FOUS11 KWBC 140655
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
255 AM EDT Fri Mar 14 2025

Valid 12Z Fri Mar 14 2025 - 12Z Mon Mar 17 2025

...The West... Days 1-3...

The active pattern continues across the West with several days of
widespread wintry precipitation impacting much of the area through
the upcoming weekend.

The period begins with a potent closed mid-level low moving across
the Southern High Plains of CO/NM. This feature is quite intense,
reflected by NAEFS 850-500mb height percentiles that are below the
minimum in the CFSR climatology across a large, and expanding,
section of the country centered along the High Plains. As this
feature shifts E/NE this afternoon, it will continue to deepen and
produce near-blizzard conditions across the Northern Plains (more
on that in the section below). Behind this departing system, heavy
snow will continue across the Southern and Central Rockies through
D1 as a combination of mid-level deformation and a trailing cold
front overlap to enhance ascent in a region of still elevated 700mb
RH across the Four Corners and into Wyoming. While this snow
should wane quickly by 00Z Saturday, and snow levels will generally
be in the 5000-6000 ft range, snow is expected to accumulate
several additional inches D1 as reflected by WPC probabilities that
are high (>70%) for 4+ inches across the CO Rockies, the northern
Sangre de Cristos, and as far north as the Big Horn Mountains of WY.

Any break in precipitation behind this significant, but departing,
system will be short lived as the active pattern sheds another
amplifying shortwave across the Pacific and into CA/OR by 00Z
Saturday /late D1/. This next impulse will be accompanied by an
impressive lobe of vorticity, with the resultant trough development
driving height falls once again from CA through the Four Corners
into D2. Available moisture with this next impulse is progged to
be much less than with the first shortwave, reflected by NAEFS PW
anomalies that are near normal east of AZ as the maximum IVT
downstream of this trough axis is modest (GEFS and ECENS
probabilities for 500 kg/m/s less than 10%) and focused generally
south into Mexico. Of course, this is pretty typical with ARs that
are oriented more W/NW than W/SW, but enough moisture and continued
lowered snow levels of generally 3000-5000 ft will allow for at
least modest snowfall accumulations. The heaviest snowfall
accumulations D1 into D2 are expected from the Sierra into the
Mogollon Rim and into the Four Corners, where WPC probabilities are
high once again for 8+ inches, and locally 1-2 feet is possible in
the Sierra and northern CA terrain near the Shasta/Trinity region.

Interestingly, as the second shortwave digs across the Four Corners
into D2, the upstream flow becomes more favorable for a surge of
moisture and heavy precipitation farther north into the Pacific and
Interior Northwest. A trough over British Columbia/Alberta
interacts with the trough moving across the Four Corners to squeeze
the flow over the Pacific, resulting in fast and zonal flow surging
into WA/OR. Impulses within this flow will help push two distinct
surface lows onshore, the first Saturday morning and the second
early Sunday morning, helping to enhance ascent. Regardless of
those surface lows, deep layer lift becomes impressive and moisture
increases as a strengthening Pacific jet streak pivots onshore and
begins to buckle, lifting meridionally on Sunday downstream of an
even more potent trough approaching the coast.

Both ECENS and GEFS probabilities exceed 70% for 500 kg/m/s IVT
reaching the coast D3, funneling moisture along a cold front which
will elongate into the region. Although the accompanying WAA with
this atmospheric river (AR) will drive snow levels to as high as
6000 ft D3 south of the cold front, considerable moisture and a
sharp gradient in snow levels will likely result in impactful
pass-level snow from the Cascades through the Northern Rockies,
with still impressive snow but generally above pass-levels farther
south from the Great Basin into the Central Rockies, as much of the
West is again covered in elevation-based snowfall. High WPC
probabilities (>70%) late D1 through D3 become widespread for 6+
inches each day from the OR Cascades northward into the Olympics
and WA Cascades, and as far east as the Salmon River/Sawtooth area,
north into the Northern Rockies, and even as far as the Tetons.
Event total snowfall is likely to be extreme in the higher terrain
of the OR Cascades where 4-6 feet of snow is possible, with
widespread 1-3 feet in the other higher elevations regions of the Northwest.

...Northern Plains... Days 1-2...

Rapidly strengthening low pressure in the lee of the Rockies this
morning will approach or exceed March sea-level pressure records
across the Northern Plains as it tracks northeast through Saturday
aftn. Not only will SLP records be approached, but 850-500mb height
anomalies are progged to fall below the all-time minimums within
the CFSR climatology according to NAEFS, further indicative of the
intensity of this system. The result of this evolution will be a
major winter storm impacting the Northern Plains late Friday
through Saturday.

As the surface low tracks northeast from the Central High Plains,
it will be driven by an overlap of impressive height falls
downstream of the primary closed low and at least modest upper
diffluence in the LFQ of a poleward arcing jet streak east of the
mid-level center. Together, these will drive intense deep layer
ascent aiding in the storm strengthening. At the same time,
confluent low-level flow east of the low will draw impressive
moisture northward on 300-305K isentropic ascent. This moisture
will surge am impressive theta-e ridge northward, which will then
rotate cyclonically and lift into a robust TROWAL, pivoting over
eastern NE, SD, ND, and western/central MN Saturday. At the same
time, an impressive deformation axis will overlap with this TROWAL,
supporting an intense axis of maximized ascent to drive heavy
precipitation rates.

Initially, the theta-e advection will surge warm air into the
region such that the column will support primarily rain. However,
as the low continues to deepen and move northeast, and mesoscale
ascent intensifies, the combination of dynamic cooling and
ageostrophic flow on NW winds into the low center will rapidly cool
the column. The guidance continues to suggest a period of mixed
precip, including sleet and freezing rain, will occur, but this
again appears to be a conceptual setup where the transition period
is quite rapid. This indicates that while some light icing is
possible, the predominant wintry p-type will be snow, which will
almost certainly become heavy at times (70% probabilities for 1"/hr
according to the HREF). This is supported as well by NAM cross-
sections indicating a region of theta-es lapse rates <0C/km in the
presence of symmetric instability, suggesting the potential for
CSI or even CI (thundersnow). Although antecedent conditions are
quite warm and some rain will begin this event, these rates should
quickly overwhelm this hostility leading to rapid snow accumulation.

Finally, impacts during the period of heavy snow are likely to be
impressive due to strong winds that may gust above 50 mph during
the period of heavy snow. While the period of heavy snow rates is
likely to be somewhat temporally modest except beneath the pivot
point of the deformation axis, the overlap of extreme rates of
1-2"/hr and these strong winds will create near blizzard conditions
and extremely dangerous travel. Snowfall will begin late D1 but
accumulations are expected to be modest before 12Z Saturday as
reflected by WPC probabilities of 30-50% for just 2 inches across
far eastern SD. Early D2 snowfall coverage and intensity ramps up,
and WPC probabilities are high (>70%) for 4+ inches near the Coteau
des Prairies and as far north as International Falls, MN. Locally,
8 inches of snow is possible beneath the pivoting deformation,
especially should any thundersnow occur. Additionally, some light
icing across western MN has a 10-30% chance of reaching 0.1+" of
ice accretion.

Weiss

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

$$
--- SBBSecho 3.20-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (21:1/175)

FOUS11 KWBC 150648
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
248 AM EDT Sat Mar 15 2025

Valid 12Z Sat Mar 15 2025 - 12Z Tue Mar 18 2025


...Northern Plains...
Day 1...

The rapidly strengthening low moving across the Northern Plains
which has been advertised the past few days will be well underway
at the start of the period. At 12Z Saturday, the surface low is
progged to be near the Twin Cities, MN, and will be lifting
northeast, reaching Ontario, Canada by 00Z Sunday. N and W of this
low center, a strong and pivoting deformation axis will be helping
to drive intense ascent, causing a changeover from rain to heavy
snow across western MN, resulting in blizzard, and near-blizzard,
conditions through the aftn.

Moisture advection downstream of this strengthening cyclone will
remain impressive through D1, with isentropic ascent surging the
accompanying theta-e ridge into a robust TROWAL which will pivot
cyclonically back into the Dakotas and Minnesota today. The overlap
of this enhanced moisture and the collocated deformation axis will
increase both moisture and ascent to cause a narrow stripe of heavy precipitation, changing rapidly from rain to snow. Dynamic cross-
sections across this region continue to suggest an axis of CSI/CI,
supported by the elevated instability within the TROWAL, to create
snowfall rates that are forecast both by HREF probabilities and the
WPC prototype snowband tool to reach 1-2"/hr. Despite hostile
antecedent conditions due to warm temperatures and rain, these
snowfall rates will rapidly accumulate, and when combined with wind
gusts of 50 mph, will likely result in blizzard and near-blizzard
conditions through the aftn. This is despite accumulations that
will in most areas be modest except within the stripe beneath the
pivoting deformation axis, where WPC probabilities are high (>70%)
for 4+ inches and locally as much as 8 inches is possible.


...The West...
Days 1-3...

The active pattern continues across the West with two separate
systems producing widespread wintry precipitation across much of
the area through early next week.

The next impulse within this active pattern will push an impressive
vorticity lobe eastward from OR through the Great Basin to start
Saturday morning, with the resultant trough development driving
height falls once again into the Four Corners on D1. PW anomalies
that are near normal east of this feature as the maximum IVT
downstream of this trough axis is modes and focused generally
south into Mexico, but available moisture is still sufficient for a
swath of moderate to heavy precipitation, with snow-levels
generally 3000-5000 ft allowing for moderate snowfall accumulations
above these levels. WPC probabilities D1 are moderate to high
(50-90%) for 4+ inches across the CO Rockies, Sangre de Cristos, White Mountains of AZ, and the Sacramentos.

As this lead shortwave digs across the Four Corners, the upstream
flow becomes more favorable for a surge of moisture and heavy
precipitation farther north into the Pacific and Interior
Northwest. A trough over British Columbia/Alberta interacts with
the trough moving across the Four Corners to squeeze the flow over
the Pacific, resulting in fast and zonal flow squeezing into
WA/OR. Impulses within this flow will help push two distinct
surface lows onshore, the first this morning and the second early
Sunday morning, helping to enhance ascent. Regardless of those
surface lows, deep layer lift becomes impressive and moisture
increases as a strengthening Pacific jet streak pivots onshore and
begins to buckle, lifting meridionally on Sunday downstream of an
even more potent trough approaching the coast.

Resultant IVT with this setup becomes moderate to strong once
again, potentially reaching 500-750 kg/m/s as moisture funnels
onshore along a cold front which will elongate into the region.
Although the accompanying WAA with this atmospheric river (AR) will
drive snow levels to as high as 6000 ft south of the cold front,
considerable moisture and a sharp gradient in snow levels will
likely result in impactful pass-level snow from the Cascades
through the Northern Rockies and eventually Sierra Nevada once
again later D2 into D3. Still impressive snow, but at generally
above pass-levels, is forecast farther south from the Great Basin
into the Central Rockies, as much of the West is again covered in elevation-based snowfall. For the entire forecast period, high WPC probabilities (>70%) for 8+ inches exist from the Sierra Nevada and
OR Cascades northward into the Olympics and WA Cascades, and as
far east as the Salmon River/Sawtooth area, north into the Northern
Rockies, and even as far as the Tetons. Event total snowfall is
likely to be extreme in the higher terrain of the OR Cascades where
4-6 feet of snow is possible, with widespread 1-3 feet in the
other higher elevations regions of the Northwest and Sierra Nevada.

During the latter half of D3 /Monday evening/ the parent trough
digging across CA and into the Great Basin responsible for the snow
axis shifting south into the Sierra late D2 will pivot eastward
towards the Four Corners/Central Rockies. This will bring a slow
end to precipitation across much of the West (at least briefly) but
may help to produce another significant central U.S. storm by the
middle of next week.


...Central Rockies and High Plains...
Day 3...

A shortwave emerging from the Pacific will amplify as it moves
across CA late Monday and then tracks progressively into the Great
Basin, reaching the Four Corners by the end of the forecast period.
This shortwave will be accompanied by an intensifying sub-tropical
jet streak which will begin to arc poleward downstream of this
amplifying trough. This will have the dual-pronged effect of
transporting moisture into the Central Rockies while also placing
favorable LFQ diffluence atop the region of greatest mid-
level height falls. The resultant deep layer ascent will likely
interact with a cold front dropping southeast into the area, and
the setup appears favorable for developing cyclogenesis in the
central High Plains by Tuesday morning. Where the strongest ascent
overlaps the greatest moisture, especially in areas of expanding
frontogenesis, a swath of heavy snow is likely. WPC probabilities
at this time are modest, just 30-50% for 2+ inches outside of
terrain features across WY, but are higher in the Big Horns and
Black Hills.

This system may become the next powerful cyclone across the Plains
through the middle of next week.


Weiss

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

$$
--- SBBSecho 3.20-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (21:1/175)

FOUS11 KWBC 310743
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 AM EDT Mon Mar 31 2025

Valid 12Z Mon Mar 31 2025 - 12Z Thu Apr 03 2025

...The West... Days 1-2...

An active stretch of wintry weather has begun from the West Coast
to the Front Range of the Rockies through Tuesday due to the
prolonged influence of a deep longwave trough over the western
U.S. set to move inland over the next few days. Extreme impacts
forecast throughout much of the central Sierra Nevada.

Focus shifts to an exceptional closed upper low in the northeast
Pacific today that slowly makes its way east towards the Pacific
Northwest by tonight. Heavy high elevation mountain snow (above
5000ft in the central Sierra and above 4000ft in both the Cascades
and northern CA ranges) will be underway, but as the upper low
inches closer to the Northwest U.S., falling heights and colder
temperatures aloft will force snow levels to lower additionally on
Day 1 to around 3000ft. The primary IVT axis will be aimed at
southern CA through Day 2, but broad onshore flow and an axis of
greater convergence just to the south of the upper low will provide
ample moisture and upslope flow for the Pacific Mts to experience
very heavy snowfall. The northern extent of the IVT will still sport
values >90th climatological percentile through the Sierra Nevada
and into the Great Basin this afternoon. From northern California
to the northern Rockies, the divergent left- exit region of a 200mb
jet streak whose wind speeds (>130 kts) are above the 99th
climatological percentile will be placed directly overhead.

The best moisture advection and upslope enhancement will be seen
from California's Siskiyou and Salmon mountains on south and west
along the spine of the Sierra Nevada. Through tonight, the flood
of Pacific moisture aloft will advance well inland through much of
the Intermountain West. Just about every notable mountain range in
the central and northern Rockies is likely to see measurable snow
into Tuesday. Meanwhile, cyclonic flow around the upper low will
still support ongoing upslope snowfall in the central and northern
California ranges through Tuesday night. Then by early Tuesday the
parent upper low is expected to dive southeast across southwest OR
and northern California, proving a final surge of Pacific moisture
within an area of cold air aloft. The bulk of the Pacific moisture
advection will be finished Tuesday night and into Wednesday, but
the core of the longwave trough will be over the Rockies with NAEFS
showing 500mb temps that are below the 10th climatological
percentile from the Sierra Nevada to the Front Range of the
Colorado Rockies. With the help of daytime heating, expect numerous
snow showers to envelope most of the western U.S. mountain ranges
given the steep lapse rates.

Through 12Z Wednesday, WPC probabilities show the northern ranges
of California (Siskiyou, Salmon, Shasta) and the length of the
central and northern Sierra Nevada have high chances (>70%) for
snowfall totals >24". Most of these ranges above 6,000ft are
currently forecast to see localized amounts between 3-5 feet
through 12Z Wednesday, with WSSI depicting Extreme impacts due to
both snowfall amounts and blowing snow. Travel delays and road
closures are likely at many of the major Sierra passes. Heavy snow
also reaches into the Oregon Cascades with high probabilities for
at least 12". Farther inland, many Intermountain West ranges
(Sawtooth, Blue, Uinta, Bear River, Wind River, Absaroka, Big Horn,
and CO/WY central Rockies) are all likely to see snowfall totals
of 6-12" with locally higher amounts in these ranges as well.
Expect hazardous travel conditions for many road ways that remain
open in these mountain ranges.

...Northern Plains and Upper Midwest... Days 2-3...

By Tuesday morning, the upper trough responsible for the barrage of
mountain snow in the West will position the 250mb jet streak's
divergent left-exit region over the Great Plains. Modest 500mb PVA
and strengthening WAA at low levels will give rise to lee
cyclogenesis east of the Rockies. Throughout the day Tuesday a
strengthening LLJ, (NAEFS shows 850mb winds reaching the 97.5
climatological percentile 18Z Tuesday from north Texas to eastern
Kansas), will deliver both increased moisture and increasing WAA
aloft. The upper trough to the west will also direct an IVT of
300-500 kg/m/s into the Upper Midwest, which is topping the 90th
climatological percentile Tuesday afternoon. There appears to be
sufficiently cold enough air present that precipitation will
transition over from rain to snow from the Black Hills on east
across South Dakota and as far east as central Minnesota Tuesday
afternoon that is largely due to strong 300K isentropic ascent and
850-700mb WAA resulting in FGEN at those mandatory height levels.
Given the early-April sun angle, snowfall will struggle to
accumulate during the day on Tuesday unless rates can reach >1"/hr.
These rates appear most likely after the 21z timeframe once WAA
strengthens and could kick start treacherous travel conditions
into the overnight period.

There are still some ongoing difference with respect to details,
including how far north the rain/snow line reaches in central MN on
Wednesday, as well as banding potential in the
northern/northwestern precipitation shield within the deformation
zone from the Dakotas through northern MN. However, most guidance
highlights a similar area of heavy snowfall extending from
northeast SD and southeast ND across much of northern MN.

The ECMWF EFI continues to depict an strong signal (values of
0.8-0.9) for a potentially disruptive winter storm from the far
eastern Dakotas on east across northern MN. This also aligns fairly
well where WSO values >50% exist on Days 2-3. Both snow amounts,
along with snow load and blowing snow, are likely to be hazards
that the Northern Plains and Upper Midwest contend with through
Wednesday, with freezing rain most likely on the eastern edge of
the WAA across northern Wisconsin and northern Michigan. Current
WPC snowfall probabilities (through 00Z Thursday) for >8" are
moderate- to-high chance (50-70%) across northeast SD, southeast
ND, and northern Minnesota. High probabilities (70-90%) for >8"
exist across the MN Arrowhead. There are also low probabilities
(10-30%) for snowfall totals >12" in this region, highest and up to
50% in the MN Arrowhead. Residents and those traveling to/from
these regions should keep close eyes on the forecast as additional
changes in the types of impacts, the severity of those impacts, and
where/when these impacts occur are likely to fluctuate for another
day or so.

*Key Messages have been issued for this winter storm, and the link
to view them is posted below.

Snell/Mullinax

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

$$
--- SBBSecho 3.20-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (21:1/175)