Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
445 AM EDT Tue Mar 28 2023

Valid 12Z Tue Mar 28 2023 - 12Z Fri Mar 31 2023

...Great Lakes into the Northeast...
Days 1-3...

Hudson Bay upper low will continue to meander in place over the
next couple of days, shedding a shortwave across the Great Lakes
tonight into Wednesday and into the Northeast Wednesday evening
into Thursday. Cold front will strengthen this afternoon and move
through the U.P. of Michigan overnight with generally light snow,
but some lake enhancement post-FROPA could accumulate to 3-4" in
the Keweenaw Pensinsula into the Porcupine Mountains. On
Wednesday, the shortwave and cold front will sharpen with an
increasing temperature gradient and some non-zero instability as
the front moves through central NY into New England. Snow and some
snow squalls are likely especially after dark with briefly heavy
snow rates -- HREF probs of >1"/hr rates are near and above 50% at
00Z/30 from the Adirondacks through central NY (SYR-ITH-ELM) which
should continue eastward especially across northern areas into
Vermont with a sharp drop in temperatures behind the front. WPC
probabilities for at least 4 inches of snow across NY/VT are low
(<40%) but this may tick up with the next cycle of hi-res
guidance. The system will continue eastward into Thursday as the
mid-level trough takes on a negative tilt and closes off over the
St. Lawrence River, allowing a brief surge in moisture into Maine
where temperatures will certainly be cold enough for snow -- at
least a couple inches but with low to moderate probabilities of at
least 4" (30-60%).

...The West/Rockies...
Days 1-3...

A deep/anomalous closed low west of Oregon this morning
(850-700-500mb heights below the 1st percentile for this time of
year) will wobble southeastward today toward the NorCal coast with
an attendant wrapped-up occlusion out ahead of the trough axis. A
modest moisture plume (precipitable water values near 1 inch along
the coast to around 0.5" near the Sierra) of up to +1 sigma will
ride the warm conveyor belt on SW to S flow into the northern CA
ranges today and steadily focus more and more southeastward into
the Sierra this afternoon/evening. IVT values (300-500 kg/m/s) per
the NAEFS will be in the top 99th percentile but the resident time
will be limited due to the progressive motion. A 120kt jet streak
will round the base of the upper low and move into NorCal and the
Great Basin tonight, with broad scale lift east and northeast of
the Sierra/OR Cascades into Wednesday. The upper low is forecast
to weaken into a sharp trough by early Thursday, with continued
height falls progressing through the Great Basin into the Rockies
on deep southwesterly flow. This will favor SW-facing mountains
all across the West until the trough axis passes through on
Thursday.

Heavy mountain snow is anticipated from the Sierra Nevada and
Trinity/Salmon Mountains of California with more modest totals
farther north through the Cascade Range into southern Washington
and over to the Blue Mountains. Between 2-4 feet of snow is likely
in many California ranges where WPC probabilities are high. Snow
levels will rise to about 4000-6000ft before falling back to
around 3000ft after the cold front passes. Farther south, the
higher terrain of the San Gabriels and San Bernadinos could see up
to a foot or so of snow. Eastward, focus of the snow will be over
parts the NV ranges into the Wasatch in UT beneath the stronger
height falls, but also over central ID due to more vigorous PVA on
the LFQ of the upper jet. Cold front will eventually reform east
of the Rockies later Thursday when snow will wind down over much
of the region. This should leave the CO Rockies in a bit of a
minimum for this system with only light/moderate snow totals
generally under a foot.


...Upper Midwest...
Day 3...

As the trough exits the Rockies late Thursday, upper jet will
strengthen over the central Plains out of the southwest pointed up
into the Upper Midwest. WAA snow will expand over northern MN into
the U.P. of Michigan as a Canadian cold front moves southward
toward the U.S. border. Models show a wide variety in both the QPF
axes and thermal profiles (snow vs rain or some wintry mix) but
would expect a broad area of snow in a west-to-east axis that will
continue beyond this forecast period. Through 12Z Friday, WPC
probabilities for at least 4 inches of snow are low (<40%) over
much of the Arrowhead of MN.


For Days 1-3, the probability of significant icing greater than
0.25" is less than 10%.


Fracasso