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FOUS11 KWBC 250743
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
243 AM EST Mon Nov 25 2024

Valid 12Z Mon Nov 25 2024 - 12Z Thu Nov 28 2024

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

Negatively tilted trough over the Upper Midwest along with a closed
low churning over south-central Canada will slide east today and
work to provide ample lift over the Upper Great Lakes during the D1
period. Additionally, broad upper diffluence within the left exit
region of a zonally oriented upper jet into the central Plains will
help create the widespread precipitation shield from northern MN
and WI into the U.P. of MI. Meanwhile, a sure low is expected to
deepen as it crosses the Great Lakes and enters southern Ontario on
Tuesday morning with the added forcing to the north of the low. Periods
of heavy snow with rates up to around 1"/hr are possible associated
from this low pressure system between northern WI to the U.P. from
late this afternoon through the evening hours per the 00z HREF
SPT. Lake -enhanced snowbands are also likely as cold west-
northwest flow follows in the wake of the low before precipitation
turns strictly lake-effect on D2. WPC probabilities are high (>70%)
across the U.P. of Michigan and medium (40-60%) across the MN
Arrowhead.

Into New England, which will be dealing with WAA and the passage of
an occluded front as the low moves into Canada Tuesday, will see some
of the colder valleys remain below freezing for a period of time
and allow for light icing. This is most likely to occur near the
White MTs of northern New England, including parts of VT, NH, and
ME. PWPF for ice >0.10 inches are about 20-40%.


...Pacific Northwest...
Day 1...

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 fall by
D2 but as QPF wanes and finally moves away from the area. WPC
probabilities are most notable across the Cascades and Blue Mts in
OR, as well as the northern CA ranges.


...Sierra Nevada...
Days 1-2...

Persistent WSW flow will continue for the next few days as waves
of anomalous moisture flow across the central and especially
southern Sierra, bringing significant heavy, wet snow to higher
elevations. Precipitation is beginning to move onshore this morning
and quickly become heavier today before peaking early 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 near 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 along with snowfall rates up to 2-3"/hr.
Snow levels will drop as QPF starts to decrease towards the end of
D2 once the shortwave to the north moves out of the Great Basin.
For the two- day period, WPC probabilities for at least 18 inches
of snow are high (>70%) above ~8000ft.


...Wasatch, CO Rockies...
Days 1-3...

Several rounds of forcing in the upper levels and a potent 140 kt
upper jet initially over UT and CO will impact the region through
Wednesday and produce widespread heavy mountain snowfall. This
strong upper flow will also take advantage of ample amounts of
atmospheric moisture advecting into CA from continued onshore flow
from the Pacific. IVT values are expected to reach all-time highs
from southern UT/northern AZ to the Four Corners per NAEFS
climatological percentiles. 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.
Snow may also bleed over into the CO Front Range as the core of the
upper jet sinks south, as well as better upslope components on the
east-facing slopes of the Sangre de Cristo and Sawatch range. Snow
levels are expected to start out at the beginning of D2 in the
6000ft-8000ft range (higher south) and rise slightly on D2 before
falling rapidly below 5000 ft on Day, but while precipitation also
weakens. In total, multiple days of very favorable forcing and
continued moisture advection from the Pacific will result in a
broad 2 to 3 feet of snow over many of the higher elevations of UT
and CO. WPC probabilities are high (>70%) for at least 24 inches of
snow during the three day period for elevations in UT and CO above
10,000ft, with low probabilities (10-30%) of 4 inches of snow
across the CO Front Range on D3.

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

Snell




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