FOUS11 KWBC 041921
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
221 PM EST Tue Mar 4 2025

Valid 00Z Wed Mar 05 2025 - 00Z Sat Mar 08 2025

...Central Plains through the Great Lakes and into the Northeast... Days 1-3..

...Powerful storm to bring blizzard conditions to portions of the
Central Plains and Upper Midwest...

A rapidly strengthening surface low pressure is clearly evident on
the WV imagery this morning across the Central Plains, and it is
this feature which will bring a powerful blizzard to portions of
the country through mid-week. This surface low will traverse
northeast and begin to occlude tonight as the anomalous upper low 
(850-700mb height anomalies falling to nearly -5 sigma and below
the minimum in the CFSR climatology according to NAEFS) stacks
vertically and crawls northeast, reaching the Great Lakes by 
Wednesday aftn. Later Wednesday, this upper low will begin to fill
and open into a negatively tilted trough, ejecting into Ontario
Thursday morning, pushing the attendant cold front and accompanying
post-frontal trough eastward through the Mid-Atlantic and Northeast.

The intensity of this upper low will help sharpen a jet streak
which will rotate through the base of the trough and then lift
meridionally from near the Gulf states, placing the favorably
diffluent LFQ atop the best height falls to result in maintaining
strength of the surface low. This impressive synoptic ascent will
also help to draw significant moisture northward from the Gulf, as
an arc of PWs above the 97.5 percentile surges northward into the
Upper Midwest. This moisture will push north within enhance
isentropic ascent, and this is progged to lift into an impressive
TROWAL, especially from IA through the Great Lakes on D1 /Tuesday
night into Wednesday/. This TROWAL is likely to overlap with a
strengthen deformation axis collocated with strong frontogenesis 
and a deepening DGZ (SREF 100mb depth probabilities 30-50%) 
suggesting a stripe of heavy snow is likely from NW MO through the 
western Great Lakes. This is additionally supported by the WPC 
prototype snowband tool indicating nearly a 100% chance of 1-2"/hr 
snowfall rates within this band.

Significant moisture, plentiful ascent, and a deep low pressure all
indicate that an impressive swath of precipitation will spread from
the Central Plains through the western Great Lakes D1, with the
primary challenge being where the snow/rain line will setup.
Guidance suggests the precip will begin as rain along and southeast
of a line from near Marquette, MI, to Minneapolis, MN, through all
of Iowa and into eastern KS. However, as the low continues to
deepen and pulls east, the resultant CAA combined with the
ageostrophic flow into the low will transition rain to snow
quickly, with the heavy snow then collapsing east through
Wednesday. The heaviest snow accumulations will occur within this
collapsing band of snow, and although amounts aren't expected to be
extreme, impressive winds of more than 50mph will create major
impacts due to blizzard conditions from snow that has a high chance
(>70%) of exceeding 6 inches from far NW Iowa through the Twin
Cities of MN and into the western U.P of MI. Locally 12+ inches of
snow is likely in the U.P.

As the low fills and moves more rapidly to the northeast and into
Canada D2, the attendant cold front and post-frontal trough will
race eastward producing a period of strong CAA on NW flow in its
wake. This will likely result in periods of moderate to at times
heavy snow, especially in the favored upslope regions in the
Central Appalachians and Adirondacks. WPC probabilities for more
than 4 inches of snow as moderate (30-50%) in the Central
Appalachians, and 30-50% for 2+ inches along the Chautauqua Ridge
and into the Tug Hill Plateau/Adirondacks.

This system has prompted the issuance of Key Messages which are linked below.

Then during D3 /Thursday night and Friday/ another potent surface
low will emerge from the Central Rockies and race eastward as it
becomes embedded within more confluent downstream flow towards the
Ohio Valley. Ridging immediately downstream of this feature will
somewhat limit moisture return from the Gulf, and the accompanying
mid-level wave is progged to weaken rapidly into the flatter flow
east. Despite that, a swath of heavy snow north of the low track is
expected as some strung out 700-600mb fgen aligns west to east to
enhance ascent from Nebraska through Iowa and potentially as far
east as lower Michigan. There is considerable uncertainty and
spread with both the timing and intensity of this second low,
leading to lower confidence in the footprint of snow and resultant
impacts. However, at this time, WPC probabilities for more than 4
inches of snow peak around 50% in northern Nebraska.
 
...California, Great Basin, Southwest, and central Rockies... Days 1-3...

A progressive upper level trough will move into the West Coast
Wednesday afternoon, then reorganize over the Rockies through
Friday. As it reorganizes, its forward motion will slow
considerably, allowing cold air to pool over the West as Pacific
moisture advects into the region, resulting in areas of heavy snow.
Multiple embedded shortwaves will rotate around the trough, which
will support more widespread snow over several Western states. On
Wednesday, a pair of jet streaks will further enhance the lift over
California, with the southern Sierras in both the left exit region
of one and the right entrance region of another. Ahead of the 
upper level trough, a surface cold front will move south and east 
off the Pacific and into the West Coast. The front will have a 
limited tropical connection, increasing the moisture flux into
southern California. IVT values will peak around 400 kg/ms. That  
Pacific moisture will rising into the 99th percentile compared to 
climatology as it moves into southern California on Wednesday. As
this moisture pushes inland, the southern Sierras will get the
first opportunity to uplift that moisture in the form of heavy
snow. WPC probabilities are the highest in this region, with a
moderate (40-60%) chance of 18 or more inches of snow.

As the trough moves inland, the moisture moving northeastward ahead
of it will cause widespread mountain snow and valley rain and snow
across much of the Intermountain West through Thursday. A surface 
low will develop over northeastern Nevada and move across Utah and
southern Wyoming through Thursday in response to the forcing from
the upper level trough and embedded shortwaves. The trough will
become more consolidated as an upper level low tries to form within
it on Thursday. This will increase the divergence in the left exit
region of the jet which will have moved into the Four Corners
region on Thursday. That jet streak will have embedded winds over
130 kt, so it will have plenty of energy for that surface low to
work with. The highest probabilities through Thursday night east of
the Sierras will be across the Uintas of northeast Utah and the
Wind River Range of central Wyoming, where there's a high (70-90%)
chance of 8 inches or more of snow. Lower (20-50%) chances of 8
inches of snow extending in a broad swath of the West from Nevada
northeast through Colorado and Wyoming, including some lower
elevations. Colder air associated with the trough will get time to
settle as the upper level trough/low stalls and reorganizes.

A surface low will develop over the Colorado Plains Thursday
evening. This low will take over as the primary forcing for the
heaviest snow as the snow shifts east across Nebraska along with
the low. By Friday morning, all organized heavy precipitation will
have moves east and out of the region. A secondary small low may
cause a brief burst of heavy snow over northern Arizona during the
day Friday for the higher elevations along the Mogollon Rim.

Weiss/Wegman

...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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