly in the higher terrain of WV where WPC probabilities for 4+ inches are locally 50-70%. Farther east,
moderate snow rates may bring some light accumulations from near
Greensboro, NC northeast through Hartford, CT. Although
accumulations will be less than 1 inch in most places, this snow
falling atop very cold ground could cause hazardous travel.


...Great Lakes... Day 1...

Lake effect snow (LES) will develop in the wake of the potent upper
low moving through the Ohio Valley and into the Mid-Atlantic today.
NW flow and accompanying CAA will produce a favorable environment
across the lakes, most notably SE of Lake Michigan early D1, and
then shifting to downstream (SE) of Lakes Erie and Ontario tonight
into Friday morning. The flow is not ideal along the long fetch of
the lakes except Lake Michigan, and duration is likely to be
relatively short, but WPC probabilities for 4+ inches of snow reach
as high as 30-50% across far SW MI near the lakeshore of Lake
Michigan, as well as along the Chautauqua Ridge east of Lake Erie.


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

A weakening shortwave moving out of the Great Basin will dig E/SE
into the Central Rockies Thursday, combining with a weak jet streak
aloft to produce widespread deep layer ascent to support wintry
precipitation from Idaho through Colorado. A surface low beneath
the shortwave will traverse across Utah and Colorado, providing
additional ascent, and although moisture is near normal according
to NAEFS PW anomalies, this will result in a swath of moderate to
heavy snow, especially in the higher terrain of the Wasatch,
Uintas, and CO Rockies, primarily above 4000 ft. The waning
amplitude of this upper trough will allow precip to wane towards
the end of D1, but WPC probabilities for 6+ inches of snow are high

70%) along the Wasatch, higher terrain of the Uintas, and into

portions of the CO Rockies.


...Pacific Northwest... Days 2-3...

A deepening trough well off the Pacific Coast will begin to advect
moisture onshore the Pacific Northwest beginning Friday night. This
moisture plume will be characterized by IVT within an atmospheric
river (AR) which has a high probabilities (>70%) of exceeding 500
kg/m/s according to both the GEFS and ECENS. Additionally, this
AR will have significant duration as probabilities for 500 IVT
exceeding 50% persist for around 48 hours from Friday night through
just beyond the end of this forecast period. The overlap of this
robust IVT and accompanying WAA with intensifying jet level
diffluence and mid-level divergence will result in expanding
precipitation, likely occurring in two distinct waves: one Friday
night into Saturday morning, and another Saturday night through Sunday.

Within the first wave, snow levels will rise gradually to 4000-6000
ft along and west of the Cascades, but cold air remaining
entrenched to the east will keep snow levels below 3000 ft across
portions of interior OR and WA. With the second surge, however,
stronger WAA will lift snow levels to 6000-8000 ft, generally
keeping significant snow accumulations above pass levels. On D2,
WPC probabilities for 4+ inches that are above 50% are confined to
the highest terrain of the WA Cascades, although Washington Pass
may receive a few inches of snowfall D2. During D3, WPC
probabilities for 4+ inches increase in the WA Cascades, but again
at only the higher elevations, while additionally spreading east
into the Northern Rockies.

Weiss

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png

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