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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