FOUS11 KWBC 020626
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
226 AM EDT Thu Oct 2 2025

Valid 12Z Thu Oct 02 2025 - 12Z Sun Oct 05 2025

...Northern Rockies...
Day 3...

An upper low over the Great Basin on Friday opens into a trough and
shifts east over the Rockies late Saturday. Lee-side cyclogenesis
over western South Dakota occurs during this time enhancing flow
from Gulf-sourced moisture over the Plains through the northern
Rockies. Snow levels through Saturday are around 9000ft over the
Absarokas, Wind Rivers, and Bighorns, above which probs for >6"
snow from 12Z Saturday to 12Z Sunday are 40-80%.


The probability of significant icing is less than 10 percent.


Jackson


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FOUS11 KWBC 030808
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 AM EDT Fri Oct 3 2025

Valid 12Z Fri Oct 03 2025 - 12Z Mon Oct 06 2025

...Northern Rockies...
Days 2-3...

A pair of shortwaves moving across the Intermountain West will
interact with well above normal amounts of atmospheric moisture in
the area to cause the area's first winter storm of the season at
the higher elevations from Glacier NP in northern Montana south and
east through the ranges of northern and western Wyoming through
Sunday. The first shortwave begins the period Saturday morning over
Utah, and then tracks northeast towards the northern Plains by
Sunday morning. As the stronger of the two shortwaves getting first
dibs at the excess moisture across the area, expect heavier and
longer duration snow across the ranges of far southern Montana and
northwestern Wyoming, including Yellowstone and Grand Teton NPs.
Snowfall amounts of 6-12 inches are expected at the highest
elevations where the snow is most persistent.

As that first shortwave tracks into the Plains Sunday morning, the
second shortwave rounding the back of the longwave trough will dive southeastward out of Alberta through much of Montana from Saturday
night through much of the day on Sunday. This shortwave will result
in a period of heavy snow across much of Glacier NP late Saturday
night through Sunday morning. This secondary forcing will keep the
snow that began with the first shortwave Saturday ongoing through
Sunday. The snow will end from north to south Sunday and Sunday
night. WPC probabilities for more than 6 inches of snow remain over
50% for the Beartooth and Absaroka Ranges.

The probability of significant icing is less than 10 percent.

Wegman



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FOUS11 KWBC 040724
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
324 AM EDT Sat Oct 4 2025

Valid 12Z Sat Oct 04 2025 - 12Z Tue Oct 07 2025

...Northern Rockies...
Days 1-2...

The weather pattern across the northern Rockies remains very
similar to previous runs of the guidance. The dynamic upper level
pattern will feature a deep upper level low over Utah ejecting
northeastward into the northern Plains by early Sunday morning.
Anomalous moisture to 2.5 sigma above normal will support waves of
showers and a few thunderstorms tracking northeastward as a
developing leeside low becomes the dominant surface feature through
the weekend. The leeside low will take advantage of a shortwave
trough also pushing northeastward. This will allow the surface low
to intensify as it makes its way to the Plains. Combined with
upslope enhancement, these ingredients all coming together will
lead to a prolonged period of heavy snow. The Beartooth, Absaroka,
and Big Horn Ranges will all come in with over 50% probabilites
for 6 inches or more of snow or more through Sunday. Behind this
first, stronger shortwave, a second shortwave will dive south out
of Alberta and into the Northern Rockies on Sunday. The presence of
this second shortwave will support maintaining heavy snow into
northern MT, especially Glacier NP. This digging second shortwave
will allow the larger longwave trough to remain in place, even
expanding and growing south and west. Similarly important to the
presence of these 2 shortwaves moving through the flow and taking
advantage of the abundant low level moisture, a large polar surface
high will effectively end the wintry threat from north to south.
This is because the polar high will also be quite dry, so it will
move the moisture feeding the snow on towards the north and east.
With that polar low in place, expect temperatures to tumble well
down into the 30s. Thus, as is very common in the wintertime, the
limited time between the advancing cold/dry air and the retreating
atmospheric moisture will favor certain areas, such as the
Beartooth Range, while keeping many others on the Plains bone dry.
With the advancing cold air, snow levels will fall to as low as
5,000 ft, but the dryness should hold any heavy snow occurring to
Sunday morning, before the snow ends from north to south.

WPC probabilities for 6 inches or more of snow remain between 50
and 90% for the Beartooth and Absarokas, while probabilities are a
bit lower, between 60 and 80% for the Wind River and Bighorn
ranges.

The probability of significant icing is less than 10 percent.

Wegman



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FOUS11 KWBC 050704
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
304 AM EDT Sun Oct 5 2025

Valid 12Z Sun Oct 05 2025 - 12Z Wed Oct 08 2025

...Northern Rockies...
Day 1...

A positively-tilted upper level trough stretching from the northern
Plains to Central California is directing much smaller shortwave
troughs around its periphery. As each shortwave moves across the
area, they've been producing an area of rain and higher elevation
snow as a surface low develops in response to the movement of the
individual shortwaves. The first shortwave is moving over the
Dakotas and taking a well-developed surface low along with it. The
surface lows have been taking advantage of an anomalous plume of
moisture from the northern Rockies to the northern Plains. This
abundance of moisture has helped support areas of snow at the
highest elevations. As the low pulls away, there may be a brief
break in the steadiest precipitation into the mountains. Meanwhile,
a second trough, not as strong but still potent, is diving south
across Idaho and will slow and turn eastward across southern
Wyoming by Monday morning. Behind this shortwave a strong polar
high will dive southward, bringing with it a much colder and drier
air mass. The combination of subsidence with the high, and the dry
air will very quickly end the snow from north to south. Into the
Beartooths and Absarokas, enough moisture may hang on to keep much
lighter snow ongoing into Tuesday morning in a few isolated areas.
By Tuesday morning, the entire longwave trough will have moved
eastward, allowing strong ridging to build into the West Coast.
This feature too will act to limit the heaviest precipitation in
all areas as subsidence works to end the snow.

With the overall forecast largely unchanged, we will see the back
edge of the precipitation move southeastward out of Canada and into
Montana by Monday afternoon. While localized upslope is a
possibility, the drier air with the advancing area of surface high
pressure will quickly win out, allowing some areas to get some
sunshine this afternoon.

WPC probabilities for over 8 inches of snow are between 60 and 80
percent for the Bighorns and Wind River Ranges through Monday
morning.

The probability of significant icing is less than 10 percent.


Wegman



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FOUS11 KWBC 090719
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
319 AM EDT Thu Oct 9 2025

Valid 12Z Thu Oct 09 2025 - 12Z Sun Oct 12 2025

...Northern Rockies...
Day 3...

By the end of the short range forecast period (Sunday morning) a
sharp, negatively-tilted upper trough is forecast to impact the
northern Rockies with moderate to heavy precipitation and lowering
snow levels. The upper trough configuration is expected to develop
as a closed-low currently off the coast of the Pacific Northwest
opens up and interacts with a diving shortwave out of British
Columbia. Additionally, this pattern is favorable for increasing
upper jet dynamics and is supported by model guidance depicting a
120-140kt southwesterly jet streak extending from the central
Great Basin to the central/northern Rockies. This places the
northern Rockies in the left-exit region of the upper jet and most
favorable for upper divergence and widespread moderate
precipitation. As the upper trough moves over the region Sat night
(500 mb heights estimated to be just below the 10th climatological
percentile per the 12z NAEFS) snow levels are expected to also fall
below 6,500ft across much of the northern Rockies by early Sunday
and below 5,000ft across the Pacific Northwest, where lighter
precipitation will be located.

The most impactful snowfall through this timeframe is expected to
be located across southwest Montana as moderate precipitation
overlaps with the lowering snow levels. Latest WPC probabilities
for at least 6 inches of snowfall are 20-40% across the high
terrain of southwest Montana above 8,000ft, which is still above
many of the major mountain passes. Additionally, WSSI-P values for
moderate impacts reach 30-40% in these areas primarily due to snow
load concerns.


The probability of significant freezing rain across CONUS is less
than 10 percent.


Snell



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FOUS11 KWBC 100748
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 AM EDT Fri Oct 10 2025

Valid 12Z Fri Oct 10 2025 - 12Z Mon Oct 13 2025

...Northern Rockies & WA Cascades...
Days 2-3...

A strong upper-level trough traversing the Pacific Northwest on
Saturday will provide ample upper-level ascent over the Northern
Rockies Saturday night and into Sunday. The EPS shows a classic
"kissing jets" setup Saturday night with the diffluent left-exit
region of a strong 120-140 kt 250mb jet over Utah and the diffluent right-entrance region of a >100kt 250mb jet over south-central
Canada both located over western Montana. 18z ECMWF depicts PWAT
percentiles are likely to remain above the 90th climatological
percentile through early Sunday morning with ample moisture aloft
to saturate the DGZ over the Absaroka, Lewis Range, and Tetons.
There remains some uncertainty with varying solutions on the
depth/phasing of two 500mb disturbances over the Northwest. The GFS
is the most amplified of the bunch and is more amplified compared
to the GEFS mean, while the ECMWF/UKMET/EC-AIFS are in the middle
ground compared to the more overly amplified GFS and weaker/sheared
CMC solution.

With a lack of sub-freezing air ahead of the trough, even with a
encroaching colder, western Canadian high pressure system on
Sunday, this setup tends to favor the higher elevations of the
Northern Rockies. While a more amplified solution like the GFS
cannot be ruled out yet, the antecedent air-mass both ahead of the
storm and trailing behind the system are not overly impressive for
mid-October. As high pressure builds in from the north, colder
temperatures and easterly upslope flow will keep snow lingering
along the Lewis Range into Sunday night. Snowfall rates could
approach 1"/hr in the Lewis Range and around Glacier Nat'l Park
Sunday morning, which could also foster hazardous travel conditions
in nearby passes.

Additionally, a separate upper low is forecast to drop south into
the Pacific Northwest on Sunday within the same longwave western
U.S. trough. This will allow for snow levels to remain rather low
for this time of year (around and slightly below 4,000 ft) across
the WA Cascades along with favorable upper ascent, which may lead
to some light accumulating snow for the major mountain passes.

WPC probabilities through early Monday depict moderate chances
(40-70%) for snowfall >8" in the peaks of the Lewis Range above
6,000ft (including Glacier Nat'l Park) on south along the
Absaroka/Tetons above 8,000ft through Sunday afternoon. Lighter
snowfall accumulations as low as 5,000ft in the Lewis
Range/northwest Montana and 7,000ft in the Absaroka/Tetons are
expected. WPC's Probabilistic Winter Storm Severity Index (WSSI-P)
show similar chances for Minor Impacts for the event in these
mountain ranges. Other mountain ranges are likely to receive light
snow totals (2-4") as far west as the Bitterroots and as far south
as the Teton and Wind River Ranges in western Wyoming.


The probability of significant freezing rain across CONUS is less
than 10 percent.


Snell/Mullinax




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FOUS11 KWBC 110736
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
336 AM EDT Sat Oct 11 2025

Valid 12Z Sat Oct 11 2025 - 12Z Tue Oct 14 2025

...Northern Rockies & WA Cascades...
Days 1-2...

Forecast remains on track for heavy snow to impact the high terrain
of the northern Rockies and WA Cascades beginning late tonight/early
Sunday morning and lasting through early next week, with certain
valley locations also potentially seeing the first snowflakes of
the season. Upper dynamics responsible for this October mountain
snow include a sharp upper trough crossing the northern Great Basin
today before taking on a negative tilt over the northern High
Plains on Sunday in tandem with a favorable duel jet structure.
This places the northern Rockies in an area of enhanced upper
divergence and lift, as well as crashing snow levels to allow for
precipitation to transition from rain to snow for areas above
5,000-6,000 ft. Snow levels are forecast to drop even lower on
Sunday across northwestern MT, where cold Canadian air noses
southward due to high pressure building to the north. This will
also allow for enhanced easterly upslope flow into the Lewis Range
and Glacier Natl Park region. Snowfall rates could peak at 1-2"/hr
in this part of northwest MT Sunday morning according to WPC's
Snowband Probability Tracker and make for difficult driving
conditions, especially at some of the major mountain passes in the
region.

Meanwhile, a separate deepening upper low will rotate southward
within the broad western U.S. trough by Sunday night over western
WA and provide additional lift along with a sinking cold front into
northern WA. Here, snow levels are expected to drop below 3,500ft
across the northern Cascades and contain light to moderate
snowfall at times into early Monday. WPC probabilities for greater
than 8" through Monday morning are high (70-905) across the Lewis,
Swan, and Mission ranges of northwest MT, as well as the northern
Cascades above 6,000ft. Probabilities for at least 8" of snow are
moderate (40-70%) across southwest MT and the northern Absarokas.


...Sierra Nevada...
Day 3...

By the end of day 3 (12Z Tuesday), the aforementioned upper low is
expected to rapidly drop south along the northern/central CA coast
and orient a corridor of 500-600 kg/m/s IVT (above the 99th
climatological percentile per the 12z NAEFS) orthogonal to the
central Sierra Nevada. There does remain some uncertainty regarding
exact track of the upper low and timing of heavy precipitation,
with the GFS and UKMET remaining on the slower end. Regardless, as
the plume of moisture approaches the central Sierra on Tuesday snow
levels are expected to remain around 6,000-7,000ft and snow ratios
should remain capped at around 8-10:1 given the moist fetch off
the Pacific. WPC probabilities for at least 8" of snow in the
central Sierra are currently moderate (30-60%) above 8,000ft through
the end of day 3, with more snow potential likely into the day 4
timeframe.


The probability of significant freezing rain across CONUS is less
than 10 percent.


Snell


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FOUS11 KWBC 120744
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
344 AM EDT Sun Oct 12 2025

Valid 12Z Sun Oct 12 2025 - 12Z Wed Oct 15 2025

...Northern Rockies & Pacific Northwest...
Days 1-2...

By the start of the forecast period (12Z Sunday), a potent
shortwave upper-level trough will be quickly exiting the northern
Rockies along with an associated strong surface low pushing
northeastward into southern Saskatchewan. Gradually increasing
heights are then expected across the northern Rockies through the
remainder of the day 1 forecast period, while a rapidly deepening
upper low drops southward along the coast of the Pacific Northwest.
At the surface, cold Canadian high pressure nosing southward into
northwest MT will allow for favorable upslope ascent in this
region, with general low-to-mid level convergence extending across
northern WA and ID along a sinking frontal boundary. This will
favor light to moderate snow across areas above 5,000ft, but with
snow levels dropping enough for flakes to reach into parts of the
MT Front Range and adjacent High Plains at times on Sunday night.
For the Cascades of WA and OR, snowfall will be tied to the
dropping upper low as snow levels start around 4,000-5,000ft
(lowest in the northern WA Cascades) before eventually lowering to
around 3,000ft in WA and 4,500ft in OR on day 2.

WPC probabilities show moderate-to-high probabilities (50-80%) for
storm-total snowfall amounts >8" from the Cascades on eastward into
the northern Idaho Panhandle to the Lewis Range of Montana.
Mountains within and around Glacier Nat'l Park even have moderate
chances (40-60%) for over 12" of snow, particularly above 7,000ft.
The WSSI shows mostly Minor Impacts due to potentially slick travel
conditions, while some passes in Montana and Glacier Nat'l Park
may contend with Moderate Impacts that could result in road
closures.

...California...
Days 2-3...

...First heavy snowfall event of the season set to impact the
Sierra Nevada beginning Monday night...

By day 2, the upper low dropping southward along the West Coast is
forecast to deepen into a robust 500mb low off the northern
California coast. This sudden amplification of the 500mb trough
(18Z ECMWF shows 500mb heights below the 0.5 climatological
percentile and nearing October records off the California coast
Monday afternoon) is due to an impressive anti- cyclonic weak break
over British Columbia. The dramatic height falls and modest
500-700mb CAA aloft will support lowering snow levels down to
around 6000-6500ft through the Siskiyou and the Sierra Nevada
starting Monday and continuing into the first half of next week.
All guidance also shows a healthy IVT topping 400 kg/m/s helping to
direct plenty of Pacific moisture at the mountain ranges. Snow
arrives over the northern California mountains on Monday with the
heaviest snowfall occurring Monday night into Tuesday over the
Sierra Nevada. Guidance has come in snowier over the past 12-24
hours, and given the lack of deep, cold air aloft, snow is likely
to be a heavy/wet snowfall along the Sierra Nevada. This could
raise concerns for potential impacts to trees and infrastructure
given this is the first significant mountain snowfall in the Sierra
Nevada this season.

WPC probabilities show moderate-to-high chances (50-80%) for at
least 24" of snowfall throughout the central Sierra Nevada (above
8,000 feet) by Wednesday morning, with maximum amounts up to 36"
possible. Latest WSSI-P shows high chances (70-99%) for Moderate
Impacts along the central Sierra Nevada with the Snow Amount, Snow
Load, and Snow Rate impacts being the main concerns for the first
significant snow of the season. Current WSSI also highlights Major
to Extreme Impacts, which would imply difficult to impossible
travel at times between Monday night and much of the day on Tuesday,
including for many major passes.


The probability of significant freezing rain across CONUS is less
than 10 percent.


Snell/Mullinax





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FOUS11 KWBC 130745
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
345 AM EDT Mon Oct 13 2025

Valid 12Z Mon Oct 13 2025 - 12Z Thu Oct 16 2025

...California & Central Nevada...
Days 1-2...

...First significant snowfall event of the season set to impact
parts of the Sierra Nevada beginning Monday night...

An upper low dropping southward along the West Coast is forecast
to deepen into a robust 500mb closed low off the California coast
by Monday night before swinging inland across central California on
Tuesday and eventually the central Great Basin on Wednesday. This
rapid amplification of the 500mb closed low (18Z ECMWF shows 500mb
heights below the 0.5 climatological percentile and nearing October
records off the California coast Tuesday morning) is due to an
impressive anti- cyclonic wave break over British Columbia. Height
falls and modest 500-700mb CAA aloft will support lowering snow
levels down to around 6000-6500ft through the Siskiyou and the
Sierra Nevada starting today and continuing through early
Wednesday, with snow levels remaining around 7,000ft to start in
the southern Sierra within the more robust precipitation axis. A
healthy IVT topping 400 kg/m/s is helping to direct a rich plume of
Pacific moisture at the mountain ranges. However, recent model
trends over the last 24 hrs have indicated a slightly further south
location of the upper- low, which lowers the QPF somewhat across
the central Sierra due to less orthogonal (more southerly) flow
into the Sierra Nevada terrain. This produces better upslope flow
into the southern CA ranges (ptype primarily rain) and southern
Sierra Nevada versus the prior forecasts centered on the central
Sierra. Regardless, snow still arrives over the northern California
mountains today with the heaviest snowfall occurring late tonight
into Tuesday over the central/southern Sierra Nevada. Given the
lack of deep, cold air aloft, snow is likely to be a heavy/wet
snowfall along the Sierra. This raises concerns for potential
impacts to trees and infrastructure given this is the first
significant mountain snowfall in the Sierra Nevada this season.
WPC's Snowband Probability Tracker shows HREF guidance identifying
2-3"/hr rates over the southern/central Sierra Nevada Monday night
into Tuesday. Look for some heavy snowfall to spill into central
Nevada's taller ranges with anywhere from 12-24" of snowfall
possible over 8,000ft.

WPC probabilities show moderate-to-high chances (50-80%) for at
least 24" of snowfall throughout the central Sierra Nevada (above
8,000 feet) through Wednesday with maximum amounts up to 36"
possible. Latest WSSI-P shows moderate-to-high chances (50-80%) for
Major Impacts along the central Sierra Nevada with the Snow
Amount, Snow Load, and Snow Rate impacts being the main concerns
for the first significant snow of the season. Current WSSI also
highlights Major to Extreme Impacts, which would imply dangerous
to impossible travel at times between Monday night and much of the
day on Tuesday, including for many major passes.


The probability of significant freezing rain across CONUS is less
than 10 percent.


Mullinax/Snell

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