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FOUS11 KWBC 241925
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
225 PM EST Sun Nov 24 2024

Valid 00Z Mon Nov 25 2024 - 00Z Thu Nov 28 2024

...Upper Great Lakes to New England...
Days 1-2...

The merger of 2 areas of upper level energy, one an upper level
low over the Canadian Prairies and the other a shortwave ejecting
out of the Colorado Rockies, will work to greatly increase the lift
over the Upper Great Lakes on Monday. A 100 kt jet streak will
support the upper level low extending southeastward, further
increasing the lift in the region. A strong surface cold front will
act to increase surface forcing for the development of a low near
the Missouri/Illinois/Iowa tripoint, tracking northeast to near
Chicago and across Michigan and into eastern Ontario by Tuesday.
With the added forcing to the north of the low, the comma-head
region of the low will be near Lake Superior...from the Arrowhead
of Minnesota to the U.P. of Michigan. Periods of heavy snow are
likely in this region, with support from the lake due to cold air
advection over the relatively warm lake resulting in localized lake
enhancement over the U.P., particularly in favored upslope areas
such as the Porcupine and Huron Mountains.

Into New England, which will be on the cold frontal/warm side of
the low as it moves into Canada Tuesday, some of the colder valleys
could see a brief period of freezing rain where the cold air
currently in place remains trapped. PWPF for ice >0.10 inches are
about 20-40% over far interior/NW Maine and northern NH.

...Pacific Northwest...
Days 1-2...

Upper low off the coast will finally move inland over SW OR and
weaken on Tuesday, with onshore flow ahead of it for D1 over the
Cascades where several inches of snow are likely. Snow levels are
around 3000-4000ft from north to south (WA to OR) and will rise
briefly tomorrow before falling again as QPF wanes and finally
moves away from the area by the end of D2.


...Sierra Nevada...
Days 1-3...

Persistent WSW flow will continue for the next few days as waves
of moisture flow across the central and especially southern Sierra,
bringing significant heavy, wet snow to higher elevations.
Moisture will start to impact the Sierra tonight and early Monday
and quickly become heavier tomorrow afternoon and peak into Tuesday
as PW/IVT approach record levels for this time of year per the
NAEFS ESAT and CFSR period. 700mb 0C line will lie W-E from central
CA eastward to the UT/AZ border, suggesting high snow levels and
generally a heavy/wet snow for many areas (and likely lower SLR
values). NBM shows snow levels around 7500ft (north) to perhaps
10,000ft (south) coinciding with the heaviest QPF. With >2-3"
liquid, even at <10:1 ratios, multiple feet of snow are likely in
the highest elevations. Snow levels will drop as QPF starts to
decrease into D3 once the shortwave to the north moves out of the
Great Basin. For the three-day period, WPC probabilities for at
least 12 inches of snow are high (>70%) above ~7000-8000ft.

...Wasatch, CO Rockies...
Days 2-3...

Several rounds of forcing in the upper levels will impact the
region through Wednesday. A zonal 140 kt upper level jet will
impinge over UT and CO on Tuesday, taking advantage of ample
amounts of atmospheric moisture advecting into CA from continued
onshore flow from the Pacific. Once the zonal jet shifts eastward,
a strong shortwave trough will approach the area on Wednesday,
resulting in much heavier snow across the Wasatch and CO Rockies.
The shortwave will be supported by a separate 120 kt zonal jet
over the Southwest. Thus, multiple days of very favorable forcing
and continued moisture advection from the Pacific will result in
a broad 2 to 4 feet of snow over many of the higher elevations.
Meanwhile where temperatures can remain below freezing,
accumulations could approach a foot through Wednesday in the
neighboring valleys.

The probability of significant ice >0.25 inches across the CONUS
is less than 10 percent.

Wegman/Fracasso

$$