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/La...

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