Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
411 AM EDT Wed Mar 29 2023

Valid 12Z Wed Mar 29 2023 - 12Z Sat Apr 01 2023

...Northeast...
Days 1-2...

Shortwave over the U.P. of Michigan will swing southeastward
through southern Ontario and into Quebec/NY this evening, carrying
a strong cold front from west to east. Sharpening trough axis and
increasingly negative tilt will promote an area of light snow with
an embedded heavier band of snow squalls likely in the vicinity of
the front as the low-level thermal gradient strengthens and steep
lapse rates support some instability. Though most accumulations
will be an inch or two, some of that may fall quite quickly per
HREF probs that still indicate a moderate (40-70%) chance of
>1"/hr rates over parts of central and upstate NY/Adirondacks into
the Green Mountains. Into Thursday, the slowing upper trough will
close off as a surface low forms just east, of Maine, prolonging
snow over northeast portions of the Pine Tree State. WPC
probabilities of at least 4 inches over the next two days are
generally no higher than 50% except for favored upslope/peak
locations in the mountains.

...The West/Rockies...
Days 1-2...

The deep closed low just west of NorCal this morning will continue
to weaken over the next two days as it moves into the central CA
coast early Thursday and opens/widens into a trough over the Great
Basin. Its wrapped occlusion and cold front have progressed into
the central Valley and will continue southeastward through the
Sierra today. Downstream divergence aloft (to the northeast along
with streams of vorticity and convergence in the lower levels will
promote moderate snows for much of central ID into western WY
where more than 8-12" is likely over the next two days. Farther
south, the moisture plume will gradually weaken, but upslope into
the central/southern Sierra as well as into the SoCal ranges
(e.g., San Gabriels/San Bernadinos) will also yield some modest
totals of 6-12"+ as the front comes through and then aided by the
trailing height falls.

The upper trough and surface front will continue through the Great
Basin on Thursday with light to modest snow over much of the
region, focused over the NV ranges and into the Wasatch/Uintas and
Caribou Range/Tetons in southeast ID and western WY. Surface front
will eventually translate to the High Plains with some snow for
the CO Rockies (generally 6-10" or so) as an area of low pressure
deepens over eastern CO late Thursday. Two-day WPC probabilities
for at least 8 inches of snow are greater than 50% over most
mountain ranges in the West and at elevations generally above
6000-7000ft.

...Northern Plains and Upper Midwest...
Days 2-3...

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. Some icing is
possible over northern WI as the milder air rides up and over the
cold surface temperatures late Thursday night into early Friday.
Meanwhile, models show the upstream trough continuing to move east
across the Plains and likely eventually close off again over the
Corn Belt. Trends in the guidance have been toward a more robust
system but with continued discrepancies in the placement of the
heavier QPF axis and also accompanying thermal/freezing boundaries
owing in part to the track of the surface low (~980s mb). Given
the spread, confidence in the finer details of the snowfall
forecast is below-average, but there is an increasing signal for
at least moderate snowfall from South Dakota northeastward. This
includes blowing and drifting snow as winds increase around the
low pressure center. As the surface low deepens across the Upper
Midwest, strong CAA around the low will change rain to snow with
some accumulation on the backside before ending. By the end of the
period (12Z Sat 4/1), surface low will likely be over Lower
Michigan/Lake Huron/southern Ontario with a northeasterly fetch
across Lake Superior into the U.P., enhancing snowfall early
Saturday (and continuing thereafter). There, WPC probabilities of
at least 8 inches of snow D2-3 are highest (>70%) from northern WI
eastward across the U.P. of Michigan. Two-day probabilities of at
least 4 inches are much wider and encompass much of SD/MN/WI due
to the large ensemble spread.

...Pacific Northwest...
Day 3...

Heights falls from the Gulf of Alaska will move southeastward into
the Pacific Northwest Friday into early Saturday at the nose of a
110kt jet. Modest moisture plume will precede the cold front
(precipitable water values rise to near 0.50" along the coast)
with SSW to SW flow in the low/mid-levels. Snow levels will
briefly rise to around 3000ft in the WAA before falling back to
near 2000ft, maintaining snow at the pass levels across the
Cascades. WPC probabilities for at least 8 inches of snow are >50%
over the Olympics and WA Cascades with slightly lower values over
the OR Cascades.


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

Fracasso/Pereira