Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
251 PM EDT Fri Mar 14 2025

Valid 00Z Sat Mar 15 2025 - 00Z Tue Mar 18 2025


...Northern Plains...
Day 1...

Rapidly strengthening low pressure analyzed over the central Plains
this afternoon will approach or exceed March sea-level pressure
records across the Northern Plains as it tracks northeast through
Saturday aftn. Not only will SLP records be approached, but
850-500mb height anomalies are progged to fall below the all-time
minimums within the CFSR climatology according to NAEFS, further
indicative of the intensity of this system. The result of this
evolution will be a major winter storm impacting the Northern
Plains late Friday through Saturday with a combination of heavy
snow and gusty winds.

As the surface low tracks northeast from the Central High Plains,
it will be driven by an overlap of impressive height falls
downstream of the primary closed low and at least modest upper
diffluence in the LFQ of a poleward arcing jet streak east of the
mid-level center. Together, these will drive intense deep layer
ascent aiding in the storm strengthening. At the same time,
confluent low-level flow east of the low will draw impressive
moisture northward on 300-305K isentropic ascent. This moisture
will surge am impressive theta-e ridge northward, which will then
rotate cyclonically and lift into a robust TROWAL, pivoting over
eastern NE, SD, ND, and western/central MN Saturday. At the same
time, an impressive deformation axis will overlap with this TROWAL,
supporting an intense axis of maximized ascent to drive heavy
precipitation rates.

Initially, the theta-e advection will surge warm air into the
region such that the column will support primarily rain. However,
as the low continues to deepen and move northeast, and mesoscale
ascent intensifies, the combination of dynamic cooling and
ageostrophic flow on NW winds into the low center will rapidly cool
the column. The guidance continues to suggest a period of mixed
precip, including sleet and freezing rain, will occur, but this
again appears to be a conceptual setup where the transition period
is quite rapid. This indicates that while some light icing is
possible, the predominant wintry p-type will be snow, which will
almost certainly become heavy at times (70% probabilities for 1"/hr
according to the HREF). This is supported as well by NAM cross-
sections indicating a region of theta-es lapse rates <0C/km in the
presence of symmetric instability, suggesting the potential for
CSI or even CI (thundersnow). Although antecedent conditions are
quite warm and some rain will begin this event, these rates should
quickly overwhelm this hostility leading to rapid snow
accumulation.

Finally, impacts during the period of heavy snow are likely to be
impressive due to strong winds that may gust above 50 mph during
the period of heavy snow. While the period of heavy snow rates is
likely to be somewhat temporally modest except beneath the pivot
point of the deformation axis, the overlap of extreme rates of
1-2"/hr and these strong winds will create near blizzard conditions
and extremely dangerous travel. The changeover to snow will
commence across eastern SD after 06Z Saturday before expanding
north-northeast across western and north-central Minnesota between
12Z-18Z. Recent trends have shifted this band of heavy snowfall
amounts (>4") eastward by about 75 miles or so as guidance is
consolidating on a more compact area of low pressure across the
Upper Midwest. WPC probabilities for at least 4" are high (>70%)
across southwest to north-central Minnesota, with 40-70% chances
extending into far eastern South Dakota and far southeast North
Dakota. Locally, a narrow swath of at least 8" of snow is possible
(20-40%) beneath the pivoting deformation, especially should any
thundersnow occur. Additionally, some light icing across north-
central MN has a 10-30% chance of reaching 0.1+" of ice accretion.


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

The active pattern continues across the West with two separate
systems producing widespread wintry precipitation across much of
the area through early next week.

After the significant winter storm departs into the central U.S.,
any break in precipitation across the West will be short lived as
the active pattern sheds another amplifying shortwave across CA/OR
tonight. This next impulse will be accompanied by an impressive
lobe of vorticity, with the resultant trough development driving
height falls once again from CA through the Four Corners on D1.
Available moisture with this next impulse is progged to be much
less than with the first shortwave, reflected by NAEFS PW anomalies
that are near normal east of AZ as the maximum IVT downstream of
this trough axis is modest (GEFS and ECENS probabilities for 500
kg/m/s less than 10%) and focused generally south into Mexico. Of
course, this is pretty typical with ARs that are oriented more W/NW
than W/SW, but enough moisture and continued lowered snow levels
of generally 3000-5000 ft will allow for at least modest snowfall
accumulations.

Interestingly, as this shortwave digs across the Four Corners, the
upstream flow becomes more favorable for a surge of moisture and
heavy precipitation farther north into the Pacific and Interior
Northwest. A trough over British Columbia/Alberta interacts with
the trough moving across the Four Corners to squeeze the flow over
the Pacific, resulting in fast and zonal flow surging into WA/OR.
Impulses within this flow will help push two distinct surface lows
onshore, the first Saturday morning and the second early Sunday
morning, helping to enhance ascent. Regardless of those surface
lows, deep layer lift becomes impressive and moisture increases as
a strengthening Pacific jet streak pivots onshore and begins to
buckle, lifting meridionally on Sunday downstream of an even more
potent trough approaching the coast.

Both 12Z ECMWF and GFS depict 500-700 kg/m/s IVT reaching the
coast D2, funneling moisture along a cold front which will
elongate into the region. Although the accompanying WAA with this
atmospheric river (AR) will drive snow levels to as high as 6000
ft south
of the cold front, considerable moisture and a sharp gradient in
snow levels will likely result in impactful pass- level snow from
the Cascades through the Northern Rockies and eventually Sierra
Nevada once again on D3, with still impressive snow but generally
above pass- levels farther south from the Great Basin into the
Central Rockies, as much of the West is again covered in elevation-
based snowfall. For the entire forecast period, high WPC
probabilities (>70%) for 8+ inches exist from the Sierra Nevada
and OR
Cascades northward into the Olympics and WA Cascades, and as far
east as the Salmon River/Sawtooth area, north into the Northern
Rockies, and even as far as the Tetons. Event total snowfall is
likely to be extreme in the higher terrain of the OR Cascades where
4-6 feet of snow is possible, with widespread 1-3 feet in the
other higher elevations regions of the Northwest and Sierra Nevada.


Snell/Weiss




...Winter Storm Key Messages are in effect. Please see current
 Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png