FOUS11 KWBC 150648
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
248 AM EDT Sat Mar 15 2025

Valid 12Z Sat Mar 15 2025 - 12Z Tue Mar 18 2025


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

The rapidly strengthening low moving across the Northern Plains
which has been advertised the past few days will be well underway
at the start of the period. At 12Z Saturday, the surface low is
progged to be near the Twin Cities, MN, and will be lifting
northeast, reaching Ontario, Canada by 00Z Sunday. N and W of this
low center, a strong and pivoting deformation axis will be helping
to drive intense ascent, causing a changeover from rain to heavy
snow across western MN, resulting in blizzard, and near-blizzard,
conditions through the aftn.

Moisture advection downstream of this strengthening cyclone will
remain impressive through D1, with isentropic ascent surging the
accompanying theta-e ridge into a robust TROWAL which will pivot
cyclonically back into the Dakotas and Minnesota today. The overlap
of this enhanced moisture and the collocated deformation axis will
increase both moisture and ascent to cause a narrow stripe of heavy
precipitation, changing rapidly from rain to snow. Dynamic cross-
sections across this region continue to suggest an axis of CSI/CI,
supported by the elevated instability within the TROWAL, to create
snowfall rates that are forecast both by HREF probabilities and the
WPC prototype snowband tool to reach 1-2"/hr. Despite hostile
antecedent conditions due to warm temperatures and rain, these
snowfall rates will rapidly accumulate, and when combined with wind
gusts of 50 mph, will likely result in blizzard and near-blizzard
conditions through the aftn. This is despite accumulations that
will in most areas be modest except within the stripe beneath the
pivoting deformation axis, where WPC probabilities are high (>70%)
for 4+ inches and locally as much as 8 inches is possible.


...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.

The next impulse within this active pattern will push an impressive
vorticity lobe eastward from OR through the Great Basin to start
Saturday morning, with the resultant trough development driving 
height falls once again into the Four Corners on D1. PW anomalies 
that are near normal east of this feature as the maximum IVT 
downstream of this trough axis is modes and focused generally 
south into Mexico, but available moisture is still sufficient for a
swath of moderate to heavy precipitation, with snow-levels
generally 3000-5000 ft allowing for moderate snowfall accumulations
above these levels. WPC probabilities D1 are moderate to high
(50-90%) for 4+ inches across the CO Rockies, Sangre de Cristos, White
Mountains of AZ, and the Sacramentos.

As this lead 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 squeezing into 
WA/OR. Impulses within this flow will help push two distinct 
surface lows onshore, the first this 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. 

Resultant IVT with this setup becomes moderate to strong once
again, potentially reaching 500-750 kg/m/s as moisture funnels
onshore 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 later D2 into D3. Still impressive snow, but at generally 
above pass-levels, is forecast 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.

During the latter half of D3 /Monday evening/ the parent trough
digging across CA and into the Great Basin responsible for the snow
axis shifting south into the Sierra late D2 will pivot eastward
towards the Four Corners/Central Rockies. This will bring a slow
end to precipitation across much of the West (at least briefly) but
may help to produce another significant central U.S. storm by the
middle of next week.


...Central Rockies and High Plains...
Day 3...

A shortwave emerging from the Pacific will amplify as it moves
across CA late Monday and then tracks progressively into the Great
Basin, reaching the Four Corners by the end of the forecast period.
This shortwave will be accompanied by an intensifying sub-tropical
jet streak which will begin to arc poleward downstream of this
amplifying trough. This will have the dual-pronged effect of
transporting moisture into the Central Rockies while also placing
favorable LFQ diffluence atop the region of greatest mid-
level height falls. The resultant deep layer ascent will likely
interact with a cold front dropping southeast into the area, and
the setup appears favorable for developing cyclogenesis in the
central High Plains by Tuesday morning. Where the strongest ascent
overlaps the greatest moisture, especially in areas of expanding
frontogenesis, a swath of heavy snow is likely. WPC probabilities
at this time are modest, just 30-50% for 2+ inches outside of
terrain features across WY, but are higher in the Big Horns and
Black Hills. 

This system may become the next powerful cyclone across the Plains
through the middle of next week.


Weiss

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

https://www.wpc.ncep.noaa.gov/key_messages/La...

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