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Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
323 AM EDT Sun Mar 16 2025

Valid 12Z Sun Mar 16 2025 - 12Z Wed Mar 19 2025

...Great Lakes... Day 1...

Guidance continues to intensify secondary low pressure developing
across the Ohio Valley early this morning downstream of a potent
shortwave trough emerging from the MS VLY. The resulting
intensification of this surface low is partly due to more
impressive downstream ridging interacting with the shortwave as it
develops a negative tilt over MO/IL/IN. Height falls and the LFQ of
an almost due-north advecting jet streak will help lower surface
pressures, and the the surface low is progged to deepen rapidly as
it lifts across the L.P. of MI and into Ontario province.

Moisture downstream of this surface low will be impressive, with
isentropic ascent lifting north from the Gulf surging PWs to above
the 99th percentile across the Mid-Atlantic, with the increasing
cyclonic flow pushing the accompanying theta-e ridge westward into
Michigan as a modest TROWAL. The overlap of this TROWAL with an
intensifying deformation axis will result in impressive omega,
partially driven by an intersection of fgen into the DGZ, to cause
rapid cooling of the column and resultant heavy snowfall. The
system is progressive off to the northeast, so the duration of
heavy snow is likely to be limited, but both the HREF and WPC
prototype snowband tool indicate a corridor of 1+"/hr snowfall
rates across the western L.P. for the first 6-10 hours of the
forecast. While the intensity has gotten stronger with recent model
runs, the placement has been quite consistent, and despite
antecedent conditions that are not favorable for snow
accumulations, these intense rates should still result in several
inches of snowfall as reflected by WPC probabilities for 2+ inches
that range from 10-30% near Chicago, IL, to above 70% near Traverse
City, MI, with 4-6" possible in the vicinity of the latter.

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

The period begins with the Pacific Coast sandwiched between an
anomalous ridge to the east across the Intermountain West, and an
amplifying trough well to the west over the Pacific Ocean. Between
these two features, pinched mid-level flow will surge moisture
eastward into the Pacific Northwest, with IVT progged to approach
750 kg/m/s, funneling along a cold front and focused into OR. As
the upstream trough over the Pacific deepens and pivots eastward,
it will shed periodic lobes of vorticity into the Northwest, with
an accompanying surface low likely moving towards the OR/CA
border at the start of D2 /12Z Monday/. This surface low will
briefly intensify ascent (which will already be impressive through
height falls, PVA, upslope flow, and LFQ jet-level diffluence),
while also pushing the strongest moisture channel southward into
CA. While this suggests the highest PW anomalies will pivot south
as well towards the Sierra, sufficient upper level flow will
maintain elevated moisture well inland through D2, providing the
impetus for continued moderate to heavy snowfall across the terrain as
far east as the NW WY ranges.

For D1, the heaviest snowfall is likely within the plume of highest
IVT along the cold front from the OR Cascades eastward into the
Salmon River/Sawtooth ranges, and south into northern CA and the
Sierra. Here, WPC probabilities are high (>70%) for more than 8
inches of snow, with locally 2-3 feet possible in the Cascades and
near the Shasta/Trinity region, although snow-levels indicate the
heaviest accumulations will remain above 5000 ft before falling in
tandem with weakening snow intensity. During D2, the heaviest snow
becomes primarily focused in the Sierra where a high risk (>70%)
is progged by WPC probabilities for more than 12 inches, leading to
2-day snowfall as much as 2-4 feet in the higher elevations above
4000 ft. This will still result in significantly impacted travel
across the Sierra passes Monday into Tuesday.

Most of the forcing and moisture advect east by D3, so heavy snow
should wane across much of the West as it focuses instead into the
Central Rockies, but additional snowfall of a few inches is
possible (10-30%) in the Wasatch and Northern Rockies.

...Central Rockies to the Central Plains... Days 2-3...

The next in this impressive series of significant low pressure
systems to impact the Plains will develop late Monday in the
Central Rockies and then strengthen into the Central Plains Tuesday.

This system will initially develop downstream of a large trough
which will cross out of CA and the Great Basin, reaching the Four
Corners states Tuesday morning. Downstream height falls and PVA
will help drive large scale ascent into the region, which will be
additionally enhanced by a subtropical jet streak strengthening
through the primary trough axis. As this jet streak pivots
poleward, the LFQ will overlap with the mid-level height falls to
help drive surface pressure falls, and lee cyclogenesis the latter
portion of D2 /after 00Z Tuesday/. However, this jet streak will
outrun the primary forcing in response to vorticity shedding
rapidly to the east, with secondary troughing and a more
pronounced, negatively tilted, trough developing over the Central
High Plains later Tuesday. This will result in secondary jet streak
development, with even more impressive diffluence overlapping the
more structured height falls/mid-level divergence. It is this
secondary development around 00Z Wednesday that will potentially
become a powerful cyclone with heavy snow across the Central Plains
and into the Upper Midwest by the end of the forecast period.

Although the first low is weaker, it will interact with a cold
front dropping out of the interior Northwest, leading to some
enhanced ascent through fgen and upslope flow across Wyoming on D2.
Moisture streaming into the wave from the E/SE, as well as aloft
from the S/SW will manifest as precipitation across WY, but with
the heaviest snowfall likely occurring in the terrain of the Big
Horns, NW WY ranges, and along the axis of greatest fgen over
N-central WY D2. However, total snowfall is expected to be modest
this period except above 4000 ft in the NW WY ranges and Big Horns,
where WPC probabilities for 6+ inches are above 70%.

Then during D3, snowfall becomes much more expansive and heavy as
the secondary, stronger, low develops and spreads snow from the
Central Rockies into the Central Plains. There is still
considerable placement and intensity uncertainty as reflected by
broad spread in the D3-D4 clusters, but the GFS members appear to
be the deepest and fastest solutions, with the CMC/ECMWF ensembles
supporting slightly less spread and are favored at this time. With
significant spread still in place, WPC probabilities are somewhat
compromised, but still support a high risk (>70%) of 6+ inches from
the Wind Rivers through the Laramie Range and into much of the CO
Rockies where heavy snow rates are likely both through
fgen/upslope flow. Farther east, increasing deformation over the
Central Plains by the end of the period will result in increasing
coverage of heavy snowfall, and while current WPC probabilities for
4+ inches are only around 10% across NE/KS, these will likely
increase and spread northeast into D4.

Additionally, as this secondary low intensifies, the pressure
gradient is progged to become quite intense, regardless of the
model preference, and WSSI-P is already featuring a widespread
30-50% probability of moderate blowing snow impacts, suggesting
the potential for near-blizzard conditions in some areas, but those
details still need to be ironed out as the event approaches.

Weiss

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