Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
259 PM EDT Mon Mar 31 2025

Valid 00Z Tue Apr 01 2025 - 00Z Fri Apr 04 2025


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

Elongated Pacific jet coming into NorCal/northern Sierra this
afternoon will transition towards a more amplified pattern over the
next day or two as ridging builds into the Gulf of Alaska,
favoring downstream troughing into the Western CONUS. Heavy snow is
likely for the northern/central Sierra with major to locally
extreme impacts. From the Great Basin eastward, a wide area of
snowfall is forecast for the Interior West with modest snows for
the northern/central Rockies.

The day 1 period features the system that already moved onshore
with heavy snow into the Sierra decreasing a bit by tomorrow
morning behind the trough axis, but the pattern will support waves
of vorticity moving into/through the West Coast as the jet digs
farther south. Snow will quickly expand to the central Rockies
tonight in the core of the moisture plume that reaches near the
90th percentile into the San Juans. Snow levels between 5000-8000ft
to start (this afternoon at 00Z) will continue to fall behind the
cold front to around 4000ft or so (Sierra into the Great Basin) but
below 3500ft over the Cascades. By day 2, the trough axis will
broaden out as the downstream northern Plains system develops,
favoring lighter snowfall for much of the Interior West. The
exception may be around SW Montana into the Bighorns near the
backside of the developing upper low (again, tied to the northern
Plains system). Snow will also expand southward as the jet dips
into SoCal and central AZ/NM with some snow to the Mogollon Rim.

By day 3, the upper ridge over the Pacific will start to drift to
the West Coast, helping to lessen QPF/snowfall for the Cascades to
the Sierra. Inland, northern stream height falls are forecast to
move southward out of western Canada into the northern Rockies
which will favor some modest snowfall over NW MT aided by upslope
enhancement. To the south, continued SW flow across the Four
Corners will yield some light to modest snow for the San Juans to
the northern Sangre de Cristos northward to the Sawatch Range.

Total snowfall over the three-day period will likely be highest
over the northern/central Sierra and over parts of the northern
Rockies in MT. For the Sierra, WPC probabilities for at least 18
inches of snow are high (>70%) above 5000-6000ft or so (from north
to south) Inland, WPC probabilities for at least 12 inches of snow
are above 50% above about 5000-6000ft (MT), 7000ft (Tetons),
8000-9000ft (Wasatch), and 10,000ft (CO).

Fracasso


...Northern Plains, Upper Midwest, & Northeast...
Days 2-3...

The upper trough responsible for the barrage of mountain snow in
the West will position the 250mb jet streak's divergent left-exit
region over the Great Plains. Modest 500mb PVA and strengthening
WAA at low levels will give rise to lee cyclogenesis east of the
Rockies on Tuesday. Throughout the day Tuesday, a strengthening
LLJ, (NAEFS shows 850mb winds exceeding the 97.5 climatological
percentile 18Z Tuesday from north Texas to eastern Nebraska), will
deliver both rich moisture and increased levels of WAA aloft. The
upper trough to the west will also direct an IVT of 300-500 kg/m/s
into the Upper Midwest, which is approaching the 97.5
climatological percentile by late afternoon. This causes strong
300K isentropic ascent and 850-700mb WAA resulting in FGEN at those
mandatory height levels. There also appears to be sufficiently
cold enough air present, thanks to a nearby dome of Canadian high
pressure, for precipitation to transition over from rain to snow
from the Black Hills on east across South Dakota and as far east as
central Minnesota Tuesday afternoon. Given the early-April sun
angle, snowfall will struggle to accumulate during the day on
Tuesday unless rates are >1"/hr. These rates appear most likely to
occur Tuesday night once the deformation zone on the northern and
western flanks of the 850mb low consolidates, allowing for
treacherous travel conditions to rapidly unfold into the overnight
and into Wednesday.

Even as we are now inside of 48 hours from the peak of the event,
there are still some ongoing difference with respect to some
details. The GFS/CMC suite remains on the northern envelope of
track solutions, while the ECMWF/EC-AIFS are on the southern flank.
When accounting for ensemble guidance, which takes into account
snowfall between Tuesday afternoon and Wednesday night, most
guidance favors an area of heavy snowfall extending from northeast
South Dakota and southeast North Dakota across much of northern
Minnesota. The latest ECMWF EFI has shifted a strong signal (values
of 0.8-0.9) for a potentially disruptive winter storm across much
of northern Minnesota, but still sports modest 0.7-0.8 values in
eastern North Dakota.

The Northern Plains and Upper Midwest will contend winter hazards
such as heavy snow, blowing snow, and snow load on tree branches
and power lines starting late Tuesday and continuing through
Wednesday. Current WPC snowfall probabilities (through 00Z
Thursday) for >8" are moderate-to-high chance (50-70%) across
northeast SD, southeast ND, and northern Minnesota. High
probabilities (70-90%) for >8" exist across the MN Arrowhead. There
are also low probabilities (10-30%) for snowfall totals >12" in
this region as well, with the highest and up to 50% in northeast
South Dakota and the MN Arrowhead. In terms of ice, freezing rain
is likely on the eastern edge of the WAA across northern Wisconsin,
northern Michigan, and even as far east as far northeast
Pennsylvania and western New York. Most ice accumulations will be
<0.1", with the lone exception being northern Michigan where there
are low-to- moderate chances (20-50%) for ice accumulations >0.1"
on Wednesday. Residents and those traveling to/from these regions
should continue to monitor the forecast as additional changes in
the types of impacts, the severity of those impacts, and where/when
these impacts occur can still fluctuate.

Farther east, the aforementioned Canadian air-mass will be
departing off the Atlantic Canadian Maritime Wednesday night, but
boundary layer wet-bulb temperatures remain sufficiently cold
enough for some wintry precipitation in northern New England
Wednesday night and early Thursday morning. The WAA is quite strong
and the air-mass in marginally cold, so ice accumulations will be
limited to the northern Appalachian ranges (Adirondack, Green,
White) and across northern Maine. WPC probabilities show low-to-
moderate chances (10-40%) for ice accumulations >0.1" through
Thursday morning. By midday Thursday, rain will be the primary
precipitation type throughout most of northern New England (far
northern Maine the lone exception).

*Key Messages have been issued for this winter storm, and the link
 to view them is posted below.


Snell/Mullinax



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

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