Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
227 PM EST Thu Feb 22 2024

Valid 00Z Fri Feb 23 2024 - 00Z Mon Feb 26 2024

...Northeast and Appalachians...
Days 1-2...

Longwave trough expanding across the eastern CONUS will shed a
vorticity lobe and accompanying low-amplitude shortwave from the
Mid-Atlantic into New England on Friday. Modest PVA/height falls
will occur in tandem with weak but present LFQ diffluence within a
zonally oriented jet streak positioned to the south to drive
cyclogenesis across the Mid-Atlantic. This surface wave will
deepen slowly as it shifts E/NE, generally remaining south of New
England as it pushes offshore, but the accompanying warm/moist
advection downstream will expand a shield of precipitation into
the Northeast. The antecedent environment is marginal for wintry
precipitation outside of higher terrain features and across
central/northern New England, and the features overall are
transient. However, the overlap of synoptic ascent with some
increasing mesoscale lift through fgen should produce at least
periods of heavy snow across VT/NH/ME Friday, with some light
freezing rain also possible from the Catskills and points north.
Total accumulations are expected to be modest however, as
reflected by WPC probabilities for more than 2 inches of snow
confined to the highest terrain and peaking around 40% in the
White Mountains and highest terrain of central ME.

Additionally, a secondary impulse digging through the longwave
trough will intensify and drop southward from the Great Lakes into
the Central Appalachians late Friday into Saturday. The downstream
airmass is again modest from a thermal perspective, but brief
intense ascent due to steep lapse rates beneath this cold pool
combined with sharp PVA could result in briefly heavy snow in the
terrain from WV through NC, with some upslope flow in the wake of
this feature enhancing ascent. WPC probabilities for more than 2
inches are generally 30-40% in the higher terrain of WV, southwest
VA, and into the NC Mountains. Locally more than 4 inches is
likely where upslope flow enhances ascent into the highest terrain.



...Pacific Northwest to the Northern Rockies...
Day 3...

A modest shortwave will cross the Pacific Northwest Saturday
evening, leaving confluent and nearly zonal flow in its wake on
D3. Within this confluent flow, a secondary and more intense
shortwave will approach and drop southward along the British
Columbia coast, producing even more pinched flow into WA state
late, with downstream divergence spreading into the Northern
Rockies and interior Pacific Northwest. At the same time, a
Pacific jet streak peaking above 110kts will arc onshore with
accompanying diffluence driving additional deep layer ascent into
the region. The onshore flow will advect higher moisture onshore
D3 as well, reflected by IVT reaching above 300 kg/ms, but this is
generally near normal IVT according to the NAEFS ensemble tables.
Despite the average moisture, the intensifying deep layer ascent,
aided by upslope flow, will result in waves of heavy
precipitation, focused in the terrain, from the Olympics eastward
through the Northern Rockies. Heavy snow accumulations are likely
above 3000-3500 ft, with WPC probabilities for more than 6 inches
of snow exceeding 80% in the WA Cascades, and 50-80% in the
Olympics and Northern Rockies. With snow levels around 3000-3500
ft, impactful snow is likely at many passes including Stevens,
Snoqualmie, and Marias Passes.

Weiss