Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
147 PM EST Wed Nov 08 2023

Valid 00Z Thu Nov 09 2023 - 00Z Sun Nov 12 2023

...Colorado Rockies...
Day 1...

A shortwave rotating through the base of a positively tilted
trough will shear out to the east and weaken through Thursday
morning as most of the associated energy moves into the Plains. A
strengthening jet streak reaching 150kts over the Upper Midwest
will leave favorable diffluence within the RRQ over CO, which will
combine with the modest height falls and PVA to drive ascent
through D1. A cold front well south of the region will move across
NM, leaving post-frontal NE flow into CO which will upslope
favorably into the terrain and into a region of enhanced mid level
RH. This will result in periods of moderate snow, with a slow wane
of coverage and intensity by Thursday as the best ascent weakens.
WPC probabilities for 4+ inches are confined to D1, and highest
across the southern Front Range including the Palmer Divide, into
the Sawatch Range, and as far south as the Sangre de Cristos where
they exceed 40%. Locally, more than 8 inches of snow is possible
in the highest terrain. Some light snowfall is also possible along
the I-25 corridor, but accumulations are expected to be minimal.


...Northern Great Lakes, Northeast...
Days 1-3...

The first wintry event of the season will impact the Northeast and
New England tonight through Thursday with a mix of snow, sleet,
and freezing rain.

A mid-level closed low will amplify over southern Manitoba into
southern Ontario tonight through Thursday morning, driving
downstream divergence while shedding vorticity impulses to the
northeast. This downstream divergence will combine with increasing
upper diffluence within the LFQ of a SE to NW oriented jet streak
which will intensify across the Upper Midwest, reaching 150kts
within its core. This will result in a surface low occluding near
the Boundary Waters of MN, with secondary low development
occurring near Chicago tonight. This second low will then track
ENE into New England, with a leading warm front lifting across the
region ahead of it.

This warm front will be accompanied by impressive WAA on strong
southerly flow, which will drive an axis of enhanced mid-level
fgen concurrent with moisture isentropic ascent to produce a band
of heavy precipitation expanding west to east tonight. The
antecedent airmass is cold and dry, so wet-bulb cooling of the
column should allow precipitation to begin as snow in most areas.
This snow will likely be heavy at times as the DGZ deepens,
reflected by SREF probabilities for 50mb of depth reaching above
50%, coincident with its moistening and during the period of most
intense fgen ascent. Additionally, the impressive WAA will help
develop an isothermal layer beneath this DGZ, suggesting a
favorable setup for aggregate maintenance, and the HREF point
probabilities for 1"/hr snowfall reaching 30%. Although this band
should be transient, and p-type should transition quickly to IP/ZR
as the column warms, WPC probabilities for more than 2 inches of
snow reaching 50-80% from the Adirondacks through the NE Kingdom
of VT and into the White Mountains of NH and ME.

After the p-type transition to IP/ZR, the setup could support
significant accumulations of sleet reaching 1/2", and freezing
rain, especially in the higher elevations. There appears to be no
strong source for cold/dry advection to drain into the area to
offset the latent heat release of freezing, but elevated areas
with their colder surface temperatures could accrete more than
0.1" of ice as reflected by WPC probabilities reaching 70% in the
Adirondacks, and 30-50% in the Greens/Whites. Lower elevations are
likely to see more of the sleet accumulations due to a more
elevated warm layer, but eventually all areas should transition to
rain before precipitation ends by early on D2.

W/NW flow in the wake of the low brings lake effect snow off Lake
Superior Thursday and Friday. Due to marginal thermals, the only
notable snow risk at this point appears to be in the Porcupine
Mountains of the western U.P. where WPC probabilities for more
than 4 inches reach 10%.


...Pacific Northwest into the Northern Rockies...
Days 2/3...

A pair of shortwaves rotating onshore through otherwise pinched
and fast flow will drive weak atmospheric rivers (AR) onshore with
associated cold front to produce ascent. This will result in two
rounds of snow this forecast period, but with accumulations
generally confined to higher elevations.

The first of these impulses will shift across WA state late
Thursday night into Friday morning coincident with high
probabilities (>90%) of IVT exceeding 250 kg/m/s according to
CW3E. This IVT will be transient as it shifts in conjunction with
a surface cold front racing eastward with minimal jet-level
support for ascent. Still, PVA/weak height falls combined with
convergence along the front and modest upslope ascent as flow
turns zonal will result in a period of moderate to heavy snow,
generally above 4000 ft. WPC probabilities for more than 4 inches
of snow reach 40-60% in the WA Cascades, with some light
accumulations possible at Snoqualmie and Stevens Passes.

A secondary IVT maxima will follow quickly in the wake of the
first as the subsequent shortwave digs along the WA/B.C. border
and moves east during Saturday. The core of this secondary trough
is displaced north of the leading impulse, but associated
diffluence along the LFQ of an approaching Pacific jet streak is
more intense. This will again result in sufficient lift to wring
out precipitation across the region, but with the farther north
trough, snow levels are likely to be higher with this second wave
than the first. This will result in heavy snow, especially in the
northern WA Cascades where WPC probabilities for more than 4
inches exceed 80%, and significant snowfall is likely at
Washington Pass. Additionally, WPC probabilities for more than 4
inches reach 10-30% near Glacier N.P. in the Northern Rockies.

Weiss