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

Valid 00Z Wed Mar 29 2023 - 00Z Sat Apr 01 2023

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

A shortwave emanating from a closed low over Hudson Bay is
forecast to dig south across the Upper Midwest-northern Great
Lakes, driving a sharpening cold front with generally light snow
across the region tonight into Wednesday.  Overall, expect
accumulations to be light, however some lake-enhanced higher
totals are possible across portions of the U.P. of Michigan. 

On Wednesday, the shortwave will continue to amplify and the cold
front strengthen.  The increasing temperature gradient, along with
non-zero instability, are expected to support a brief period of
heavy snow as the front moves through central NY into New England.
Snow and some snow squalls are likely especially late in the day,
with briefly heavy snow rates by Wednesday evening and continuing
into the overnight.  The HREF probs continue to show snowfall
>1"/hr rates near and above 50% at 00Z Thu from the Adirondacks
through central NY (SYR-ITH-ELM), which should continue eastward
especially across the northern areas into Vermont, northern New
Hampshire, and northern Maine.  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 -- producing at least a couple inches of snow 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 (850-700-500mb heights
below the 1st percentile for this time of year) will sink
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
continue to translate south from NorCal through the state tonight.
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 CA with more modest totals farther
north through the Cascade Range into southern WA and over to the
Blue Mountains. Between 2-4 feet of snow is likely in many CA
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.


...Northern Plains and Upper Midwest...
Day 3...

As the trough exits the Rockies late Thursday, an upper jet will
strengthen over the central Plains, extending from the Southwest
toward the Upper Midwest. Supported in part by mid-level energy
and favorable upper jet forcing along the top of the downstream
ridge, WAA snow will expand over northern MN into the U.P. of MI
Thu evening into the overnight.  Consensus of the guidance shows
the heavier WAA precipitation sliding off to the east along with
the better forcing into southern Ontario on Friday.  Meanwhile,
models show the upstream trough continuing to move east across the
Plains.  The 12Z guidance continued to show a significant amount
of spread with respect to the amplitude of this system, with the
GFS and GEFS Mean more eager to develop a closed low over the Corn
Belt/mid MO Valley on Friday.  Given the spread, confidence in the
finer details of the snowfall forecast is below-average.  However,
as the system amplifies, precipitation changing over to snow
within the associated comma head, with impacts over portions of
the northern Plains, is likely.  The latest WPC guidance indicates
central SD as the area most likely impacted by significant
snowfall totals by the end of the period, with probabilities of at
least 4" 30-60% by 00Z Sat.

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

Pereira/Fracasso