Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
323 AM EST Sun Mar 10 2024

Valid 12Z Sun Mar 10 2024 - 12Z Wed Mar 13 2024

...Eastern Great Lakes, Northeast, and northern Mid-Atlantic...
Days 1-2...

An anomalous closed mid-level low will track east from the Great
Lakes this morning, ejecting off Cape Cod by Monday morning while
continuing to amplify. This intense low will arc an inverted
mid-level trough to its NW as it passes into Canada, with this
secondary wave moving out of New England by Monday night. This
will result in a prolonged period of strong synoptic lift, aided
by the LFQ of a poleward arcing subtropical jet streak to drive a
rapidly intensifying surface low pressure. This surface low will
lift northeast across New England Sunday, moving into Nova Scotia
by Monday. This evolution will produce two distinct areas of heavy
snow into early next week.

The first will be Sunday from Upstate NY into ME where strong warm
and moist advection will surge an expanding area of precipitation
northward. The attached theta-e ridge will rotate cyclonically and
lift into a TROWAL over New England, which will occur in tandem
with impressive mid-level deformation to enhance mesoscale ascent
from northern VT into central/northern ME. Although the column
will be marginally cold for snow, and warming in the intense WAA,
this moisture lifting into the deformation axis and beneath the
TROWAL will support intense snowfall rates to help dynamically
cool the column. While the heaviest snowfall is likely above 3000
ft in the Adirondacks, northern Greens, Whites, and into northern
ME, where any banding can occur north of the cyclone, even lower
elevation snowfall is likely. Additionally, with snowfall rates
1-3"/hr possible within a region of favorable CSI, impacts due to
snow load are likely as SLRs remain well below climo during this
period of WAA, producing significant impacts as reflected by
WSSI-P.  Within this WAA snow, which will generally be confined to
NH/ME today, WPC probabilities for more than 4 inches reach 80% or
more, with locally up to a foot of snow possible in the highest
terrain.

As the strong low shifts to Nova Scotia Sunday night into Monday,
strong CAA in its wake accompanied by further vorticity streamers
rotating through the flow will combine to re-invigorate ascent
within a cold column, aided most impressively along north facing
slopes of the Adirondacks and far northern New England where
upslope becomes intense, and evaluation of Froude numbers indicate
sub-critical flow which will result in the heaviest snow axes
along or upstream of the terrain crests. Moisture from the open
eastern Great Lakes will increase as well thanks to this strong
CAA, driving impressive late-season LES downstream of Lakes Erie
and Ontario. The SLRs will be much higher with this secondary
snowfall, especially in LES regimes, and as the saturated column
cools into the DGZ. This will create heavy snow amounts in the
favored NW LES belts along the Chautauqua Ridge and Tug Hill
Plateau, with additional maxima likely in the Adirondacks and
Greens. For LES regions, WPC probabilities for more than 4 inches
are above 80% east of Lakes Erie and Ontario, with 10 or more
inches likely in a few areas. For the pronounced upslope snow in
the Adirondacks and Greens, WPC probabilities are high for more
than 8 inches of snow, and moderate snowfall accumulations are
also likely as far south as the central Appalachians around WV.

Snow Squalls: As the northern stream upper low crosses the
northern Mid-Atlantic today, a reinforcing cold front and
subsequent afternoon convective activity will produce snow
showers. A tight surface pressure gradient will result in strong
wind gusts, so a progressive snow squall with the frontal passage
and subsequent showers producing squall-like visibility concerns
and flash freeze conditions with rapidly cooling air. The
environment appears favorable for at least isolated snow squalls
from southern Upstate NY southward through PA to central WV in the
morning, then over central/eastern PA into northern NJ and
northern Maryland in the afternoon. Rapid reductions of visibility
and flash freezing of roadways (less so with flash freeze by the
I-95 corridor) in gusty conditions will make for difficult travel
conditions.



...The West...
Days 1-3...

Multiple shortwaves, one on D1 and then next D3, will move onshore
the Northwest through the forecast period, continuing an extended
period of unsettled weather with heavy snow each day across much
of the Northwest terrain down to the central Sierra Nevada.

The initial trough will emerge from the British Columbia coast and
advect onshore WA/OR this afternoon. Downstream of this impulse,
increasingly confluent flow and WAA will surge moisture into the
region, acted upon by height falls and PVA as the shortwave moves
inland. This feature is progged to weaken quickly as it moves
eastward D1, somewhat in response to rapidly shortwave ridging
development immediately in its wake owing to the short amplitude
of this progressive pattern, and the next shortwave clipping its
heels. This second shortwave is likely to be more impressive than
the first, but admittedly the guidance has been generally too
aggressive with the intensity of most of these features the last
few cycles. Still, as this next shortwave approaches Tuesday
morning, it should again be accompanied by rapid height falls and
PVA, but this one should have more intense ascent thanks to the
accompanying Pacific jet streak providing LFQ diffluence. This jet
will also transport more moisture ahead of the best forcing, but
NAEFS IVT anomalies peak only around +1 sigma, with IVT above 250
kg/ms lifting northeast into the northern Great Basin from CA.
Snow levels stay generally 2000-3000ft over the PacNW through the
forecast period and more like 3000-4000ft over the northern
Rockies and 3000-5000ft over the northern Sierra Nevada.

WPC probabilities D1 for more than 6 inches of snowfall are
highest (above 80%) in the WA Cascades and Olympics which will be
in closer proximity to the strongest ascent and combined best
upslope flow, but probabilities exceeding 50% extend south along
the Cascades and into the Shasta/Trinity/Siskiyou region of CA,
and as far inland as the Northern Rockies as the weakening impulse
shears out to the east. By D2, coverage of heavy snow wanes as the
area will be between the two impulses, with eastern portions of
the Northwest under the influence of weak ridging. However, the
subsequent shortwave which moves onshore D3, will begin to spread
moisture and downstream divergent ascent D2, resulting in moderate
to high (50-80%) probabilities for more than 6 inches into the WA
Cascades and Olympics. During D3 this trailing impulse and
accompanying jet streak pivot eastward, leading to an expansion
once again of high probabilities for more than 6 inches of snow
reaching as far east as the Great Basin and into the Tetons of WY
and Wasatch of UT.


Weiss/Jackson