FOUS11 KWBC 020719
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
319 AM EDT Wed Apr 2 2025

Valid 12Z Wed Apr 02 2025 - 12Z Sat Apr 05 2025

...Northern Plains & Upper Midwest... Day 1...

...Major winter storm to bring heavy snow and gusty winds to
portions of the Dakotas and Minnesota today. Key Messages have
been issued for this system and are linked below...

The storm system responsible for today's heavy April snow is
currently organizing across the central Plains early this morning
with an initial impulse of WAA snow showers lifting northward
across the Dakotas and Upper Midwest. At the start of the forecast
period, broad and deep upper troughing will be situated over the
Intermountain West as an ejecting shortwave lifts northward in
conjunction with a ridge building over the East Coast. As this
shortwave amplifies, it will close off over eastern SD this
afternoon and then continue to deepen while sliding northeast
across northern MN this evening. For context, the 850-700mb heights
associated with this low pressure system are forecast to drop
below the 0.5 percentile and even near the 18Z record minimum
within the NAEFS CFSR climatology. This indicates a strong system,
and as the strongest height falls and PVA from the mid-level
evolution overlap with the increasingly intense diffluence within
the LFQ of a pivoting jet streak, a strong and intensifying surface
cyclone will track from eastern Nebraska through the Arrowhead of
MN before exiting into Canada Thursday aftn. With this track, the
heaviest snowfall will occur just to the north and northwest of the
low center.

Two main areas most likely to experience heavy snowfall rates

1"/hr and accumulating snow despite the early-April sun are along

the MN Arrowhead and north shores of Lakes Superior, as well as a
corridor from the SD/ND/MN border intersection through northern MN.
The latter region will fall with a potent deformation axis
overlapped with an increasingly impressive TROWAL, especially
across from west-central through northern MN, causing heavy snow
rates for which the WPC prototype snowband tool and the HREF
probabilities suggest will reach 1-2"/hr. Even these kinds of rates
could overwhelm the warmth due to April sun, leading to rapid snow accumulations. These impressive snowfall rates may also occur for
several hours (roughly 14Z-21Z per the 00Z HREF) as the upper low
crosses overhead and pivots the TROWAL over west-central MN and
the eastern Dakotas, which led to an increase in snowfall here
compared to the prior forecast. Here, WPC probabilities are
moderate (50-70%) for 6+ inches of snow, with locally 8-10 inches
possible. The Arrowhead of MN will also be a focus for locally
heavier snowfall as reflected by a 70-90% chance for 8 inches here,
but there remains some uncertainty into how effectively lake
enhancement can occur.

South of this axis of heavy snow, a swath of mixed precipitation,
including freezing rain, is likely. The heaviest icing is expected
across and parts of lower MI, where WPC probabilities for 0.1" of
ice or more range from 10-30%.


...Northeast... Days 1-2...

The same system that will bring the major winter storm to the
Northern Plains/Upper Midwest will bring some wintry weather to the
Northeast as well. As the primary surface low over MN occludes to
the east into the Great Lakes, an attendant warm front will begin
to lift northeast downstream of this secondary low pressure. As
this warm front lifts into the Northeast, it will spread increasing
moisture across Upstate NY and northern New England, especially
after 00Z Thursday. The accompanying lift through WAA will ascend isentropically, especially along the 290K-295K surfaces, which
will push PWs to above the 99th climatological percentile according
to NAEFS, while at the same time driving a warm nose above 0C
northward. The impressive ascent along this warm front will result
in axis of heavy precipitation, but with p-type gradually changing
from snow to a mix to rain, even into northern Maine. The fast
progression of this event, combined with heavy precipitation rates,
and the transitioning p-type will limit overall amounts, but
impacts will still be notable due to snow, sleet, and freezing
rain. WPC probabilities for more than 4 inches of snow are modest
at just 30-50% in the higher terrain of NH and ME, but more than
0.5" of sleet is possible across a larger portion of those areas.
Additionally, freezing rain has a 50-70% chance of accreting to
more than 0.1" of ice across the Adirondacks, southern Greens, the
Northeast Kingdom of VT, and northern NH into neighboring ME.


...Rockies into the CO Front Range... Days 1-3...

Widespread light to moderate snow D1-D2 potentially becomes more
significant across the Southern/Central Rockies late D3.

For D1 and D2 /12Z Wednesday through 12Z Friday/, the entire
region will be covered by large but diffuse troughing extending
from the Pacific Coast through the High Plains. Beneath this large
and slow moving trough, waves of vorticity will rotate through,
leading to periods of enhanced ascent within steep low level lapse
rates and aided by upslope flow. In general, this forcing will be
modest across the region, but a local exception will exist across
the Northern Rockies and maybe as far south as Yellowstone NP as a
cold front digs out of Canada leading to some increased fgen and
enhanced upslope flow in its wake by the end of D1 into D2. This
will cause heavier snow rates and at lower snow levels (falling to
2000-3000 ft) leading to accumulations that have a high risk (>70%
chance) of exceeding 6 inches around Glacier and northern Absarokas.
Elsewhere on D1 and D2, moderate to locally heavy snowfall across
much of the terrain from the Four Corners and UT ranges into the CO
Rockies has 30-50% chances of exceeding 6 inches, primarily above
8000 ft. This is in response to a shortwave rounding the base of
the trough in the Southwest pivoting over the Four Corners.

Then during D3, more impressive and consolidated ascent begins to
manifest across the central and southern Rockies, generally from WY
through CO and into NM as the tail shortwave within this larger
trough begins to amplify. There is good agreement that this
shortwave will close off and dig south into the Deserts of AZ,
leading to increasingly impressive mid-level divergence downstream.
At the same time, a subtropical jet streak will intensify and
rotate around the base of this amplifying closed low to place
favorable LFQ ascent into the eastern Rockies, while additionally
the cold front from D2 across the northern Rockies drops south
across WY and CO. The overlap of this baroclinic gradient with the
increasing synoptic ascent could result in an impressive area of
expanding precipitation D3 and bleeding into D4, with snow levels
starting around 6000 ft and dropping below 4000 ft and snow into
the High Plains. There is still uncertainty into the exact
evolution of this event and how far heavy snow extends into the CO
Front Range. Current WPC probabilities on D3 are high (60-90%) for
more than 8 inches of snow in the Sangre de Cristos and eastern San
Juans, with slightly lower into the central and northern CO Mts.

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

$$
--- SBBSecho 3.20-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1337:3/103)