FOUS11 KWBC 010804
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Tue Apr 1 2025

Valid 12Z Tue Apr 01 2025 - 12Z Fri Apr 04 2025


...Northern Plains & Upper Midwest... Days 1-2...

A major winter storm is set to develop today and strengthen 
through Wednesday across the Northern Plains as a deep upper trough
traverses across the Great Basin and ejects a closed low over the 
region. This upper level setup places a developing surface cyclone 
over the central Plains this afternoon into a favorable left-exit 
region of a potent 150kt 250mb jet diving across the Southwest. 
Throughout the day today, a strengthening LLJ, (NAEFS shows 850mb 
winds exceeding the 97.5 climatological percentile 18Z this 
afternoon 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 by late this afternoon. Given the early-April sun angle, 
snowfall will struggle to accumulate during the day unless rates 
are >1"/hr. These rates appear most likely to occur tonight into 
early Wednesday once the deformation zone on the northern and 
western flanks of the 850mb low consolidates, allowing for 
treacherous travel conditions to rapidly unfold. 00z HREF depicts 
an initial wave of of WAA advection heavy snow (max rates around 
1"/hr) lifting northward this evening from the Dakotas across 
central MN into northern WI. Then by about 11z/Weds the surface low
lifts north to become vertically stacked while anomalous IVT 
advect from the southeastern flank to produce heavy deformation 
snow bands with rates up to 2"/hr across the eastern Dakotas and 
northern MN.

Guidance has consolidated on the heaviest snowfall occurring from
the eastern SD-ND border through central/northern MN. The latest 
ECMWF EFI has shifted a strong signal (values of 0.8-0.95) for a 
potentially disruptive winter storm across much of northern 
Minnesota, but still sports modest 0.7-0.8 values in eastern North 
Dakota and even northeast WI.There also remains modest signal in
hires CAMs and a few global models for QPF enhancement along the
north shores of Lake Superior in the MN Arrowhead due to strong
east-northeast flow and an associated upslope component. The 00z
HREF was particularly aggressive (probably too much so) and will
need to be monitored for an axis of particularly heavy snowfall.

These aforementioned areas will contend winter hazards such as 
heavy snow, blowing snow, and snow load on tree branches and power 
lines today through Wednesday night. A few locations, including
Fargo, ND (9.9" and Duluth, MN (12.7" could near 1-day April 
snowfall records per the 75th percentile NBM. Current WPC snowfall
probabilities (through 12Z Thursday) for >8" are moderate- to-high
(50-70%) across northeast SD, southeast ND, and northern 
Minnesota. High probabilities (70-90%) for >8" exist across the MN 
Arrowhead, where moderate probabilities (40-70%) for snowfall 
totals >12" exist as well. For WI and MI, moderate probabilities 
(40-60%) are found across northeast WI into the south-central MI 
U.P.. In terms of ice, impactful freezing rain is most likely on 
the eastern edge of the WAA across northern Wisconsin and into the 
MI L.P.. Most ice accumulations will be <0.1" due to the rapid 
warming of the low- mid levels, with the lone exception being 
northern Michigan Mitten, where there are low-to- moderate chances 
(30-60%) 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.

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


...Northeast... Days 2-3...

Farther east along and north of a lifting warm front, 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 and parts of the Interior
Northeast 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 moderate chances (30-60%) 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). 


...California through the Intermountain West... Day 1...

One additional day of heavy mountain snow is forecast across the
CA ranges and Sierra Nevada through the central Great Basin as an
upper low dives across northern California into the Southwest. This
pattern during the Day 1 period provides broad onshore flow with a
weak focus (and 300 kg/m/s IVT) into the Klamath Mts during the day
today. Snow levels will also remain relatively low and around
3000-4000ft (below major passes) given the upper low crossing 
directly overhead. Strong westerly flow also provides upslope 
enhancement across the central Great Basin ranges of NV and UT, 
where snow levels are expected to remain around 4000 ft. WPC
probabilities for an additional foot of snow are high (70-90%)
across the central Sierra Nevada. Moderate chances for >8" exist in
the northern California Mts, central Nevada ranges, and into the UT
ranges as well.


...Rockies... Days 1-3...

Multiple systems are forecast over the next few days to impact the
Rockies with moderate snowfall due to a deep upper trough entering
the region and an associated upper low closing off/lingering over
the Southwest through Thursday. For Day 1, strong westerly flow
provides prime upslope potential across the CO Rockies,
particularly for the first half of today. Snow levels start the day
around 5000-6000ft crash to below 4000ft tonight, but also as
Pacific moisture advection decreases and the column gradually dries
out. Meanwhile, as an upper low ejects into the northern Plains a
upper trough inflection leads to some modest convergence across the
MT ranges into the Bighorns.

For Day 2 there appears to be a bit of a lull in activity as the
initial upper low ejects eastward and a trailing upper low digs 
into the Southwest (cutting off the greatest moisture flux to the 
south). However, deep troughing (200 mb heights below the 0.5th 
climatological percentile at the start of Day 2 per the 12z NAEFS)
will allow for steeping lapse rates and scattered snow showers in 
the high elevations. 

By Day 3 a shortwave dives south into the northern Rockies and
provides upslope flow in the western MT and northwest WY ranges,
but with moisture relatively meager. Meanwhile, a lobe of vorticity
rounding the base of the deep southwestern U.S. trough ejects into
the central Rockies and spreads light-moderate snowfall potential
into the region, with snow also possible into the CO Front Range on
Thursday. Overall WPC probabilities for Day2-3 are moderate for >6"
both days across most of the Northern and Central Rockies spanning
from northwest MT to the San Juans of CO.

Snell

...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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