FOUS11 KWBC 120807
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
407 AM EDT Thu Mar 12 2026

Valid 12Z Thu Mar 12 2026 - 12Z Sun Mar 15 2026

...Cascades Through The Northern Rockies... Days 1-2...

...Washington Cascades Blizzard today...

Active winter weather continues across the WA Cascades and northern
Rockies through Friday night with heavy precip and high winds
especially tonight.

Atmospheric river (AR) plunging onshore WA/OR will continue with
high probabilities (>90% chance) of exceeding 250 kg/m/s for IVT
continuing into Friday. This AR has been persistent, and will
maintain a westerly direction through landfall transporting
moisture orthogonally into the WA/OR coasts. This prolonged AR will
bring copious moisture onshore, with pronounced spillover reaching
all the way to the Northern Rockies as reflected by still high
probabilities (>90% chance) for IVT exceeding 250 kg/m/s today and tonight.

This moisture will be wrung out as heavy precipitation through
orographic enhancement into the Olympics, Cascades, and Northern
Rockies, with additional heavy precipitation reaching the lower
elevations through mid-level isentropic ascent due to WAA. However,
a low pressure moving out of Alberta, Canada and into the Northern
Plains by Thursday aftn will drape a cold front southward, with
this front become elongated west to east causing some additional
upslope and mesoscale ascent, while additionally creating a sharp 
gradient in snow levels. North of this front, snow levels will
waver between 1500-2500 ft, creating significant impacts at many of
the Cascades and Northern Rockies passes, especially during D1 when
precipitation will be the heaviest. Farther south, snow levels will
remain elevated as high as 6000-7000 ft in Oregon, resulting in far
weaker winter related impacts through the period.

The precipitation will likely be nearly continuous across the
terrain, with only as low S/SE trend through Friday evening.
However, by Saturday, the ascent should advect away to the east
while the best moisture shunts to the southeast, bringing an end to
wintry precipitation across the area. Before that happens, periods
of heavy snow combined with strong winds will result in blizzard
conditions across the Olympics, WA Cascades, and parts of the
Northern Rockies near Glacier NP and the Lewis Range. With snow
levels so low, travel across the passes will be extremely impacted
as 1-2+"/hr snowfall combines with strong winds. Where this snow
accumulates most robustly, WPC probabilities are high (>70%) for at
least an additional 24", with locally 3-5 feet of snow expected in
the Cascades and highest Northern Rockies elevations, leading to
continued challenging travel across the passes. 

...Great Lakes into the Northeast... Days 1-2...

A fast moving, compact, but intense low pressure will race eastward
near the United States/Canada border from this morning through
Saturday morning, bringing a stripe of heavy snow and strong winds
to parts of the area.

This low will initially develop over the British Columbia province
Wednesday evening, but being to deepen rapidly as it approaches
North Dakota Thursday morning. This low will be driven by a potent
shortwave emerging from the Canadian Rockies and then deepening
into a closed low despite modest amplitude of the feature thanks to
being embedded within pinched westerlies and fast mid-level flow.
As this shortwave deepens, likely reaching peak amplitude 00Z/Fri -
12Z/Fri, it will work in tandem with the LFQ of nearly zonal, but
strengthening, jet streak, to produce rapid ventilation aloft to
result in the surface low deepening quickly. This low is likely to
track from western ND to eastern WI on Thursday, and then continue
east, ejecting away from the Maine coast on Saturday morning.

As this low progresses east, especially into the Great Lakes, it
will interact with a moistening column thanks to residual IVT from
an atmospheric river spreading eastward from the Pacific Northwest.
PWs are progged to reach +2 sigma, with this moisture then being
advected cyclonically into the low, reflected by a strengthening
theta-e ridge /TROWAL/ lifting northward into the western Great
Lakes. Impressive deep layer ascent will then couple with
increasing isentropic ascent and 850-700mb fgen (in response to
intensifying WAA) to produce an environment will support heavy snow
rates for which the HREF suggests has a 70-90% chance of exceeding 1"/hr,
especially along the Arrowhead and into the U.P. of MI. Although 
there is still some latitudinal uncertainty as to where the 
heaviest snow will occur, there is high confidence in a stripe of 
heavy accumulations from northern MN through northern WI, the U.P.
of MI, and into the northern L.P. of MI. This is reflected by WPC
probabilities that are high (>70%) for at least 6 inches from far
northern MN into much of the U.P. and northern L.P. of MI, with 
locally more than 12 inches possible across the Iron Ranges of the Arrowhead.

Additionally, impacts will be considerable as these intense
snowfall rates combine with gusty winds that may reach 30-50 mph to
produce near blizzard conditions and dangerous travel. The WSSI-P
indicates a 20-40% chance of at least moderate impacts across the
Upper Midwest/Great Lakes, so the Thursday evening and Friday
morning commutes will likely be extremely challenging.

As this low skirts eastward on Friday into Saturday, it initially
occludes over MI and then pivots rapidly into New England Friday
night. During this time, the most significant ascent beings to wane
as the upper low opens and the best synoptic ascent no longer
overlaps within the LFQ of the accompanying jet streak. Impressive
WAA at 850mb will surge northeast from the Mid-Atlantic states into
the Northeast, helping to expand precipitation further despite a
reduction in available PW (compared to areas farther west). This
will likely still result in a swath of moderate snowfall from
Upstate NY through coastal ME, with again some continued
uncertainty into the latitudinal placement of the heaviest amounts.
The heaviest snow is likely in the upslope regions of the
Adirondacks D2 where WPC probabilities are high (>70%) for at least
6 inches, including across the Tug Hill Plateau, with lesser, but
still significant snowfall of 2-4" expected in the higher terrain
of the Green and White Mountains.

...Central Appalachians & I-95 Corridor... Day 1...

A tricky forecast for snowfall will develop from the Central
Appalachians into the Mid-Atlantic states today. A powerful cold
front will race eastward from the Ohio Valley/Central Appalachians
tonight, to exit the Mid-Atlantic coast by late this afternoon.
Precipitation developing along the front will expand northeast and
become heavy within a plume of PWs above +3 sigma. This will
support primarily heavy rain as the column will be much too warm
for any wintry p-types. However, the intense CAA along and
immediately behind the front will drive tremendous ascent and
rapidly cool the column, aided by dynamic cooling as precipitation
falls from aloft. Cold air chasing moisture is rarely a setup that
supports snowfall, especially after record high-temperatures from
Tuesday/Wednesday. However, a potent jet streak will try to
strengthen just as the cold front moves offshore, placing
sufficient deep layer lift atop the front to potentially change
precip over to heavy snow, aided by additional ascent downstream of
the primary southern stream shortwave/PV anomaly opening into the
Mid-Atlantic. This could support snowfall that could be heavy at 
times thanks to strong omega into the DGZ. 

The combination of intense lift into the DGZ at the same time as 
the column cools (with steep lapse rates below 850mb) could support
a changeover to heavy snowfall, at least briefly, from northern NC
through Long Island. The timing of the changeover (afternoon)
combined with the antecedent hostile conditions (for snow
accumulation) will limit most impacts. However, the high-res is in
decent agreement that there will be a changeover to heavy snow with
rates potentially exceeding 1"/hr at times. This will likely not
materialize as 1"/hr accumulation, and any accumulations should be
limited to grassy surfaces as reflected by WPC probabilities that
are generally 10-30% for 1" of snow (higher in the higher terrain). However,
a wide range in WSE plumes suggests the potential for some 
accumulation, especially along and east of I-95 or across the 
higher terrain of the Blue Ridge, which could impact travel for the
evening commute as reflected by the WSSI-P supporting at least 
modest probabilities for minor impacts.

...Northern High Plains through the Upper Midwest... Day 3...

...Increasing confidence in a major winter storm this weekend...

The next in a series of shortwaves embedded within rapid flow
across the northern tier will move onshore the British Columbia
coast Friday and then advect rapidly southeast while deepening. The
core of this shortwave and its associated vorticity is expected to
dive across the Northern Rockies Saturday night and then reach the
central High Plains by Saturday night/Sunday morning. This
deepening trough across the High Plains will move across a residual
baroclinic gradient draped from the Northern Rockies into the
Central Plains, to help drive surface cyclogenesis in the lee of
the Rockies. This low is then likely to rapidly intensify as it 
tracks northeast into Sunday, aided by coupling of upper level jet
streaks. Although there continues to be spread among the various
ensemble clusters, there is high confidence in this low developing
and reaching the Central Plains by the end of the forecast period.

This low will develop within impressive moisture, both in response
to persistent IVT spreading Pacific moisture onshore and across the
West, but also in response to an expanding theta-e ridge emerging
from the Gulf downstream of the amplifying trough axis. As this low
deepens, it will wrap this moisture northward through intensifying
isentropic ascent/WAA, leading to a large swath of heavy snow. 

This heavy snow may begin as early as Friday across the northern 
High Plains in response to impressive fgen driven both by the
low-level front and 700mb WAA and the response to the peripheral 
RRQ of the zonally oriented jet streak aloft. It is this jet
streak overlapping the strengthening 700mb WAA, as it moves 
eastward, which will allow snow to expand in a narrow corridor 
across the High Plains into the Upper Midwest through Saturday
morning, and in some places there may be two rounds of moderate to
heavy snow. However, the most significant snowfall is expected to
begin Saturday night and then continue beyond this forecast period
from the Northern Plains into the Great Lakes. While it remains a
bit too early for specific forecast amounts, 2-day WPC
probabilities for at least 8" of snow are above 70% across the High
Plains of MT, and 30-70% for parts of SD and MN, and snowfall 
rates above 1"/hr appear probable due to a synoptic evolution that 
suggests impressive banding is likely.

These snowfall rates combined with an intense pressure gradient due
to a strong high positioned just to the northwest, will likely
cause significant blowing and drifting of snowfall, along with
limited visibility. WSSI-P is already projecting 70+% probability 
for at least moderate impacts (mostly on D4 into the Upper Midwest 
and Great Lakes) and this event could materialize as a blizzard in 
some areas with widespread substantial impacts from Saturday into 
early next week.

Winter storm key messages have been initiated for this event, and
are linked below (Key Message 1).

Weiss

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

https://www.wpc.ncep.noaa.gov/key_messages/La...

$$
--- SBBSecho 3.28-Linux
 * Origin: Capitol City Online (618:250/10)