FOUS11 KWBC 021904
QPFHSD

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

Valid 00Z Thu Apr 03 2025 - 00Z Sun Apr 06 2025


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

...Major winter storm continues to bring heavy snow and gusty winds
through Thursday afternoon. Key Messages have been issued for this
system and are linked below...

The system which is expected to bring a late season major winter
storm to the Dakotas and Minnesota is organizing this morning as
reflected by an increase in reflectivity across the region in the
vicinity of increasing 925-700mb PVU. This system will deepen today
in response to a mid-level closed low deepening as it moves across
Minnesota and into Ontario by Thursday morning. This low will
become quite intense, reflected by NAEFS 850-700mb height
anomalies falling below the 1st percentile in the CFSR database,
with similar 500mb anomalies dropping below 2.5 percentile. This is
reflective of a strong system, with surface low deepening
encouraged by the robust height falls and concurrent/overlapping
upper diffluence as a strengthening jet streak pivots poleward
downstream of the mid-level low. The guidance has come into better
agreement today with the track of this surface low, and it is
expected to track from near Minneapolis this aftn through the
western U.P. of MI by Thursday morning.

This evolution will have a two-pronged impact on the winter weather
impacts. First, this slightly more northern track will help push a
dry slot and a warm nose northward leading to an axis of light
sleet and freezing rain, but accumulations are expected to be minimal.

More significantly, NW of the surface low, a potent deformation
axis is progged to develop and pivot from eastern ND/SD through
north-central MN. This deformation axis will occur concurrently
with an intensifying TROWAL as theta-e advection on moist
isentropic ascent increases and wraps cyclonically around the
system. The TROWAL will be most impressive from generally 18Z today
through 06Z Thursday across N-Central MN, and cross-sections within
this region suggest a threat for CSI to enhance snowfall rates.
This is additionally supported by both the WPC prototype snowband
tool and HREF snowfall rate probabilities which indicate a moderate
to high risk (50-80%) for 1+"/hr rates, with locally 2"/hr rates
possible. This will overwhelm antecedent warmth and the April sun,
leading to rapid snowfall accumulation beneath this deformation,
and WPC probabilities are moderate (30-50%) for an additional 4+
inches after 00Z, with storm total snowfall of 6-10" in some areas.
While there is still some uncertainty into the exact placement of
this deformation, and the gradient on the NW and SE side should be
pretty significant, this band of heavy snow will be impactful
before the entire system exits to the northeast by late Thursday
morning/early Thursday aftn.

...Northeast... Days 1 & 3...

The same system that will be producing heavy snow across the Upper
Midwest D1 will occlude to a secondary low and attendant warm
front farther east. This warm front will lift northeast into
Upstate NY and New England tonight into Thursday, bringing a period
of heavy precipitation through Thursday aftn. The front will lift
rapidly northward, so the duration of precipitation will be
limited, and p-types will vary widely as the warm nose causes a
change from snow, to sleet, to freezing rain, and eventually rain
before ending (likely ending as freezing rain the highest terrain
of NH and northern ME). Despite this, the multitude of
precipitation types will create hazardous conditions, especially in
the higher terrain from the Adirondacks through Vermont, New
Hampshire, and Maine where WSSI-P probabilities indicate a 10-30%
chance for moderate level impacts.

The impressive fgen along this front should result in periods of
heavy precipitation rates as well, regardless of the p-type. This
could result in a few inches of snow and sleet, before changing to
freezing rain. Total accumulations will be generally modest, but
WPC probabilities indicate a high risk (>70%) for at least 4 inches
of snow in the highest terrain of NH and ME, and a 50-70% chance
for at least 0.1" of ice in the Adirondacks and portions of
NH/VT/ME as well, ending by 00Z Friday.

Another round of mixed precipitation is likely as moisture funnels
northward on return flow through the Mississippi Valley and into
New England Saturday. At this time any significant wintry
precipitation accumulations are expected to be confined to the
highest terrain of northern New England, with a mix of snow and
freezing rain likely. Total accumulations are expected to be modest
however, as reflected by WPC probabilities for 0.01" of ice that
are generally just 10-30%.

...Intermountain West... Days 1-3...

Expansive mid-level trough will remain entrenched across much of
the West through the weekend, but evolution of embedded shortwaves
will lead to amplification and wavelength shortening by Friday.
Before this occurs, generally modest ascent and modest moisture
will lead to widespread but light snow across much of the terrain
from the Northern Rockies southward through the Four Corners
states. Within this broad ascent, there is likely to be two areas
of more consolidated ascent and heavier snowfall D1: the Northern
Rockies and from the Wasatch to the San Juans. In the Northern
Rockies, a cold front sagging southward will cause some enhanced
fgen and post-frontal upslope on E/NE winds. This will more
efficiently wring out available moisture leading to heavy snow in
the vicinity of Glacier NP where WPC probabilities are high (>70%)
for at least 6 inches of snow. Additionally on D1, a subtle
shortwave ejecting from the southern stream will work together with
downstream mid- level divergence and some upslope ascent to
increase snowfall from the Wasatch into the San Juans where WPC
probabilities are 70-90% for 4+ inches of snow.

More widespread, generally light, snow occurs across the
Intermountain West terrain on D2, but some focused heavier snowfall
is likely in the vicinity of the MT/WY border, including the
Absarokas, Wind Rivers, and in the vicinity of Yellowstone NP where
the sinking cold front helps to enhance snowfall. WPC probabilities
in this region reach as high as 30-50% for 6+ inches of snow.

Then late D2 into D3, the amplifying trough and concurrent closing
off of an upper low across the Four Corners will yield much more
significant snow across the Southern Rockies and into Colorado.
While there is still uncertainty as to how this system will evolve,
and trends have been for a slightly farther south track, confidence
is increasing that heavy snow will become widespread across
northern NM and into CO. This will be in response to increasing
synoptic ascent driven by both downstream divergence and increasing fgen/upslope flow as the cold front continues to sag southward.
This will additionally cause isentropic lift to surge moisture
northward, and as this pivots back to the west it will yield an
expanding area of heavy snowfall, especially in the terrain from
the Front Range, to the Palmer Divide, the Raton Mesa, and the
Sangre de Cristos. Additionally, as snow levels crash behind the
front, impactful snow may spread into the High Plains including the
urban I-25 corridor, with at least moderate snow potentially
pivoting into the TX/OK Panhandles late in the forecast period and
into D4. At this time, WPC probabilities begin to rise late D2,
reaching 30-50% for 4+ inches along the Front Range, before
expanding and increasing D3 to 70-90% from the San Juans to the
Raton Mesa and along the Sangre de Cristos. Additionally, with
snowfall expanding and snow levels falling, WPC probabilities
indicate a low risk (10-30%) for at least 4 inches as far east as
the western TX/OK Panhandles and across the northeast NM High Plains.

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

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