FOUS11 KWBC 130729
QPFHSD
Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 AM EDT Fri Mar 13 2026
Valid 12Z Fri Mar 13 2026 - 12Z Mon Mar 16 2026
...Cascades Through The Northern Rockies...=20
Days 1-2...
...Washington Cascades Major Winter Storm continues today...
Impressive atmospheric river (AR) continues one more day with a
plume of IVT with high probabilities (>90%) for exceeding 500
kg/m/s continuing to advect onshore around 45N near the WA/OR
border. This AR has been ongoing for more than a day already, and
will continue one more day before finally sinking southward Friday
aftn. This prolonged fetch of IVT will cause significant spillover
into the Northern Rockies as well as reflected by still significant probabilities for IVT exceeding 250 kg/m/s, so while the most
intense snowfall will continue across the Olympics and Cascades,
the Northern Rockies will also experience heavy snow for at least
one more day.
The nearly zonal direction of this AR will result in exceptional
orographic enhancement as it directs nearly orthogonally into the
Olympics and Cascades, with additional large scale ascent being
produced via isentropic ascent on the WAA within the AR plume and
modest LFQ diffluence as the Pacific jet streak sinks southward. At
the surface, a stationary front will waver in a west-to-east
fashion through tonight before finally sinking southward as a cold
front in response to a shortwave digging out of British Columbia.
This shortwave will also be responsible for weakening and shifting
the narrowing IVT plume southward, with the West-WRF indicating IVT
falling below 250 kg/m/s into northern CA by Saturday morning.
There is high confidence in heavy snow continuing today and tonight
before precipitation slowly wanes on Saturday with much lighter
snow. The more challenging aspect is what will happen with snow
levels across the region. There will be a very sharp gradient in
snow levels from north to south, with the surface front acting as a
demarcation between very low snow levels in the WA Cascades
(1000-2000 ft) and much higher across OR (6000-7000 ft). As the
front digs south in conjunction with the shortwave, snow levels
will crash across OR as well, but with much lighter precipitation
during that transition. This suggests that the heaviest snow (and
most notable impacts) will be across the WA Cascades and Olympics,
with considerable impacts also expected in the Northern Rockies,
and lesser snowfall and impacts expected across the OR Cascades.
This is reflected by WPC probabilities that are high (>70%) for at
least an additional 12 inches in the WA Cascades and parts of the=20
Northern Rockies. At least 4 inches of additional snowfall D1 is=20
expected (>70%) in the Blue Mountains, the higher elevations of NW=20
WY near Yellowstone NP, and the Sawtooth/Salmon River ranges. BY
D2, snowfall persisting across the Cascades wanes rapidly leaving
just light additional accumulations, but heavy snow will continue
across the Wind Rivers, Absarokas, Tetons, Big Horns, and other
neighboring terrain where WPC probabilities predict a high risk
70%) for 8+ inches of snow.
...Great Lakes into the Northeast...
Days 1-2...
Intense but compact low pressure will cross the Great Lakes today
bringing blizzard, and near-blizzard conditions, to many areas.
This low is being driven by a potent shortwave which is progged to
briefly close off near the MN Arrowhead tonight (pre-forecast
period) and then slowly open as it pivots eastward on Friday. This
will result in the occlusion of the accompanying surface low over
MI, leading to a brief slowing of the low before it then picks up
speed and races east into New England Saturday before exiting into
the Canadian Maritimes.
This event will be considerable for the Great Lakes. The
combination of the deep surface low, impressive upper shortwave,
and the LFQ of zonally oriented jet streak arcing from the Pacific
Northwest will drive intense deep layer lift. Within this region of
deep layer omega, impressive mesoscale ascent will aid through fgen
thanks to WAA lifting into a TROWAL, and a deformation axis which
will pivot N/NW of the surface low. Additionally, low-level flow
across Lake Superior from the east will lift orographically into
the Arrowhead/Iron Ranges bringing locally even heavier snow.
The setup is favorable for intense snowfall rates, despite SLRs
that may become fractured due to strong winds leading to blizzard
and near-blizzard conditions. The snow character may be drastically
different in some places too, with WAA leading to a heavier-wet
snow across the L.P. of MI, and more near-climo or fluffier/lighter
snow across the U.P. and into the Arrowhead. Regardless, the HREF
probabilities for 1"/hr snowfall reach above 70-80% in many areas,
with 2+"/hr snowfall rates possible at times thanks to CSI. Where
this snow is most intense and lingers the longest on D1, WPC
probabilities predict a high chance (>70%) of at least 6 inches
(after 12Z this morning) for the Huron Mountains of the U.P. as
well as near the Tip of the Mitt, with lighter accumulations
surrounding these areas.
As the low continues to pivot eastward late D1 into D2, a=20
resurgence of WAA will expand precipitation into New England, and a
secondary low may develop near the coast of ME Saturday morning.
Total forcing for ascent and available moisture will likely be less
than areas upstream (in the Great Lakes), but periods of heavy snow
both within WAA and along an inverted trough/occluded front
upstream from the secondary low pressure will result in areas of
heavy snowfall with rates again potentially reaching 1"/hr. WPC
probabilities across the Northeast are as high as 90% for 4+ inches
in the Tug Hill and southern Adirondacks, with probabilities
reaching 50-70% for the Chautauqua Ridge, and parts of the higher
terrain of VT/NH/ME. A local maxima is also possible along the
coast of ME where onshore flow ahead of the secondary low pressure
advects higher moisture onshore.
...Northern High Plains through the Upper Midwest...
Days 1-3...
...Blizzard likely this weekend with widespread major impacts and
potentially record-breaking snowfall for the Upper Midwest and=20
Great Lakes...
Guidance is consolidating around a rapidly strengthening low which
will develop in the lee of the Rockies Saturday night and then
track northeast through Monday before occluding and then exiting
after the forecast period. While the primary low is going to be the
one that brings the most substantial impacts, this system will, in
reality, driven by two distinct surface lows embedded within the
amplifying mid-level pattern and accompanying coupled jet streaks.
The event really begins today across the High Plains as moisture
streaming onshore within an impressive IVT plume (atmospheric
river) continues to push well inland leading to a surge in
PW/column moisture across Montana. The accompanying upper jet
streak arcing zonally across the region working in tandem with a
surface stationary front will cause snowfall to break out and
spread WNW to ESE across the state. Overall ascent during D1 across
this area appears modest as noted by omega fields and regional
soundings, but the impressive PW anomalies (nearing +2 sigma) will
support periods of moderate snowfall from MT through parts of SD.
The heaviest accumulations are likely in the higher terrain, with
otherwise modest accumulations expected D1. Where the heaviest snow
occurs, generally just east of the Continental Divide, WPC
probabilities are high (>70%) for at least 4 inches of snow for
much of central and southern MT, with locally more than 8 inches
possible in the Little Belt range.
The more significant portion of this event begins Saturday /D2/ as
lee cyclogenesis begins in more impressive fashion east of CO/WY. A
shortwave digging onshore the British Columbia coast Friday night
will begin to amplify as it digs into the Central Rockies Saturday
night. The rapid amplification of this shortwave will drive
impressive height falls downstream, with impressive jet streak
intensification occurring as a result. The overlap of height falls
with this jet development will lead to a rapidly deepening surface
low tracking from CO to IA before 12Z Sunday. As the jet begins to
kink southward, it will continue to produce heavy snow, with an
expansion across MT due to mid-level divergence downstream of the=20 approaching shortwave. At this time the snowfall rates should
become more intense through 00Z/Sun across MT, with additional
snowfall of 4+ inches possible (50-70%) for much of the eastern
half of the state.
After 00Z Sunday is when this event truly becomes a major winter
event with widespread considerable impacts. As the surface low
begins to lift northeast from the Central Plains it will encounter
dual moisture streams, one from the decaying AR pushing east, and=20
a second as the theta-e ridge begins to arc northward on=20
intensifying WAA/southerly flow out of the Gulf. The overlap of=20
robust deep layer ascent with maximizing moisture return will=20
deepen the low substantially, and help expand a considerable=20
precipitation shield, with heavy snow arcing from SD through the=20
Great Lakes by the end of D2, and then slowly pivoting northeast=20
while continuing through D3 as the surface low reaches MI by=20
12Z/Monday.
The synoptic setup is extremely favorable for a classic Midwest
winter storm with widespread intense snowfall. Embedded bands=20
with snowfall rates of 1-3"/hr are expected as reflected by high=20
potential for CSI and even CI in model cross-sections Sunday aftn=20
into Monday which drives intense omega into the TROWAL. Although=20
guidance has trended slightly north tonight, there is high=20
confidence in a laterally translating band of heavy snow from SD=20
into MN Sunday morning, with the environment transiting into one
that matches the conceptual model for a pivoting band over eastern
MN and WI Sunday evening into Monday, It is in this area where the
heaviest snowfall is expected, and potentially record, or near-
record 2-day snowfall is possible from eastern MN through WI and
northern MI. Elsewhere, snowfall rates of 1"/hr or more are still=20
likely at times, just with lesser duration. These snowfall rates=20
combined with gusty winds reaching 50 mph will create blizzard and near-blizzard conditions, with major to extreme impacts progged by
the recent WSSI-P. Even in areas that get lower snow totals, the=20
blowing snow potential will still result in difficult travel, and=20
it is likely that most forms of transportation will be treacherous
at best, and impossible at worst, through the event across a large
area.
2-day WPC probabilities ending 12Z Monday are impressive, and
suggest a high likelihood (>70%) for at least 12 inches from the
Buffalo Ridge eastward through the Twin Cities, across much of WI,
and through all of the U.P. of MI. Within this axis, WPC
probabilities for at least 24" are already above 50%, and some
locations may approach 3 feet before the event winds down Tuesday.
Outside of this axis of heaviest snow, a broad swath of at least 6
inches is likely (>70%) from far western SD through central IA and
northeast into central MI.=20
Finally, on the S/SE side of this snowfall swath, an axis of mixed precipitation is likely, leading to icing accretion from central IA
through southeast WI and into lower MI. The most significant icing
is expected for lower MI where WPC probabilities for at least 0.25"
of ice reach as high as 30-50%.
This storm could result in record snowfall in some places, and=20
with SLRs being slightly below climo, combined with the strong=20
winds, power outages and long-lasting impacts are expected. For=20
these reasons, Winter storm key messages are in effect, and are=20
linked below (Key Message 1).
Weiss
...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!4CUm3VHbp2eO8zz-iqQs-CRbN5QXMmbP5hCf54RPoQaUg= 2fX7_B0T69JzXePtJCSfYDZOvjQXHU45DO1_ExR7dKcoiw$=20
$$
=3D =3D =3D
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