FOUS11 KWBC 170755
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 AM EDT Mon Mar 17 2025

Valid 12Z Mon Mar 17 2025 - 12Z Thu Mar 20 2025

...Central Rockies, Central Plains, Upper Midwest... Days 1-3...

...Another significant winter system likely to bring blizzard 
conditions to portions of the Plains and Upper Midwest...

This system manifests as two waves of low pressure, a modest lead
wave, followed immediately by the more impressive cyclone. These
will generate from an anomalous trough which will amplify across 
the Four Corners Tuesday /D2/ leading to impressive height
anomalies that are progged to fall below the 1st climatological
percentile between 700-500mb across the Central Plains by
Wednesday. This deepening and intense amplitude will yield strong
downstream ascent through height falls and mid-level divergence,
and as the subtropical jet streak amplifies it will begin to shift
almost due poleward, placing intense LFQ diffluence atop the region
of greatest height falls.

Together this will produce an environment extremely favorable for a
rapidly deepening cyclone in the lee of the Rockies. However,
before this occurs, a weaker wave is progged to develop near SE WY
Tuesday morning in response to more modest height falls, but in the
presence of increased baroclinicity as a cold front drops southward
from the Northern Rockies/High Plains, aided by the ageostrophic
response (fgen) of the RRQ of a departing jet streak into the Great
Lakes. This first low will eject rapidly east and weaken, but the
strong ascent and plentiful moisture will create heavy snow across
the Central Rockies, especially in WY where low-level upslope
ascent into the best frontogenesis will produce heavy snow rates.
WPC probabilities for more than 4 inches of snow are high (>90%)
across much of NW WY and into the Big Horns, with some elevated
probabilities for 1-2" extending into the lower terrain of eastern
WY as well.

During D2, however, is when the significant low develops. As the
synoptic pattern evolves as described above, lee cyclogenesis will
occur, and this low will be located beneath extremely favorable
synoptic ascent for rapid surface height falls, and nearly all the
ensemble groupings suggest SLP falling to the 980s Wednesday
morning over IA. There is generally good clustering of the ensemble
member low-pressures, but D3 clusters still suggest that the GEFS
is a bit deeper and faster with the progression, although the
spread appears to have decreased in the past 24-hrs. This leads to
high confidence in a rapidly strengthening low pressure, with
moisture steadily increasing as 300K isentropic ascent surges 
theta-e northward and into the system, eventually lifting into a 
TROWAL to provide moisture and elevated instability.

As has been the case with the past few of these March systems, the
antecedent airmass is warm, and the moist/warm advection funneling
in the higher PW air will cause the precipitation to begin as rain 
in many areas, especially outside of terrain and into the Plains. 
However, this should have minimal affect on snowfall accumulations,
as extreme dynamic cooling combined with ageostrophic flow into 
low and cold advection as the low moves east will cause a rapid 
p-type transition from rain to snow, with snow likely becoming 
quite heavy within the deformation axis NW of the low Wednesday. 
Snowfall rates that may reach 1-2"/hr (or more? some signals for CI
in a deep and saturated DGZ over IA indicating possible thunder-
snow) will overcome the warm ground quickly. Thus, the limiting 
factor to snowfall amounts appears to be more the speed of this 
system as it remains progressive, and CIPS analogs indicate the 
75th% is only generally 4-8", higher into WI/MI where leading jet-
streak snowfall may add on to amounts before the main system. This 
is additionally reflected by WPC probabilities for 4+ inches that 
are below 30% D2, but increase to greater than 70% from western IA 
northeast into the U.P. of MI D3. These probabilities have trended 
upward, and locally 6+ inches of snow is likely within this band.

Finally, despite snowfall that may be modest in many areas outside
of the Upper Midwest, the intense winds driven by 850mb northerly 
flow of 60 kts will produce strong gusts for which DESI indicates 
has a 90% chance of exceeding 35 mph, and even a 50% chance for 50 
mph across the Plains. This has led to high (>70%) probabilities 
for moderate blowing snow impacts from WSSI-P, and despite snow 
that may start as the heavier/wet variety, will transition to 
higher SLR and broken aggregates leading to likely blizzard 
conditions in some areas, especially from central Kansas northeast 
into southern Minnesota.

By the end of the forecast period, this low will be well into
Ontario, but lingering snow and gusty winds are expected to still
be ongoing across MI, with some lake-enhanced snow across the U.P.,
western L.P. and maybe even into northern Indiana into early D4.

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

Shortwave trough positioned just west of northern CA at 12Z Monday
will deepen into a closed mid-level low as it advects onshore and
drops subtly southeast Monday morning. The core of this low is
progged to track across the northern Great Basin this evening, but
a strong lobe of vorticity immediately in its wake will dig farther
south, amplifying the trough axis into southern CA by the end of
D1. Downstream of this trough, squeezed mid-level flow will produce
moisture confluence, lying directly beneath a strengthening and
meridionally advancing subtropical jet streak. This setup will 
support impressive moist advection, reflected by PWs surging to 
above the 90th percentile across CA and the Great Basin according 
to NAEFS ensemble tables.

Within this moistening column, and beneath the deep layer ascent, a
wave of low pressure will move southeast along a baroclinic 
gradient/cold front that will be driven eastward by the parent 
trough evolution. This will maintain the focus of greatest moisture
just south of the front, but will drive some enhanced mid-level 
isentropic ascent above and behind the boundary. This additional 
lift, combined with periods of upslope flow into terrain features, 
will result in widespread precipitation D1 from the Cascades 
southward through the northern CA terrain and into the Sierra, as 
well as points well east across the Great Basin and into the
Wasatch/Tetons/Wind Rivers. 

Snow levels will vary considerably across the West during this 
period due to the passage of the cold front. However, within the
region of greatest moisture/ascent and heaviest resultant snowfall,
snow levels should be generally 4000-6000 ft from the Sierra 
through the Great Basin and into the Central Rockies. Snow levels 
crash behind the cold front, allowing for at least moderate 
snowfall but with more substantial pass level impacts in its wake. 
WPC probabilities for more than 6 inches of snow are moderate 
(30-50%) D1 across the Sierra and WA Cascades, but high (>70%) 
across the Wasatch and into portions of western WY.

Additionally, along this front, impressive 0-2km fgen and increased
low-level RH will move into a corridor of elevated instability (NAM
SBCAPE 100-400 J/kg). This will occur within a region of high SnSq
parameter above 2 from eastern NV Monday evening through UT, WY,
and CO by Tuesday evening. This setup could produce convective 
snow showers or even snow squalls, which is additionally supported
by simulated reflectivity in the high-res guidance. While any snow
squalls won't produce tremendous snowfall amounts, dangerous travel
is likely through snow-covered roads and rapidly changing visibility.

This first system moves into the Central Rockies during D2,
becoming a significant winter storm in the Plains, and leaving a
brief respite much of D2 across the West within this otherwise
continued active pattern. This break will be short lived, however,
as yet another strengthening mid-level trough moves across the
Pacific and approaches the Pacific Northwest D3, likely coming 
onshore Wednesday evening. Although the guidance varies 
considerably with timing of the associated mid-level waves within 
this next trough, they all agree that it will be potent, and 
potentially negatively tilted, as it moves onshore during D3. The 
downstream affects of this will be increased synoptic lift through 
mid-level divergence, height falls, and intensifying upper 
diffluence, all to pivot renewed moisture and forcing back onshore.
The antecedent airmass is cool, so snow levels will are only 
expected to rise to around 3500 ft within the core of the strongest
WAA, and otherwise be generally 2000-3000 ft, suggesting travel 
across area passes will again become problematic. This feature will
likely be transient, but have sufficient moisture to produce heavy
snow once again from the Olympics and WA Cascades southward into 
the Sierra, with some spill over occurring before the end of the 
period as far east as the Salmon River/Sawtooth/Northern Rockies.

WPC probabilities for 6+ inches of snow on D3 feature a high risk 
(>70%) across the Olympics, Cascades, and Shasta/Trinity region of 
northern CA< with lower probabilities below 50% continuing into the
Sierra and east across parts of the higher elevations of Idaho.

Weiss

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

https://www.wpc.ncep.noaa.gov/key_messages/La...

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