Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
339 PM EDT Thu Mar 12 2026
Valid 00Z Fri Mar 13 2026 - 00Z Mon Mar 16 2026
...Cascades Through The Northern Rockies...
Days 1-2...
...Washington Cascades Major Winter Storm through Friday...
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 begins 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.
...Northern High Plains through the Upper Midwest...
Days 3-3.5...
...Increasing confidence for 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 some spread among the
various ensemble clusters, there is high confidence in this low
developing and reaching the upper Great Lakes 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 into Monday from the
Northern Plains into the upper Great Lakes. While it remains a bit
too early for specific forecast amounts, 3-day NBM probabilities
for the entire storm ending Tuesday include warning level (6") of
snow at the 5th percentile (meaning 95% chance of more than this)
in a swath from the southeast corner of Minnesota through central
Wisconsin and then northeast across much of the northern Mitt and
far western U.P. of Michigan. 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. The parent low feeding the system will be in an
intensifying phase as the track turns north of east after the
center crosses the Mississippi River Sunday evening. Very heavy
snow and persistent banding will be in place as the upper level
shortwave trough driving the upper level dynamics of the storm
Sunday night becomes negatively tilted, and the RRQ of the rapidly
strengthening upper level jet streak curls to a SSE direction, thus
maximizing upper level divergence. Snowfall rates greater than 2
inches per hour are likely in the strongest bands.
Snow efficiency will be a big challenge with this storm. The
combination of it being mid-March, so there's about 12 hours of
daylight for any snow to contend with when accumulating, as well as
increasing wind concerns, especially Sunday into Monday as the
storm continues to intensify. The winds work to break up the
snowflakes, lowering accumulation efficiency. The cold air coming
south behind the storm is quite dry, as usual, so that should work
to prevent there being much snow at very cold temperatures, which
would increase snow liquid ratios.
Finally, NBM mean snow totals are over a foot from southwest
Minnesota across all but far northwestern Wisconsin, and all but
southeast Michigan. 2 foot means cover much of central and
northeastern Wisconsin, the northern Mitt, and most of the western
half of the U.P. Thus, this is expected to be a historic and
extremely impactful winter storm due to both snowfall amounts and
high winds blowing the snow around. Frequent whiteouts are expected
with blizzard conditions certain along the shorelines of Lakes
Michigan and Superior, and likely for many inland areas.
Winter storm key messages have been initiated for this event, and
are linked below (Key Message 1).
Wegman/ 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
