FOUS11 KWBC 220653
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
253 AM EDT Tue Apr 22 2025

Valid 12Z Tue Apr 22 2025 - 12Z Fri Apr 25 2025


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

A drier stretch is expected to prevail today outside of a few high
elevation snow showers near Yellowstone before the next shortwave
slides over the northern Rockies Wednesday night into Thursday.
This feature will spread snow across southwestern MT and
northwestern WY ahead of a much deeper upper trough approaching the
West Coast by the end of D3. Broad west-southwesterly flow will
aid in overspreading Pacific moisture into the region, while also
rising snow levels to around 6,000-8,000 ft during this time frame.
Amounts will be generally light, focused on the Absarokas, Wind
River, and Bighorns, where WPC probabilities for at least 8 inches
of snow through 12Z Friday are low (20-40%) at the highest
elevations.

The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.


Snell




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FOUS11 KWBC 221914
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
314 PM EDT Tue Apr 22 2025

Valid 00Z Wed Apr 23 2025 - 00Z Sat Apr 26 2025


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

Shortwave moving into/through Oregon this evening/overnight will
spread a new round of generally light snow across southwestern MT
and northwestern WY as it deepens over the northern Great Basin.
Upper jet will be positioned over the northern Plains, aiding in
ascent over the region where temperatures are mostly mild,
confining snowfall to areas above 7000ft or so. This feature will
weaken as it pushes through Idaho and into western Montana Thursday
night with snow tapering off by Friday morning. WPC probabilities
for at least 8 inches of snow are moderate (40-70%) over the
Absarokas and Wind River ranges and generally less than 50% over
the Bighorns.


...Northern California...
Day 3...

A deep upper trough in the northeastern Pacific will dig
and deepen into a closed low just west of NorCal by Friday, sending
in some moisture to the region. Snow levels will start high,
confining generally light snow to areas above 7000ft where WPC
probabilities are low (10-40%) through 00Z Sat. More snow is
expected thereafter.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.

Fracasso


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FOUS11 KWBC 230654
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
254 AM EDT Wed Apr 23 2025

Valid 12Z Wed Apr 23 2025 - 12Z Sat Apr 26 2025


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

Shortwave moving through OR this morning will spread a new round
of generally light snow across southwestern MT and northwestern WY
as it deepens over the northern Great Basin tonight. Upper jet
will be positioned over the northern Plains, aiding in ascent over
the region where temperatures are mostly mild, confining snowfall
to areas above 7000ft or so. This feature will weaken as it pushes
through ID and into western MT Thursday night with snow tapering
off by Friday morning. WPC probabilities for at least 8 inches of
snow are moderate (40-70%) over the Absarokas and Wind River ranges
and generally less than 40% over the Bighorns.


...Oregon & California...
Day 3...

A potent upper trough in the northeastern Pacific will dig and
deepen into a closed low just west of NorCal by Friday, sending in
some moisture to the region. This upper low is then forecast to begin
moving inland over central CA Friday night, with some lingering
uncertainty regarding exact timing. Snow levels will start high and
around 7000ft, but fall to around 5000ft by 12Z Saturday just as
the precipitation shield becomes more expansive in response to the strengthening upper jet over SoCal. WPC probabilities on day 3 for
at least 6 inches are low (10-40%) across the southern OR Cascades
into the NorCal ranges and central Sierra Nevada. More snow is
expected into the day 4 timeframe.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.


Snell





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FOUS11 KWBC 231915
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
315 PM EDT Wed Apr 23 2025

Valid 00Z Thu Apr 24 2025 - 00Z Sun Apr 27 2025


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

A shortwave moving through the northern Great Basin tonight will
spread generally light snow across central ID, southwestern MT, and northwestern WY Thursday as it deepens a bit then lifts
northeastward into southwestern Montana tomorrow afternoon. Right
entrance region of a retreating upper jet will promote broad ascent
over the region where temperatures are mostly mild, confining
snowfall to areas above 7000ft or so. Snow will taper off by Friday
morning as the feature weakens over western Montana. WPC
probabilities for at least 8 inches of snow over the next 36 hours
are moderate (40-70%) over the Absarokas and Wind River ranges and
generally less than 50% over the Bighorns.


...Oregon & California...
Days 2.5-3...

A potent upper trough in the northeastern Pacific Thursday will
dig and deepen into a closed low just west of NorCal by Friday,
sending in some moisture to the region as early as Thursday
evening. This upper low is then forecast to move inland over
central CA Saturday morning, with a trend to the south since
yesterday. Snow levels will start high (over 7000ft), but fall to
below 6000ft by early Saturday as the precipitation shield expands
in response to the strengthening upper jet over SoCal. WPC
probabilities for at least 6 inches of snow are low (~10%) for the
Oregon Cascades due to the decrease in QPF, low (10-40%) over the
northern CA ranges, and moderate (40-70%) over the northern/central
Sierra Nevada (generally above 7000ft). Snow is expected to
continue into the day 4 timeframe.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.


Fracasso



$$

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FOUS11 KWBC 240714
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
314 AM EDT Thu Apr 24 2025

Valid 12Z Thu Apr 24 2025 - 12Z Sun Apr 27 2025


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

A shortwave trough over the Great Basin (axis currently over the
NV/ID border) will shift ENE over the Yellowstone region this
evening. Some moist flow ahead of the trough will lift over a
stationary front over southern WY and continue to bring light to
locally moderate mountain snow until the trough crosses this
evening to higher portions of the Absarokas, Wind River, and
Bighorns, where Day 1 probabilities for an additional >4" snow
after 12Z are 10-30%. Snow levels in this mild airmass are
generally 7000ft or higher in northern WY.


...Sierra Nevada...
Days 2/3...

A negatively-tilted upper trough reaches the central CA coast
tonight as an upper low develops offshore and tracks to the central
CA coast through Friday night. This low then drifts to southern NV
through Saturday night. Moisture inflow and topographical lift over
the central Sierra become sufficient for higher elevation snow by
later Friday afternoon with a prolonged light to locally moderate
snowfall then into Sunday. Snow levels begin around 6500ft, but
drop to as low as 5000ft under the upper low. Day 2.5 WPC
probabilities for >6" are 30-50% for the Sierra Nevada generally
above 7000ft in 24hrs (00Z Sat to 00Z Sun).


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.


Jackson


$$

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FOUS11 KWBC 241934
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
334 PM EDT Thu Apr 24 2025

Valid 00Z Fri Apr 25 2025 - 00Z Mon Apr 28 2025


...Sierra Nevada...
Days 1-3...

Digging trough west of 130W this evening will slip into the CA
coast overnight with light snow breaking out tomorrow over the high
Sierra as initial height falls cross the mountains. Upper low will
eventually come ashore on Saturday with downstream light to
occasionally moderate snow for the Sierra as snow levels lower to
around 5000-6000ft with the approach of the cold core. By Sunday,
the upper low will cross through the Great Basin, with snow ending
from west to east. WPC probabilities for at least 8 inches of snow
over the next three days are >50% above 6500ft or so.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.

Fracasso

$$

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FOUS11 KWBC 250713
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
313 AM EDT Fri Apr 25 2025

Valid 12Z Fri Apr 25 2025 - 12Z Mon Apr 28 2025


...Sierra Nevada...
Days 1-2...

Positively-tilted trough approaching the central/northern CA coasts
will provide some onshore flow and High Sierra snow today as
initial height falls cross the mountains with snow levels 7000ft or
higher. Slight southward trend continues in upper low track that
reaches the central CA coast Saturday with downstream light to
occasionally moderate snow for the Sierra through Saturday evening
as snow levels lower to around 5000ft. By Sunday, the upper low
will cross through the Great Basin, with Sierra snow ending from
west to east. Day 2 WPC probabilities for >6" are 20-30% in the
5500-6500ft range and 30-60% above 6500ft or so.


...Great Basin and Northern Rockies...
Day 3...

The upper low that crosses the Central CA Coast Saturday pivots
northeast from southern NV to central WY Sunday through Sunday
night. Left exit jet dynamics aid lift ahead of the low where
sufficient moisture is present for valley rain and mountain snow
with snow levels around 6500ft. Day 3 snow is most likely over the
NV ranges, northern Wasatch, with Sunday night snow bands
developing over the Absarokas where there's potential for brief
bursts of heavy snow.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.


Jackson


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FOUS11 KWBC 251712
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
112 PM EDT Fri Apr 25 2025

Valid 00Z Sat Apr 26 2025 - 00Z Tue Apr 29 2025


...Sierra Nevada and Great Basin...
Days 1-3...

An impressive closed mid-level low (NAEFS 500mb height anomalies
falling below the 1st percentile) will move onshore southern CA
Saturday aftn before slowly filling while lifting northeast through
the Great Basin on Sunday. Downstream of these robust height
anomalies, significant synoptic lift will occur downstream through
height falls, mid-level divergence, and overlapping upper
diffluence as the LFQ of a strengthening subtropical jet streak
digs through the trough axis and ejects meridionally. The overlap
of this ascent into a region moistened by 700mb S/SW flow and an
accompanying ribbon of IVT above the 90th percentile (from NAEFS)
will result in widespread precipitation Saturday morning through
Sunday afternoon.

Snow levels during this time will be quite low in
response to the anomalous upper low, falling to around 4500 ft,
which is nearing the 10th percentile. This suggests that snow will
accumulate across much of the CA and Great Basin terrain above this
level through Sunday, but at least light accumulations or mixing
with snow is possible much lower due to steep lapse rates and
dynamic cooling. The heaviest accumulations, however, should remain
across the Sierra, where D1 and D2 probabilities for 6+ inches
exceed 70%, and 30%, respectively, and locally more than 12 inches
is possible before precipitation wanes late D2.

As this upper low continues to track northeast, the accompanying
moisture and ascent will move across the Great Basin, bringing some
light to moderate snow to the higher elevations of NV through
Monday morning. WPC probabilities D3 reach as high as 50% for 6+
inches across the Ruby Mountains.


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

The closed low ejecting from the Great Basin will fill and open
into a positively tilted trough Monday as it shifts east into the
Rockies. Despite this evolution, deep layer ascent will remain
impressive as periodic lobes of vorticity swing northeast
downstream of the trough axis, and work together with mid-level
divergence and upper diffluence in the LFQ of a jet streak arcing
into the Northern Plains. Together, this will cause pressure falls
over CO/WY leading to surface cyclogenesis, and this low will then
track northeast into the Dakotas and Minnesota by the end of D3.
While there is good model agreement in the general synoptic setup
and evolution, low-clusters from the various ensembles still
feature considerable spread in both timing and placement,
additionally reflected by modest differences in the primary 500mb
EOF on the D3 WPC clusters.

The primary driver of these variances appears to be the speed at
which this trough opens and ejects, with 2/3 of the ECENS
suggesting a lower ejection, while 50% of the GFS members make up
the faster end of the envelope, and by the end of D3 there are
height differences among the cluster means of more than 80m across
the Central High Plains. At this time, a solution somewhere between
the camps as a consensus is probably best.

While the exact placement of the low and its associated synoptic
forcings will be critical, a cold front digging south through the
Northern Rockies Sunday night into Monday will likely interact with
an inverted surface trough, leading to enhanced ascent across parts
of MT and WY D3. This is likely to occur regardless of the low
evolution, but a slower system may produce more enhanced ascent and
stronger accompanying deformation than a faster ejecting system.
Either way, impressively combined upslope with fgen should result
in heavy snow rates, with dynamic cooling response to the column
occurring even outside of the higher terrain. Snow levels will be
generally low anyway, (near the NBM 10th percentile after
coordination with the local WFOs), suggesting that while the
heaviest accumulation will remain above 5500 ft, significant
accumulations are possible to 4500 ft, and lighter accumulations
even lower than that. This is reflected by WPC probabilities that
are high (>70%) D3 across the Big Horns, Absarokas, and other
terrain around Yellowstone NP, but also much more widespread
moderate probabilities for 2" expanding across much of the ID/MT/WY
juncture.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.


Weiss


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FOUS11 KWBC 260856
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
456 AM EDT Sat Apr 26 2025

Valid 12Z Sat Apr 26 2025 - 12Z Tue Apr 29 2025


...California and Nevada...
Days 1-2...

Upper low crosses the central CA coast this morning before slowly
filling while lifting northeast, reaching south-central NV by
Sunday morning. Lift ahead of the low, aided by left jet exit
dynamics, overlapping sufficient Pacific moisture will allow
expanded precip coverage over CA into this evening with the focus
over NV and the eastern side of the Sierra Nevada overnight through
Sunday.

Snow levels will be 5000-5500ft through Sunday morning. 00Z HREF
mean hourly snow rates are generally 1-1.5"/hr from 12Z to 00Z on
the Sierra Nevada.Day 1 snow probs for >6" are 50-80% for the
Sierra Nevada and highest Transverse Ranges in SoCal. Eastern
slopes of the Sierra Nevada are favored for the heaviest snow with
50-80% probs for >12" over the White Mtns and other ranges along
the NV border up through the eastern side of Tahoe.

As this upper low continues to track northeast, the accompanying
moisture and ascent will bring light to moderate snow to the
higher elevations of NV through Sunday night. Day 2 WPC
snow probs are 50-80% >6" across the Ruby Mountains of NV.


...Northern Rockies...
Days 2/3...

The closed low drifting from the Great Basin Sunday night will
fill and open into a positively-tilted trough Monday as it reaches
WY. Despite the weakening, deep layer ascent will remain
impressive as periodic lobes of vorticity swing northeast
downstream of the trough axis, and work together with mid-level
divergence and upper diffluence in the left exit region of the jet
streak arcing into the Northern Plains. Together, this will cause
pressure falls over CO/WY leading to surface cyclogenesis on the
High Plains.

A cold front digging south through the Northern Rockies Sunday
night into Monday will approach the Plains surface low, leading to
enhanced ascent across parts of MT and WY. This slow moving system
will allow upslope flow with fgen, resulting in heavy snow rates,
with dynamic cooling response to the column occurring even outside
of the higher terrain. Snow levels will be generally around 6000ft.
Day 2 snow probs for >8" are 30-40% in the eastern Sawtooths in ID
and 50-80% over the northern Absarokas in MT. This shifts east for
Day 3 over the eastern Absarokas and Bighorns in WY.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.



Jackson



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FOUS11 KWBC 261813
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
213 PM EDT Sat Apr 26 2025

Valid 00Z Sun Apr 27 2025 - 00Z Wed Apr 30 2025


...California and Nevada...
Day 1...

An intense closed mid-level low (500-700mb heights below the 1st
percentile according to NAEFS) will move onshore southern
California just before the start of the forecast period, with
gradual filling/weakening occurring thereafter as it lifts
northeast. As this low moves from CA through the Great Basin, it
will drive deep layer ascent through height falls and downstream
500mb divergence, which will overlap at least modestly with a
strengthening jet streak wrapping around the downstream side of the
trough. Together, this will result in periods of moderate to heavy precipitation, which will fall as snow in the higher elevations.
While most of the snow accumulations will be above 5500 ft across
the Sierra and into the Ruby Mountains, where WPC probabilities
for 6+ inches reach 70-90%, the anomalously cold pool aloft and
steep lapse rates may allow for at least light accumulations down
below the NBM 10th percentile snow level, or as low as 4000 ft
before the system ejects to the northeast and ends on D2.


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

The closed low emerging from the Great Basin will pivot into WY/UT
and then begin to fill/open into a trough as it shifts east slowly
into Tuesday. DUring this evolution, the primary trough axis will
become positively tilted and split as northern stream energy ejects
into the Great Lakes while a southern stream shortwave stalls over
the Desert Southwest. This development will result in prolonged
deep layer ascent beneath the slowly advancing trough, with
additional, but weakening, ascent through the LFQ of a downstream
jet streak helping to produce surface cyclogenesis from CO into MN
during D2. As this low tracks northeast, a cold front will dig out
of Canada, leading to enhanced post-frontal ascent into the terrain
and increased fgen in the vicinity of the front. This will work in
tandem with some increased fgen and deformation which develops on
the NW side of the surface low to enhance ascent and increase the
intensity of precipitation from western ND through eastern ID.

The guidance has trended a bit slower and warmer with the
evolution, leading to snow levels and an atmospheric column that
are warmer, at least at precipitation onset around 00Z-06Z Monday.
Thereafter, however, the combination of the cold front and the
increasing ascent through the mesoscale forcing (fgen and
deformation) will help dynamically cool the column and lower snow
levels such that heavy snow rates will accumulate efficiently in
both higher terrain and some of the elevated valleys. There is
still uncertainty into how much snow can accumulate in the lower
levels, especially below around 4500 ft (NBM 10th percentile snow
level). However, it is likely that a lower-than-climo SLR will
result in heavy wet snow that should be impactful to many areas as
snowfall rates potentially reach 1-2"/hr at times, and this is
reflected by WSSI featuring moderate to locally major impacts,
focused in the Absarokas, Big Horns, and in the vicinity of
Yellowstone NP. These areas are also where the highest WPC
probabilities for 6+ inches exist, reaching above 90%, with locally
1-2 feet possible in the Absarokas.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.



Weiss


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FOUS11 KWBC 270836
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
436 AM EDT Sun Apr 27 2025

Valid 12Z Sun Apr 27 2025 - 12Z Wed Apr 30 2025


...Great Basin and Northern Rockies...
Days 1-2...

Upper low over central NV will slowly track to northern UT through
today where it will stall and fill through tonight. Mountain snow
continues over the higher NV ranges with snow levels of
6000-7000ft. The Ruby Mtns stand out in NV for impacts with Day 1
snow probs for >8" 50-90%.

A reinforcing trough ejects over the northern High Plains tonight
into Monday which allows a slowly pivoting swath of moderate to
locally heavy precip this afternoon through Monday morning from
central ID through southern MT. Snow levels in this swath start
high, above 8000ft through this evening, but decrease overnight to
5000 to 6000ft over the northern Absarokas. The resulting
positively-tilted trough axis shifts south Monday bringing some
moderate snow to terrain in WY. Day 1.5 snow probs for >8" are
70-90% in the Red Lodge area of the northern Absarokas in southern
MT (where storm totals over 2ft are locally likely) and 50-80% in
the Madison/Gallatin ranges, Absarokas east of Yellowstone, and the
Bighorns.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.



Jackson



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FOUS11 KWBC 271831
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
231 PM EDT Sun Apr 27 2025

Valid 00Z Mon Apr 28 2025 - 00Z Thurs May 1 2025


...Great Basin and Northern Rockies...
Days 1-2...

An upper low traversing the Great Basin today will be responsible
for periods of higher elevation snow through the northern Nevada
ranges this afternoon and evening. Snow levels will be as low as
6,000ft in some cases, but the heavier amounts are likely to resume
above 7,000ft. As the upper low weakens and moves east tonight,
mountains snow will ensue within the more remote reaches of the
Wasatch and Uinta of Utah, while minor amounts are possible as far
east as the peaks of the Colorado Rockies through Monday morning.
Additional snowfall accumulations of 2-6" are forecast in the

6,000ft ridge lines of northern Nevada through early Monday

morning.

The ranges most likely to see the heaviest snowfall will be in the
Absaroka, the Tetons, and the Big Horns. These ranges will reside
favorably beneath the the best PVA at 250-500mb and a pivoting axis
of >90th climatological percentile 500-700mb moisture content
aloft. 850-700mb winds will back of out of the NE-E as the upper
low slides south of these ranges and a surface cold front passes
through tonight. While the cold front does provide a brief
infection of CAA aloft, the cold air source is not sufficient
enough to produce much more than minor snowfall accumulations
below 7,000ft. The heaviest amounts will be confined to elevations
at/above 8,000ft in the ranges listed above, with some locally
heavier amounts as low as 7,000ft in the Big Horns. Snow tapers off
by Monday afternoon and evening as the upper low races east
towards the Northern Plains Monday evening. WPC probabilities
highlight high chances (>70%) for snowfall totals >12" in the
Absaroka above 7,000ft with some totals in the Absaroka-Beartooth
Wilderness seeing totals above 24 inches in some cases above
9,000ft. WPC probabilities show moderate-to-high chances (50-70%)
for snowfall totals >12" in the Big Horns above 7,000ft, while
similar chances for >6" of snow exist in the Tetons above 8,000ft.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.


Mullinax






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FOUS11 KWBC 280833
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
433 AM EDT Mon Apr 28 2025

Valid 12Z Mon Apr 28 2025 - 12Z Thu May 01 2025


...North-Central Rockies...
Day 1...

Positively-tilted trough axis from the western Dakotas to Nevada
will sag south today as a leading shortwave tracks to the Upper
Midwest while a filling low drifts south over Utah. The surface low
associated with the leading wave is over SD with comma head/cold
conveyor belt flow resulting in a swath of moderate to locally
heavy precip across southern/eastern MT. Snowfall rates above the
6500ft snow levels in southern MT around 7000ft will rapidly rise
through this morning, particularly in the Red Lodge portion of the
northern Absarokas where rates will soon exceed 2"/hr per the 00Z
HREF. This precip swath will shift south over WY today where snow
levels will also be around 7000ft and rates in the higher terrain
will generally be 1-2"/hr. Day 1 snow probs for >8" additional
snow after 12Z are 40-70% in the Absarokas and 60-90% in the
Bighorns.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.


Jackson



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FOUS11 KWBC 281834
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
234 PM EDT Mon Apr 28 2025

Valid 00Z Tue Apr 29 2025 - 00Z Fri May 02 2025


...Northern & Central Rockies...
Days 2-3...

After light-to-moderate snow quickly ends across WY by the start
of day 1 following the departure of a postively-tilted trough
sliding eastward into the Upper Midwest, a separate upper shortwave
and associated cold front is forecast to swing over the northern
Rockies on Wednesday. This will lead to an area of broken
precipitation with an upslope emphasis on the highest peaks and
northward facing slopes as the system slides south into the CO
Rockies on Thursday. Snow levels will fall below 8,000ft in western
MT and northwest WY shortly behind the cold front and as low as
6,000ft. However, these relatively lower snow levels also coincide
with a drier column and only lighter precipitation. For the
central Rockies, snow levels are expected to remain around
8,000-9,000ft through the end of day 3. WPC probabilities for at
least 6 inches of snow during the days 2-3 time frame are moderate
(40-70%) across the high elevations of southwest MT, WY, and CO.
This primarily includes the Absarokas, Bighorns, Wind River Range,
and Medicine Bow Mts above 9,000-10,000ft.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.


Snell






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FOUS11 KWBC 290834
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
434 AM EDT Tue Apr 29 2025

Valid 12Z Tue Apr 29 2025 - 12Z Fri May 02 2025


...Montana, Wyoming, and Colorado...
Days 1-3...

The next system of note in the West is an upper shortwave and
associated cold front that shifts south over the northern Rockies
from MT this afternoon through Wednesday. Snow levels of
6000-7000ft are expected in western MT and closer to 8000ft in
northwest WY tonight. The parent trough shifts east over the
northern Plains Wednesday night as the cold front slowly shifts
south over CO into Thursday. However, snow levels remain around
8,000-9,000ft through Thursday.

Day 1 snow probs for >6" are 30-50% in ranges around Glacier NP.
Day 1.5 snow probs for >6" are 30-70% over the Absarokas, Tetons,
and Bighorns. Day 2.5 snow probs for >6" are 40-80% for the Front
Range in CO.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.


Jackson



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FOUS11 KWBC 291801
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
201 PM EDT Tue Apr 29 2025

Valid 00Z Wed Apr 30 2025 - 00Z Sat May 03 2025


...Northern/Central Rockies...
Days 1-2...

Positively-tilted trough over southwestern Canada/northern Divide
this evening will carry a cold front through the region with colder
air in its wake and lowering snow levels. Precipitation will
translate from northwest to southeast Days 1-2 from southwestern MT
through Wyoming then into the CO Rockies. By Wednesday evening,
the trough will moving into the High Plains as heights rebound into
Day 3, ending any snowfall over the region. Snow levels will start
around 8000ft near/ahead of the cold front then drop to
6500-7000ft during the morning hours behind the front, only to rise
again over 7000ft during the day. This will confine the most snow
to the mountain ranges like the Absarokas, Tetons/Wind River,
Bighorns, and CO Rockies. WPC probabilities for at least 8 inches
of snow for this period are at least 50% above 10,000ft or so.

The probability of at least 4 inches of snow on Day 3 is less than
10%.

The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.


Fracasso


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FOUS11 KWBC 300838
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
438 AM EDT Wed Apr 30 2025

Valid 12Z Wed Apr 30 2025 - 12Z Sat May 03 2025


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

Positively-tilted trough over the northern Rockies this morning
with a vort lobe over the Bitterroots will drift south through
Thursday. Focus for precip is along a surface boundary ahead of the
upper trough axis with light to moderate rates over WY today and CO
tonight Thursday. Snow levels will be elevated - around 8000ft
in WY and 9000ft in CO. Day 1 snow probs for >6" are around 30% in
the southern Absarokas/Wind River and 40-60% over the Bighorns. Day
1.5 snow probs for >6" are 40-70% through the Front Range and the
Mosquito Range.


The probability of any freezing rain across CONUS is less than
10%.


Jackson



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FOUS11 KWBC 301814
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
214 PM EDT Wed Apr 30 2025

Valid 00Z Thu May 01 2025 - 00Z Sun May 04 2025


...Central/Southern Rockies...
Days 1-2...

Cold front associated with a shortwave over the Northern Plains to
the central Rockies will continue southeastward through Colorado
and into New Mexico Thursday. QPF has decreased in the past 24
hours but some light snow over the high mountains will fall,
generally above 8000-9000ft, where WPC probabilities for at least 4
inches of snow are >50% above 10,000ft or so. Snow will end from
north to south over the San Juans and Sangre de Cristos Thursday
evening.

...Sierra Nevada...
Days 1&3...

Combination of a small upper low off the central/southern CA coast
tonight into Thursday with a trough axis across the Great Basin
will bring some light snow to the high Sierra tonight/Thursday.
Amounts may only be an inch or two with low probabilities of at
least 4 inches (<40%) as snow levels remain above 9000ft.

After a break on Day 2, a deeper trough with origins in the
northeastern Pacific will bring in a more robust moisture plume to
the West Coast on Day 3 (Saturday) that will continue into the
medium range. Through 00Z Sun, snow levels will be high (>9000ft)
with the initial surge in (modest) moisture, yielding low
probabilities for at least 4 inches of snow (<30%) in the central
Sierra as well as across the highest peaks of the Oregon Cascades.


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

Fracasso


$$

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FOUS11 KWBC 010703
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
303 AM EDT Thu May 1 2025

Valid 12Z Thu May 01 2025 - 12Z Sun May 04 2025


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

A potent closed low will dig along the CA coast Saturday into
Sunday, with rapid amplification of this feature expected by Sunday
morning. This is reflected by NAEFS height anomalies falling to
below the 1st percentile with respect to 700-500mb heights over
southern CA by 12Z Sunday. Although the accompanying jet streak
will intensify as well as it pivots around the upstream side of
this trough and through the base, the overall evolution will keep
the most intense ascent displaced south/east of the Sierra, but
sufficient lift and increasing moisture will still result in
periods of snowfall D3. Snow levels during this time will begin
around 8000-9000 ft, so only the higher Sierra Passes will begin as
snow, but as the upper low digs across CA, snow levels are progged
to fall to as low as 7000 ft, bringing at least some accumulations
to Donner Pass as well. Overall, significant snowfall accumulation
will be confined to above pass level, where WPC probabilities for
more than 6 inches of snow are as high as 50%, but light snow down
to most Sierra Pass levels is also likely before precip winds down
by Sunday morning.


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

Weiss



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FOUS11 KWBC 011822
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
222 PM EDT Thu May 1 2025

Valid 00Z Fri May 02 2025 - 00Z Mon May 05 2025


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

A potent closed low will dig along the CA coast Saturday into
Sunday, with rapid amplification of this feature expected by Sunday
morning. This is reflected by NAEFS height anomalies falling to
below the 1st percentile with respect to 700-500mb heights over
southern CA between 12Z Sunday and 00Z Monday. Although the
accompanying jet streak will intensify as well as it pivots around
the upstream side of this trough and through the base, the overall
evolution will keep the most intense ascent displaced south/east of
the Sierra, but sufficient lift and increasing moisture will still
result in periods of snowfall D3. Snow levels during this time
will begin around 8000-9000 ft, so only the higher Sierra Passes
will begin as snow, but as the upper low digs across CA, snow
levels are progged to fall to as low as 7000 ft, bringing at least
some accumulations to Donner Pass as well. Overall, significant
snowfall accumulation will be confined to above pass level, where
WPC probabilities for more than 6 inches of snow are as high as
60%, but light snow down to most Sierra Pass levels is also likely
before precip winds down by Sunday midday.


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

Weiss/Snell






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FOUS11 KWBC 020709
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
309 AM EDT Fri May 2 2025

Valid 12Z Fri May 02 2025 - 12Z Mon May 05 2025


...Sierra Nevada...
Days 2-3...

An anomalous closed mid-level low (700-500mb heights reaching -3
sigma by Sunday aftn) will dive along the CA coast before turning
into the Four Corners states Sunday night. Impressive height falls
and downstream divergence will combine with a sharpening jet streak
to produce deep layer ascent across CA and into the Great Basin
late Saturday through late Sunday. This will result in expanding
coverage of precipitation, with snow likely in the higher terrain
of the Sierra. Initially, snow levels will be quite high, generally
8000-9000 ft, but as the closed low pivots south of the region,
colder air will funnel in and drop snow levels to 7000-7500 ft.
This will keep most of the heavy snow accumulations above the area
Passes, but at least light accumulations are possible even as low
as Donner Pass before precip wanes late Sunday. WPC probabilities
across the Sierra peak around 50% for 6+ inches of snow D2 into D3,
with the highest amounts expected in the higher terrain of the
southern Sierra where locally 8-10 inches is possible.


...Central Rockies...
Day 3...

A complex mid-level synoptic evolution will occur late in the
forecast period across the Intermountain West as the extremely
amplified flow across the CONUS exhibits minimal change into early
next week. Dual closed lows, one over the Ohio Valley and another
across the Four Corners will meander with little movement, as flow
stalls over the country. The important low for winter purposes will
be the one drifting across the Four Corners, which will shed
multiple lobes of vorticity around it as nearly a carousel of
shortwaves rotates southward from Canada and towards Baja and
around the primary closed low. The interaction of these shortwaves
with the primary low and secondary developing subtropical jet
streak will result in expanding precipitation, with snow likely in
the higher terrain of the San Juans before the close of D3 /12Z
Tuesday/. More significant precipitation, including more widespread
heavy snow across the Rockies, is possible beyond this forecast
period as a cold front drops through the region as well, but at
this time, for D3, WPC probabilities indicate a 70-90% chance of 6+
inches, focused in the higher elevations of the San Juans.



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

Weiss


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FOUS11 KWBC 021848
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
248 PM EDT Fri May 2 2025

Valid 00Z Sat May 03 2025 - 00Z Tue May 06 2025


...Sierra Nevada...
Days 1-3...

An anomalous closed mid-level low (700-500mb heights reaching -3
sigma by Sunday aftn) will dive along the CA coast before turning
into the Four Corners states Sunday night. Impressive height falls
and downstream divergence will combine with a sharpening jet streak
to produce deep layer ascent across CA and into the Great Basin
late Saturday through late Sunday. This will result in expanding
coverage of precipitation, with snow likely in the higher terrain
of the Sierra. Initially, snow levels will be quite high, generally
8000-9000 ft, but as the closed low pivots south of the region,
colder air will funnel in and drop snow levels to 7000-7500 ft.
This will keep most of the heavy snow accumulations above the area
Passes, but at least light accumulations are possible even as low
as Donner Pass before precip wanes late Sunday. WPC probabilities
across the Sierra peak around 50% for 6+ inches of snow D2, with
the highest amounts expected in the higher terrain of the southern
Sierra where locally 8-10 inches is possible.


...Central Rockies...
Days 2-3...

A complex mid-level synoptic evolution will occur late in the
forecast period across the Intermountain West as the extremely
amplified flow across the CONUS exhibits minimal change into early
next week. Dual closed lows, one over the Ohio Valley and another
across the Four Corners will meander with little movement, as flow
stalls over the country. The important low for winter purposes will
be the one drifting across the Four Corners, which will shed
multiple lobes of vorticity around it by the end of D2 as nearly a
carousel of shortwaves rotates southward from Canada and towards
Baja and around the primary closed low. The interaction of these
shortwaves with the primary low and secondary developing
subtropical jet streak will result in expanding precipitation, with
snow likely in the higher terrain of the northern/central Rockies
as well as the UT ranges by the close of D3 /00Z Tuesday/. Snow
levels are expected to remain rather high (9000-10000ft) outside of
the northern Rockies by Monday evening, where levels drop to
around 8000ft. More significant precipitation, including more
widespread heavy snow across the Rockies, is possible beyond this
forecast period as a cold front drops through the region. At this
time, primarily for D3, WPC probabilities indicate a 70-90% chance
of 6+ inches, focused in the higher elevations of the San Juans
above 11000ft with low probabilities (10-30%) for 12+ inches.



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

Weiss/Snell





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FOUS11 KWBC 030645
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
245 AM EDT Sat May 3 2025

Valid 12Z Sat May 03 2025 - 12Z Tue May 06 2025


...Sierra Nevada...
Days 1-3...

Light to at times moderate precipitation will persist across the
Sierra and Great Basin through Sunday as an anomalous closed low
drifts across CA and into the Four Corners region. Although this
feature will be impressive, the best ascent through height falls
and the overlapping of a subtropical jet streak rotating through
the base will occur east into a warmer column. This suggests that
the only significant accumulating snow will be across the Sierra,
generally above 7000 ft, with the heaviest accumulations in the
high southern Sierra. In this area, WPC probabilities indicate a
low to medium chance (30%) for 6+ inches both D1 and D2.


...Northern and Central Rockies...
Days 2-3...

The closed low drifting across the Desert Southwest Sunday will
lift slowly northeast into the Four Corners and then pause nearly
in place through the remainder of the forecast period. This slow
evolution is due to a highly amplified pattern across the CONUS
with dual closed lows bookending an impressive omega block across
the center of the Nation. With little impetus for features to move,
this will result in an extending period of unsettled weather
across the Rockies as height falls in the vicinity of the closed
low, modest diffluence within the LFQ of a modest but favorably
oriented upper jet streak, and a southeast advancing cold front
interact to provide broad and long-lasting lift to the region.

As this pattern evolves across the West, moisture will steadily
increase as a surge of theta-e lifts out of the Gulf and ascends
isentropically along the 305-310K surfaces as far north as CO and
UT as it wraps cyclonically around the low and into the Great
Basin. This will be most impressive late Monday into Tuesday thanks
to pinched and nearly unidirectional southerly flow through the
column helping to advance moisture return. Forcing impinging into
the moistening column will lead to rapid expansion of
precipitation, with much of the Intermountain West experiencing
rain and high-elevation snow beginning 00Z Monday, with periods of
heavy precipitation rates likely.

Snow levels during precipitation expansion will be quite high,
generally 9000-11000 ft across much of the Rockies. The exception
will be in the vicinity of a cold front dragging into ID/MT late
D2 into D3, behind which snow levels will crash to 4000-5500 ft.
These lowest snow levels are progged by the NBM to remain generally
north of Wyoming, but farther south into the Central and Southern
Rockies, snow levels will also fall beneath the core of the upper
low, reaching to 7000-8000 ft. These lowering snow levels will help
expand the wintry precipitation areas, with periods of heavy snow
becoming more widespread late D3, especially from ID through WY
where fgen along the front will drive more pronounced mesoscale
lift. It is across these areas, generally from Glacier NP southward
through the Absarokas, towards Yellowstone NP, and into the Wind
River range where snowfall will be heavy, as reflected by WPC
probabilities that reach as high as 50% for 6+ inches on D3.
Additionally, heavy snow accumulations exceeding 6" are likely D2
(70-90%) and possible D3 (30-50%)in the higher terrain of the San
Juans where storm-total snowfall of 10-15" is possible.

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

Weiss


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FOUS11 KWBC 031828
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
228 PM EDT Sat May 3 2025

Valid 00Z Sun May 04 2025 - 00Z Wed May 07 2025


...Sierra Nevada...
Days 1-2...

Digging upper trough just off the CA coast this evening will
deepen and close off as it moves into/through SoCal overnight and
early Sunday. The best dynamics will quickly move into the Interior
West, but light upslope-driven snow will continue over the
central/southern High Sierra with snow levels generally near/over
8000ft. Main upper low will move through AZ into the Four Corners
though trailing streams of vorticity from the north will keep light
snow going over the Sierra until late Monday. WPC probabilities
for at least 8 inches of snow are generally 20-60% for the High
Sierra (above 10,000ft).


...Northern and Central Rockies...
Days 1-3...

Combination of the incoming upper low through the Southwest and a
slowly-moving trough axis in the northern stream pushing its way
across Montana will promote a period of unsettled weather for much
of the Interior West/Rockies through the period, including higher
elevation snow. The upper low is forecast to only move from
central AZ Monday morning to around the Four Corners region by
Tuesday morning then onto the western High Plains by the end of the
period as the upper jet carves out breaks down over the region.
This will favor an influx of moisture from the southeast
(TX/western Gulf) in combination with rotating vort maxes around
the main upper low atop a slowly-moving cold front at the surface
to the north (in addition to the one to the south ahead of the main
upper low). Snow levels will be high in the central/southern
Rockies, generally above the Front Range around 8000-10,000ft
(lowest beneath the cold core upper low) but lower farther north
behind the cold front to around 5000-7000ft across MT into WY. Snow
will be light to moderate and should maximize into the San Juans
via southerly flow but then into the CO Rockies and Sangre de
Cristos as the upper low moves eastward and the flow turns to more southeasterly to easterly and northeasterly later in the evolution
of the system. Snow to the north will be driven by the surface
front and low-level convergence, but with more limited moisture and
lighter snowfall amounts.

For the three-day period, WPC probabilities for at least 8 inches
of snow are above 50% across the terrain of southern UT into CO
above 10,000ft or so and are at least 40% for at least 18 inches of
snow above 12,000 ft. To the north, probabilities are a bit lower
for at least 8 inches of snow through the period (20-50%), mostly
over the Wind River Range, Uintas, and across SW MT.


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


Fracasso


$$

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FOUS11 KWBC 040722
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
322 AM EDT Sun May 4 2025

Valid 12Z Sun May 04 2025 - 12Z Wed May 07 2025



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

Stagnant but impressively amplified upper pattern characterized by
dual closed lows across the CONUS will result in increasing
precipitation beneath the slow moving vortex across the western
CONUS beginning today. The core of this low will emerge from the
Desert Southwest today before drifting slowly across the Four
Corners through Tuesday. On D3 /Tuesday into Wednesday/ a northern
stream impulse pushing a cold front southward will phase with the
larger gyre across the Four Corners bringing more widespread height
falls to the region, while also amplifying moist advection out of
the Gulf. This will lead to more impressive precipitation focused
across CO and northern NM, with snowfall occurring in the higher
terrain of the Front Range and Sangre de Cristos above generally
9000 ft. It is likely that the heaviest snowfall of the period
will occur D3 in this area as upslope flow combined with modest
fgen in the vicinity of the cold front and the large scale, but
broad, synoptic lift combine. This will result in heavy snow, for
which WPC probabilities suggest have a 70-90% chance of exceeding 8
inches in the Front Range and Sangre de Cristos, with 2-day
snowfall of 1-2 feet possible above 9000 ft. With periodic heavy
snow rates dragging down some colder air, especially in areas of
pronounced upslope, some light accumulations are possible in the
highest terrain of the Palmer Divide as well, impacting I-25.

Otherwise, during D1 and D2, heavy snow is likely across the San
Juans, generally above 10,000 ft, with more moderate snowfall
expected in the Sangre de Cristos (for the latter, this is separate
from the heavier snow D3). WPC probabilities across these areas are
high (>70%) for 6+ inches D1, and moderate (30-50%) D2.

Additionally, lighter snow accumulations are progged from the
Northern Rockies through the Absarokas and into the terrain around
Yellowstone NP, including the Wind River Range from D1 into D2
where WPC probabilities suggest there is a 30-50% chance for at
least 4 inches, and locally more than 8 inches is possible.

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


Weiss



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FOUS11 KWBC 041851
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
251 PM EDT Sun May 4 2025

Valid 00Z Mon May 05 2025 - 00Z Thu May 08 2025


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

The very blocky upper pattern (typical for May during our
atmospheric spindown into summer) is manifesting itself as two
closed lows over the CONUS, the western one of which will be
responsible for a wintry thump of snow over the Rockies. As of
Sunday evening, the upper low will move from the CA deserts/CO
River Valley northeastward through AZ on Monday then into northern
NM Tuesday before finally moving into the Plains Wednesday. The
southern track of the upper system will keep the largest negative
anomalies (height and temperatures) over northern Mexico and
astride the southern US border, meaning snow levels will be on the
higher side over the Rockies (9000ft +/- 500ft). Later, colder air
will eventually advect in from the northeast (modestly) which
should decrease snow levels a bit to around 8000ft (and a bit lower
at times). Significant snow is expected for the mountains in CO
and northern NM with moderate to major impacts near/above the
treeline.

The storm system will progress in phases, with the initial WAA-
driven snow first on southerly flow D1 to be followed by jet/PVA-
driven snow as the upper low approaches northern NM D2 as the flow
turns to more easterly, maximizing upslope potential on the east
side of the Rockies. Finally by D3, as the system unwinds/extends,
and modest snow to start will gradually wind down but not fully end
until just after the end of this D3. Areas in favor of the most
snow include San Juans D1, Sangre de Cristos northward to the
higher elevations of the Front Range D2-3. There, snowfall rates of
1-3"/hr are possible (>50% probs for 2"/hr in the HREF) D2. WPC
probabilities for at least 8 inches of snow over the next three
days are highest (>50%) above 10,000ft with >50% probs for at least
18 inches above 12,000ft. As some colder air filters in early
Wednesday, some snow may fall into the higher elevations of the
I-25 corridor (Palmer Divide and Raton Mesa along the CO/NM
border).

Farther north, an incoming cold front will aid in producing some
mountain snow for the Uintas, Wind River Range, Absarokas and
Bighorns, and southwestern Montana where snow levels will be a bit
lower (6000-8000ft). There, WPC probabilities for at least 8 inches
of snow Days 1-2 are moderate to high (50-80%).

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


Fracasso



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FOUS11 KWBC 050653
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
253 AM EDT Mon May 5 2025

Valid 12Z Mon May 05 2025 - 12Z Thu May 08 2025


...Intermountain West...
Days 1-2...

The interaction of an anomalous and slow moving closed mid-level
low with a surface cold front wavering across the Central Rockies
will result in widespread heavy precipitation through Wednesday
before everything kicks out to the east during D3. The primary
mechanisms for ascent will be the intense closed low (500-700mb
height anomalies below the 1st percentile according to NAEFS) and a
cold front that will stall generally from the Dakotas across
Colorado and into the Great Basin. Intensifying and veering low to
mid level flow will encourage the impressive draw northward of
moisture, leading to robust isentropic ascent at 305K from the Gulf
into the Rockies on the downwind side of this feature. With
moisture confluence becoming impressive, ascent through height
falls and convergence along the front, aided by both strong upslope
flow (especially into the Sangre de Cristos and Front Range) and
elevated frontogenesis will wring out this moisture as expansive
and heavy precipitation from Montana through New Mexico.

Additionally, impressive easterly flow leading to the pronounced
upslope flow will correspond with some reduction (and isolated
folding) of the theta-e surfaces leading to some elevated
instability. This enhanced forcing combined with a lowering trend
in snow levels (NBM 10th% down to 7500 ft) could allow for more
impactful snow into lower elevations of the foothills, especially
across the Palmer Divide and Raton Mesa, including the I-25
corridor in that vicinity. After coordination with WFO BOU, opted
to raise SLRs a bit in this region for the potential, with some
impacts becoming more probable even east of the Front Range and
Sangres.

WPC probabilities across this area are high (>70%) for 6+ inches
both D1 and D2 in the San Juans, Sangre de Cristos, and Front
Range, with 1-2 feet of snow likely up around 9,000 ft, and locally
as much as 3+ feet above 10,000 ft. While accumulations in the
foothills and lower elevations down to 7,000 ft will be much less,
heavy snow rates with low-SLR could cause impacts even into the
I-25 corridor, especially Tuesday evening/night.

Farther north, snow levels will crash behind the cold front as it
sags southward, so while across WY and southern MT they too will
begin around 9,000 ft, by D2 snow levels are expected to fall to as
low as 6,000 ft, especially in MT and in the vicinity of
Yellowstone NP. Across this area, WPC probabilities are high
(70-90%) for 6+ inches D1 across the Absarokas and surrounding
elevations of southern MT, with probabilities reaching 50% across
the Wind Rivers, Big Horns, and in the vicinity of Yellowstone NP.

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


Weiss


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FOUS11 KWBC 051841
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
241 PM EDT Mon May 5 2025

Valid 00Z Tue May 06 2025 - 00Z Fri May 09 2025


...Intermountain West...
Days 1-2...

A deep upper low over AZ will slowly wobble eastward over the next
two days as an influx of Gulf moisture wraps in from the
southeast. This will set up a major winter storm for the high
elevations of the central/southern Rockies with snowfall rates of
1-3"/hr at times and total accumulations well over 1-2ft at the
highest peaks. The environment will be mild overall given the late
season timing and focus of colder air well south (central AZ/NM),
meaning that much of the I-25 corridor will remain snow-free
(except for the typical higher elevations along the corridor in the
Palmer Divide and Raton Mesa). The system will become elongated
and stretch eastward tomorrow which will signal the end of modest
snow, but lighter snow will persist into the evening/overnight and
largely end by Thursday morning.

The system will come in two phases for many areas, with the lead-
in WAA-driven lighter snow early D1 (tonight) and the dynamically-
driven heavier snow tomorrow by the afternoon through the evening
as the upper low approaches the central CO/NM border from the WSW.
Models indicate a robust system with strong isentropic ascent at
the 305K level from the Gulf and strong/anomalous/record easterly
flow at 700mb by midday tomorrow (per the CFSR period). As can be
typical with strong systems, dynamical cooling may bring down snow
levels from ~8000-8500ft to 7500ft or so, reaching farther down the
Front Range toward the high elevations of the I-25 corridor before
then rising as the upper low pulls away.

Favored areas for the heaviest snowfall are over the San Juans
this evening/overnight on southerly to southeasterly flow, then
transitioning to the east/upslope side of the CO Rockies and into
the Sangre de Cristos. Total snow may be 2-3ft (perhaps more) at
the highest elevations including Pike's Peak and along the spine of
the Sangre de Cristos. Impacts will be moderate to major per the
WSSI. WPC probabilities for at least 8 inches of snow over the next
two days are >50% above 8500ft or so and >50% above 10,500ft or
so. Depending on the time of day and amount of dynamical cooling,
there are low probabilities (10-40%) of at least 2 inches of snow
as low as 6500ft or so.

Farther north, snow levels will continue to fall behind a cold
front currently through WY with precipitation trailing to its
northwest through Montana, tied to northern stream
troughing/vorticity diving south-southwestward tonight/early
tomorrow. This rain/snow will continue to progress southward, with
snow levels around 6000-8000ft from north to south, favoring snow
over portions of southern MT, the Absarokas, and into the Bighorns
and Wind River Range. There, WPC probabilities of at least 8 inches
of snow are at least 30% (highest in the Absarokas).

The probability of at least 4 inches of snow on day 3 are less
than 10%.

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


Fracasso


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FOUS11 KWBC 060728
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 AM EDT Tue May 6 2025

Valid 12Z Tue May 06 2025 - 12Z Fri May 09 2025


...Central and Southern Rockies...
Days 1-2...

Pinwheeling upper lows within a larger gyre centered over the Four
Corners will continue to produce significant winter weather in the
higher elevations of the Rockies through Wednesday morning.

The first, and less impressive, area of snow is expected to drift
south across WY today as a cold front driven southward by a
vorticity center drops through the area. Height falls and
convergence along the front will produce ascent, with post-frontal
upslope aiding ascent. PWs this far north will be somewhat less
impressive than points farther south, but an impressive stream of
moisture emerging from the Gulf will lead to PW anomalies above the
90th percentile into WY on D1. With snow levels fall from as high
as 9000 ft near Cheyenne to around 5000 ft around Yellowstone,
significant accumulations are likely in the favored upslope
terrain, especially above 7000 ft, before precip wanes after 00Z
Wednesday. Additional snow after 12Z Tuesday (beginning of D1) will
be modest, but WPC probabilities suggest a low-risk (10-30%) for an
additional 2-4 inches of snow.

Farther south, a more significant snowfall is expected from the
Front Range through the Sangre de Cristos, potentially including
portions of the I-25 urban corridor. The larger upper low swinging
across NM will pivot slowly eastward leading to height falls to
drive synoptic lift, while moisture surging northward from the Gulf
pivots NW as a theta-e ridge into the region. Low-level flow will
become increasingly from the east, leading to additional ascent
through upslope flow and isentropic ascent as weak high pressure
builds down the terrain. This will induce impressive lift from the
Sangres northward into the Front Range, which could produce
snowfall rates of 1-3"/hr at times as reflected by the WPC
prototype snowband tool. These rates will be confined to the
highest terrain, generally above 8500 ft, but folding theta-e
surfaces within the terrain and in the presence of the
strengthening barrier jet and accompanying ascent will likely lead
to dynamic cooling to allow snow accumulations falling to as low as
7000 ft, below the NBM 10th percentile. While the heaviest snow
accumulations will remain above 9,000 ft, light but impactful snow
is possible into the Foothills including the Palmer Divide and
Raton Mesa, with hazardous travel possible along portions of I-25.

As far as accumulations, this does appear to be a significant event
and the latest EFI suggests a higher than 95% probability of an
extreme event with SoT exceeding 2 in the Sangre de Cristos. In
this area, heavy snow accumulating to above 8 inches is likely

70%) from the Front Ranges through the southern Sangre de

Cristos, and including portions of the San Juans and eastern CO
Rockies. 1-2 feet of snow is likely above 9,000 feet, with 3 feet
or more expected in the highest peaks such as Pike's Peak. Farther
east, light snow of a few inches remains possible, especially
across the Palmer Divide and Raton Mesa, impacting the portion of
I-25 that traverses those crests. Most of this snow occurs D1, as
by D2 only lingering probabilities for 4+ inches continue across
the San Juans.


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


Weiss


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FOUS11 KWBC 061831
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
231 PM EDT Tue May 6 2025

Valid 00Z Wed May 07 2025 - 00Z Sat May 10 2025


...Central and Southern Rockies...
Days 1-2...

Upper low over northeastern NM this evening will continue eastward
and pull away from the Rockies, allowing any moderate/heavy snow
to taper to light snow into Wednesday. For this evening, the strong
moisture plume from the western Gulf will support snowfall rates
of 1-2"/hr over parts of the CO Rockies into the Sangre de Cristos
before lessening in intensity and becoming lighter Wednesday
morning. For Wednesday itself, troughing will remain over the
central Rockies and a trailing smaller/compact upper low over the
CA/AZ border will act to prolong the rebound in heights, allowing
some light snow to persist over the San Juans tomorrow afternoon.
By tomorrow night into Thursday, heights will rise as will snow
levels, confining only light snow to the highest mountain peaks
above 11,000ft or so.

WPC probabilities for at least 8 inches of snow after 00Z tonight
are >50% above 10,000ft or so. Some snow is possible down to around
7000ft this evening.

The probability of at least 4 inches of snow on day 3 is less than
10 percent.

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


Fracasso


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FOUS11 KWBC 070646
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
246 AM EDT Wed May 7 2025

Valid 12Z Wed May 07 2025 - 12Z Sat May 10 2025


...Central and Southern Rockies...
Day 1...

Closed low over the Central Rockies will finally begin to fill and
lift off to the east today, while a secondary closed mid-level
impulse rotates cyclonically around it, moving over the Four
Corners tonight while dissipating. This evolution will result in
the primary forcing beginning to weaken over CO and NM due to
weaker lapse rates aloft, a lack of height falls, and decreasing
upslope flow. While this will force the most significant
precipitation to wind down quickly after 12Z this morning, the
secondary feature moving into the region will maintain at least
modest ascent and steepened lapse rates into tonight. This will
shift the focus of the heaviest snowfall from the Front Range and
Sangre de Cristos Tuesday night/Wednesday morning, to the San
Juans. Snow levels will remain elevated, around 8000-9000 ft,
keeping the heaviest snow confined to the highest terrain, and WPC probabilities suggest a moderate risk (50%) for at least an
additional 6 inches of snow across the San Juans.


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


Weiss



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FOUS11 KWBC 071814
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
214 PM EDT Wed May 7 2025

Valid 00Z Thu May 08 2025 - 00Z Sun May 11 2025


...Colorado...
Day 1...

Lingering vorticity on the western side of the departing upper low
will still trigger some light snow over the San Juans this evening
before tapering off tonight/early Thursday. WPC probabilities for
at least an additional 4 inches of snow after 00Z are low (10-40%)
above 11,000ft.

For days 2-3, the probability of at least 4 inches of snow is less
than 10%.

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

Note - this is the last regularly scheduled discussion (QPFHSD)
for the 2024-2025 winter season. Unless a significant winter
weather threat emerges, this discussion will next be updated on or
about September 25, 2025.


Fracasso



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