Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
351 AM EST Wed Jan 31 2024

Valid 12Z Wed Jan 31 2024 - 12Z Sat Feb 03 2024

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

Deep upper low well west of WA/OR will be the driver for
precipitation in the West from the coast to the Rockies over the
next few days as an Atmospheric River works its way through
California and the Great Basin/Southwest. Wrapped up occluded
system will initially favor a S-N plume of moisture D1 along the
WA/OR/NorCal coast that will transition to a SW-to-NE orientation
as the strong subtropical Pacific jet noses its way to the SoCal
coast by Friday evening (250mb wind speed perhaps nearing top 0.1%
of events at NKX per the ECMWF forecast and SPC climo page). This
will help split the system into two parts... the northern portion
tied to the upper low that will drift off the WA coast until
Saturday, spreading height falls across the Pac NW into the
northern Great Basin/northern Rockies, and the second with the
brunt of the AR into CA and the Great Basin/Southwest. Surface
front will stretch/elongate and eventually break, with the
southern portion being the main focus for heavier snow. Although,
even to the north, central Idaho will likely see the heaviest snow
in the region as moisture lingers in the area D2-3. There, WPC
probabilities for at least 8 inches of snow are high (>70%) over
the Blue Mountains in OR and across the central ID ranges, with at
least a 50% probability of at least 12 inches.

To the south, mild influx of Pacific air and moisture will support
high snow levels to start (7000-8000ft) with the highest QPF
rates, with lowering snow levels advancing southeastward as the
cold front pushes inland tonight and especially Thursday. With IVT
values reaching around 400 kg/m-s into the Sierra foothills, heavy
snow (2-4"/hr) will fall over higher elevations initially but then
lower to around 6000ft under the trough axis. Multiple feet of
snow are likely in the Sierra above 7000-8000ft, with WPC
probabilities for at least 4 inches around 50% as low as about
5000ft. With snow levels lowering through SoCal, higher terrain
outside L.A. will see accumulating snow as well.

East of the Sierra, strongest heights falls will move through the
CA deserts, southern NV, and northern/central AZ Friday as the
cold front marches eastward. Southwest flow will favor SW-facing
terrain in southern/central NV, southern UT, and across AZ
(Mogollon Rim to the White Mountains) then into the San Juans
through early Friday. Heavier snow will continue Friday into early
Saturday (end of this forecast period) as the upper trough takes
on more of a negative tilt and perhaps closes off over
southeastern CO/northeastern NM by 12Z Sat. WPC probabilities of
at least 8 inches of snow are highest in the Wasatch and all
higher UT ranges and especially the San Juans thanks to favorable
upslope. Higher probabilities also extend into the CO Rockies
through 12Z Sat especially above 9000-10,000ft.


...Northern Maine...
Day 2...

A Canadian cold front and developing wave of low pressure along
the boundary will push southeastward out of Quebec and into
northern New England on Thursday. Southwesterly to southerly flow
will help guide moisture up and over the boundary, with snow over
northern areas of Maine and along the Canadian border of VT/NH. A
rain/snow mix is likely farther south into the milder air. Though
the low and height falls will pass directly over northern Maine,
general lack of deeper moisture and longer residence time of QPF
will prevent significant totals. Nevertheless, several inches of
snow are likely across far northern Maine with the system, where
WPC probabilities for at least 4 inches are highest (above 50%).
Lighter snow on the order of an inch or two is likely over
northern NY (Adirondacks), and the northern Greens/Whites.


The probability of significant icing greater than 0.10" is less
than 10% for days 1-3.


Fracasso