070
FOUS11 KWBC 091929
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 PM EDT Sun Mar 9 2025

Valid 00Z Mon Mar 10 2025 - 00Z Thu Mar 13 2025


...Upstate New York and Northern New England...
Day 1...

A fast moving clipper-type low will race southeast across Ontario
tonight and move offshore New England Monday morning. Forcing
associated with this clipper will be enhanced downstream due to
height falls/weak PVA ahead of the accompanying shortwave and
modest upper diffluence in the LFQ of a jet streak pivoting across
Canada. Overall, ascent is transient and modest, and available
moisture is just slightly above climo according to NAEFS, so
precipitation is not expected to be heavy. However, where ascent
maximizes, especially into the higher terrain of the upslope
regions of the Adirondacks, Greens, and Whites, snowfall rates of
0.5" to 1" per hour are likely at times. This will result in local
snowfall exceeding 4" as reflected by WPC probabilities that are as
high as 30-50% in these areas.


...Washington State to Northwest Montana...
Day 1...

Fast moving shortwave trough of modest amplitude will shed from an
amplifying longwave trough over the Pacific and race across the
Northwest tonight through Monday. Height falls associated with this
impulse will drive synoptic lift, while a subtle period of WAA on
backing mid-level flow to more S/SW enhances moisture on IVT which
is progged by NAEFS to briefly pulse above the 97th climatological
percentile. Jet-level support is modest, but sufficient deep layer
lift into the moistening column will result in periods of moderate
to heavy snow, especially in the Cascades and Northern Rockies
where low to mid level westerly flow drives additional upslope
enhancement. Snow levels above 4000-5000 ft will confine heavy
accumulations to only the highest passes and higher elevations, but
in these regions WPC probabilities for 6+ inches are as high as
70-90%.


...Pacific Coast...
Day 3...

A much stronger trough than the one from D1 above amplifies off the
Pacific Coast beginning Tuesday, driving an expansive area of
height anomalies between 700-500mb that fall to below the 2.5
climatological percentile according to NAEFS by 00Z/Thursday. While
these height falls are progged to remain just west of the coast by
the end of the forecast period, the resultant downstream shift to
S/SW flow onshore CA and the Pacific Coast will surge IVT onshore
as an atmospheric river (AR). Recent IVT probabilities from both
the GEFS and ECENS have increased, and although the GEFS remains
more amplified (40% IVT exceeding 500 kg/m/s), the ECENS has a few
members that feature IVT exceeding 750 kg/m/s. Regardless of the
maximum IVT, it is likely that heavy precipitation will spread
onshore, with elevated instability due to deep lapse rates
exceeding moist adiabatic levels as the trough advects east. This
suggests that despite general short duration of the highest IVT,
precipitation coverage and intensity will be significant.

Within the WAA which demarcates the onset of the AR, snow levels
will climb to around 3000 ft over WA state, and as high as
5000-6000 ft in CA according to the NBM deterministic forecasts.
As the accompanying surface low and attendant upper trough swing
towards the coast late (and more into D4), snow levels will drop
rapidly, becoming more impactful to many western passes. At this
time, through D3, the heaviest snow accumulations are expected
across the northern Sierra and Shasta/Trinity region of CA where
WPC probabilities feature a high risk (>70%) for 8+ inches and
locally 1-2 feet is possible. Farther north into the Cascades of WA
and OR, as well as the Olympics, lighter snows have a moderate
chance (30-50%) of exceeding 6 inches, but with snow levels around
3000 ft more area passes will experience impactful snow.

This system will likely become significant across much of the West
into the medium range, and Key Messages, linked below, have been
issued for this event.


The probability of significant icing is less than 10 percent.


Weiss



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

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

$$