FOUS11 KWBC 040859
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 AM EST Tue Feb 4 2025

Valid 12Z Tue Feb 04 2025 - 12Z Fri Feb 07 2025

...Western U.S. across the Northern Rockies... Days 1-3...

...Atmospheric River (AR) and upper low maintain heavy snow from Oregon/California border through the Northern Rockies and down the
Sierra Nevada through tonight. A separate low tracks through
California Thursday night bringing further heavy snow for the
Sierra Nevada...

A positively-tilted trough around an upper low off Vancouver Island
will continue to drift southwest off Pacific NW beneath a strong
upper ridge over Alaska through Wednesday before weakening in place
through Thursday. A zonal jet streak exceeding 130kt punches
through north-central CA today with the increasingly diffluent
left exit region over northern CA/NV today and across the
northern Great Basin/Rockies tonight. This will maintain the swath
of heavy snow over far northern CA into southern OR and northeast
through the Bitterroots. Day 1 snow probs for additional >18" are
40-70% for the Klamath/Siskiyou, Sawtooth/Salmon River and
Bitterroots with 70-90% probs for the Sierra Nevada above 6500ft.
A strong 1042mb high centered over the Canadian Prairies will
maintain the cold air on easterly upslope flow to enhance snow
rates along the leeward slopes of the Montana Rockies where Day 1
probs for >4" are 30-60%.

Low pressure ahead of the jet streak reaches the CA coast this
afternoon and northern NV tonight, bringing heavy precip to the
Sierra Nevada. Strong WAA with the low will keep snow levels
6000-7000ft. Heavy snow crosses the Teton and Wind River Ranges
late tonight into Wednesday.
Onshore flow will cause moderate snow over the Cascades and
coastal ranges Wednesday (where Day 2 snow probs for >8" are
50-80%) while the rest the Northwest enjoys a reprieve in snow.
However, the next Pacific storm system arrives Thursday with lower
heights and snow levels 3000-5000ft over northern CA Heavy snow
with prolonged 1-2"/hr rates is expected along the length of the
Sierra Nevada Thursday afternoon through the night. Day 3 snow
probs for >18" are 50-80%.

...Upper Midwest/Great Lakes... Days 2-3...

Reinforcing shortwave trough that crosses OR tonight on the nose
of a powerful zonal jet will track over the northern Great Plains
Wednesday night. Warm air advection ahead of the wave allows
snowfall to breakout Wednesday afternoon over central MN, expanding
over northern MN/WI through the evening before tracking over the
U.P. into Thursday. Day 2.5 snow probs for >4" are 20-50%, highest
over the MN Arrowhead.

Jackson

...Midwest to Mid-Atlantic... Days 2-3...

Very progressive shortwave ejecting eastward off the Pacific will
make quick headway to the east crossing the Continental Divide by
Wednesday morning with sights downstream on the Ohio Valley and
Mid-Atlantic by the end of D2. Further downstream, high pressure
will usher eastward out of the Great Lakes with a cold front
progression through the Mid- Atlantic and Northeast, solidifying a
relatively formidable polar airmass with a classic CAD wedge
situated east of the Appalachian front by Wednesday morning.
Consensus of forecast soundings across the region signal a shallow,
yet firmly entrenched airmass by Wednesday afternoon with a modest
CAA regime ongoing until later that evening. Limited blocking
across the North Atlantic will allow the surface ridge to propagate
eastward fairly quickly leading to a shift to modest return flow
by the very end of the D2 period allowing for warmer air to
protrude the boundary layer from south to north. This is important
as the approach of the aforementioned trough will couple with the
surface high off the Atlantic seaboard creating a modifying low to
mid- level airmass as the two work in tandem.

There's a growing consensus within the 00z deterministic suite for
a bout of mixed precipitation developing downstream across the
Ohio Valley and Mid-Atlantic within the increasing mid-level
difluence pattern ahead of the mean trough. Smaller shortwave
perturbations will also eject out ahead of the trough axis which
could spawn some scattered light precip ahead of the main QPF axis
that will approach more during the overnight time frame on
Wednesday night into early Thursday morning. The initial ptype may
start as a very brief period of snow for places further northeast
away from the nose of the budding 85H LLJ (50-55kts) initiating
over the Shenandoah. Any snowfall will quickly transition over to
sleet and/or freezing rain/drizzle before the main swath of precip
makes its way through the Appalachian and Allegheny front into the
rest of the Mid- Atlantic.

From here, the approach of the shortwave trough will create a
strengthening v-vector component of the wind field allowing for
enhanced meridional flow capable of advecting much warmer air
poleward within the 925-650mb layer as indicated via bufr soundings
from locations across the Mid- Atlantic. Further west over the
Appalachian front extending from northern WV up through the Laurel
Highlands, a very shallow yet stern surface cold air pattern will
lock in and become very difficult to erode despite the increasing
warm air depth being advected overhead. The main thermodynamic
process for warming will likely have to come from latent heat
release of ice accretion processes to slowly maneuver the
temperature to near freezing by the end of the D2 window when much
of the precipitation will be shutting off. Wet bulb temps in the
higher elevations and elevated valleys west of the Blue Ridge will
likely be within 24-27F creating a large gap for any latent heat
processes to effectively kick the shallow cold layer out of the
lowest portions of the PBL. This is one of the main reasons this
area is the focus for the most significant ice accumulation
forecasts thus far with agreement among much of the deterministic
suite and national blend. Areas east of the Blue Ridge will likely
see the surface pattern erode a bit faster, especially those east
of the fall line within the Piedmont. Places between the Blue Ridge
and the fall line will be slower to erode as is customary within
these synoptic scale evolutions, especially when you factor in the
fresh CAA pattern that transpired less than 24 hrs before.

Ice probs of >0.1" remain very high (80+%) across west-central PA
with the focus in-of the Laurel Highlands where now 50-70% probs
for >0.25" of ice accretion are now forecast as of the latest WWD
progs. Northern Ohio Valley doesn't carry the probabilistic maxima
for >0.25" like the mountains to the east, however >0.1" ice
accretion is sufficiently within the 50-80% range, very much a non-
trivial depiction that has been consistent within NWP outputs. Ice probabilities also exist within the Central Mid Atlantic east of
the Blue Ridge with a similar prob of 50-70% for >0.1" of ice
existing for locations northwest of the fall line across MD
extending into southern PA. Newest WWD progs have introduced small
areas in-of the higher terrain of north-central MD within the Parrs
Ridge and Catoctin Mtn domains. These areas also have
probabilities for >0.25" now upwards of 30-40% for the event,
something we'll have to monitor closely as we move closer to the
event. Lower non-zero probs (20- 50%) exist for totals >0.1"
further east into the I-95 corridor and adjacent lowlands. Setup is
delicate with multiple variables involved including depth of cold
air, strength of LLJ providing WAA pattern, as well as timing of
precip onset. These factors will determine the magnitude of ice
potential in-of the Mid-Atlantic and Ohio Valley with guidance
starting to hone in on specifics as we move closer to the event.
While there may be some snow at onset across the Mid-Atlantic on
Wednesday and throughout the Interior Northeast and New England as
WAA increases on D3, probabilities for >4" remains low (10-40%) and
confined to mainly higher elevations of NY, VT, NH and ME.

Given the relatively large geographic coverage of at least 0.10"
ice potential and the increasing chances for over 0.25" of ice
accretion in parts of WV, MD, and PA... Key Messages continue for this event.

Kleebauer

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

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

$$
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FOUS11 KWBC 180721
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
321 AM EDT Tue Mar 18 2025

Valid 12Z Tue Mar 18 2025 - 12Z Fri Mar 21 2025

...Central High Plains through the Great Lakes... Days 1-2...

...Significant storm system to bring blizzard conditions to
portions of the Central Plains and Upper Midwest. Key Messages are
in effect for this event...

This system will emerge in response to an amplifying mid-level
trough which will be crossing the Great Basin and approaching the
Four Corners starts to start the period /12Z Tuesday/. This trough
will continue to deepen in response to a sharp shortwave trough and accompanying vorticity lobe swinging through its base, such that
by the time it tracks into the Plains Wednesday, 850-700mb heights
fall below the minimum within the CFSR climatology according to
NAEFS, with 500mb heights dropping below the 2.5 percentile. This
is indicative of an exceptionally strong system producing intense
synoptic ascent. While height falls and PVA will drive a
considerable portion of the deep layer lift, a collocated
subtropical jet streak intensifying downstream and gaining rapid
latitude (arcing poleward) will place impressive LFQ diffluence
atop the region of greatest height falls. This setup will support
lee cyclogenesis with rapid intensification, and NAEFS mean MSLP
progs suggest that by 12Z Wednesday, this event has a 75-90% chance
of reaching an extreme minimum pressure.

Intense low pressures in March are not exceedingly rare, but this
one will likely result in a wide swath of heavy snow and strong
winds, producing blizzard conditions across the Plains and Upper
Midwest. While there is some uncertainty still among the various
model clusters as to the exact temporal and spatial evolution,
confidence is high that after some heavy snow in the Rockies D1, a
swath of blizzard conditions will develop farther east.

This will result in heavy snow developing first across KS/NE late
Tuesday night/early Wednesday morning, and then shifting northeast
through Wednesday across IA, SD, MN, WI, and finally the U.P. of
MI. During this time, the robust moisture funneling northward on
300K isentropic ascent will rotate into a strong TROWAL, helping to
support elevated instability and increased moisture. Where the
TROWAL is most impressive and lies atop the strongest deformation
from eastern NE through northern WI, cross-sections continue to
indicate a risk for CI and thundersnow. This will support snowfall
rates that have high chance for exceeding 1"/hr and 2"/hr (90% and
60% chance, respectively, from the HREF), which is supported as
well by the WPC prototype snowband tool. Where CI occurs, rates
could reach 3"/hr, and this will be more than sufficient to
overcome hostile antecedent conditions.

While some snow may initially melt, snow accumulation will likely
become rapid as SLRs rise due to cooling despite dendritic
fracturing on winds as high as 60 kts below the DGZ. This will
produce blizzard conditions, and the WSSI-P is forecasting a 90%
chance of moderate impacts, and 20% chance of major impacts, even
where snow amounts are more modest due to the speed of this system.
As far as snowfall amounts, WPC probabilities have increased, and
while are modest D1, are high (>70%) D2 for more than 6 inches from
far eastern NE through a continuous, but narrow, corridor as far
north as the central U.P. of MI. In this corridor, WPC
probabilities for 12+ inches have increased to as high as 30-50%,
highest from NW IA through the U.P., and WSE plumes indicate a
potential for up to 15" in some areas should this band translate
temporally over some areas a bit longer. Some higher snow totals
are also possible in the Huron Mountains of the U.P. where the
deformation axis and some lake enhanced snowfall combine.

By D3 this entire system shifts well east as the primary low moves
into Ontario and the accompanying cold front surges into the Mid-
Atlantic states. While some wrap-around snowfall will likely
continue through early Thursday, especially where lake enhancement
can occur across the U.P. and northwest L.P. of MI, as well as S/E
of Lake Michigan, additional accumulations are expected to be
modest, but have a low chance (10-30%) of exceeding 4 additional
inches in the U.P. and far NW lower Michigan near Traverse City.

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

The period begins with an amplifying mid-level trough diving across
the eastern Great Basin and into the Four Corners, pushing a cold
front eastward into the Rockies. This front will have the
potential to produce scattered convective snow showers and snow
squalls this morning across UT/CO/WY as its forcing driven by 0-2km
fgen moves east into a region of at least modestly unstable air
(100-300 J/kg of SBCAPE) and increased low-level RH. While snow
accumulations from these squalls will be minimal, brief heavy snow
rates and gusty winds could produce dangerous travel through the day.

Behind this front, remnant low to mid level fgen and increased
moisture will drive a swath of heavy snowfall across much of the CO
Rockies and northward into WY and the Black Hills of SD. Large
scale ascent through the height falls downstream of the parent
trough combined with this fgen will allow for some heavy snow
rates, especially into the terrain where upslope flow will
additionally produce lift. WPC probabilities D1 for more than 6
inches are high (>70%) across the Wasatch, CO Rockies, and much of
the terrain of WY where locally more than 12 inches is possible,
especially in the Big Horns. Where the most intense fgen is
progged, primarily along the MT/WY border (a bit north from
previous model cycles), some lower elevation snowfall is likely
which could accumulate 3-6" into the High Plains of southern MT.

As this trough moves into the Plains later D1, brief ridging
blossoms across much of the West bringing an end to precipitation.
However, this respite will be relatively short lived as yet another
deepening trough approaches from the Pacific and moves onshore
Wednesday night. Height falls, PVA, and LFQ diffluence ahead of an
accompanying jet streak will all provide deep layer lift, aided by
upslope ascent as the low-to-mid level flow south of the trough
becomes more zonally oriented. This overlap of zonal/broadly
confluent mid-level flow beneath the upper jet streak will pivot
IVT exceeding 250 kg/m/s onshore, leading to an expanding plume of precipitation beginning Wednesday night /late D2/, and continuing
into D3. Snow levels during this period will be rather low,
starting around 2000-2500 ft, and warm only slowly, reaching
3500-4000 ft by the end of the forecast period. This suggests that
pass-level impacts are likely, and where the most intense ascent
occurs, even the lower valleys could receive some light snow accumulations.

WPC probabilities for heavy snow exceeding 4 inches on D2 reach
above 70% from the northern CA terrain northward through the
Cascades and into the Olympics. On D3, moisture shunts north and
east just a bit, causing a wane in precip over the Sierra, but
expanding WPC probabilities for more than 4 inches from the
Olympics and Cascades east as far as the NW WY ranges and CO
Rockies where they exceed 70% in the higher terrain (especially in
the Cascades, Tetons, and Park Range of CO).

...Interior Northeast... Day 3...

Guidance has trended stronger with a developing low pressure
Thursday evening across the northern Mid-Atlantic states. While
there is still considerable uncertainty at this time range in the
strength of this system, most ensemble systems indicate a rapidly
developing low pressure between 00Z Friday and 12Z Friday lifting
across southern New England.

The synoptic setup is favorable for this development. The same
trough responsible for the Plains/Midwest blizzard will continue to
shift east, and while the primary shortwave will lift into Canada,
a secondary southern impulse is progged to deepen over the Central Appalachians. This vorticity lobe will help split the jet stream,
leading to an increasingly coupled pattern as the subtropical jet
streak pivots over the Carolinas and the polar jet stream lifts
towards Greenland. The LFQ of the former and the RRQ of the latter
will overlap the greatest height falls, indicating a strong
likelihood of low development along a surface cold front/baroclinic
gradient. By 12Z Friday, the deterministic global models prog SLP
to be sub 990mb, and this rapid intensification within the synoptic
setup will likely cause a strong deformation axis to develop NW of
the surface low. While the antecedent column is marginal for wintry precipitation, strong ascent in the vicinity of this deformation
will result in rapid dynamic cooling, and rain changing to snow,
possibly heavy, across Upstate NY and interior New England.

Being 6th period, confidence in amounts and placement is still
modest, but a band of heavy snow is becoming more likely, with
significant accumulations possible, especially in higher terrain.
This is reflected by current WPC probabilities that are above 50%
for 2 inches and 10-30% for 4 inches, highest in the Adirondacks
and Catskills.


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

$$
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