FOUS11 KWBC 130856
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 AM EST Thu Feb 13 2025

Valid 12Z Wed Feb 12 2025 - 12Z Sat Feb 15 2025

...Great Lakes and Northeast... 
Day 1...

Low pressure moving through the Ohio Valley this morning will 
continue northeastward into southeastern Ontario, with broad WAA- 
driven snow over northern New England D1 and an icy mix where 
boundary layer sub-freezing temperatures persist beneath the 
warming layer ~750-850mb (e.g., North Country, Green and White 
Mountains). By this afternoon, a forming area of low pressure near 
the Gulf of Maine will help maintain a colder column over much of 
Maine, favoring more snow for the Pine Tree State via northerly 
flow. After 00Z, the whole system will depart into Atlantic Canada.
WPC snowfall probs for >4" are highest (>50%) in the higher 
elevations of the Adirondacks and Green/White Mountains, as well as
over the northern half of Maine. Freezing rain probs for >0.1" ice
are low (10-20%) over parts of central New England.

In its wake, NW/WNW/W flow across the mostly unfrozen Great Lakes 
will support lake effect snow as 850mb temps of -15 to -20C move 
across the region. WPC probabilities for >6" low (10-50%) over the 
eastern U.P. of MI but are high (>70%) south of Buffalo with some 
moisture flux through the ice and especially southeast off Lake 
Ontario just north of SYR up to FZY. Snowfall totals locally over a
foot are possible.


...Sierra Nevada, Pacific Northwest through the Rockies... 
Days 1-3...

...Atmospheric River with heavy Sierra Nevada snow affects 
California tonight into Friday. Key Messages are linked below... 

Upper low approaching 130W this morning is directing an 
atmospheric river (AR) into NorCal then shifts south down the coast
to SoCal tonight as the low opens/splits into a slow-moving trough.

The atmospheric river has IVT around 500 kg/m/s (near the MAX in 
the CFSR climo period) making for a wave of moderate to heavy 
topographically- enhanced precip over the Sierra Nevada. Snow 
levels around 6000-7000ft across CA this morning will drop back to 
5000-6000ft under the trough axis tonight (Sierra) and around 
4500ft (Shasta/Trinity). SLRs will be low to moderate given the 
strong WAA, resulting in plenty of Sierra cement (>5ft and perhaps 
>8ft at the highest peaks). Snowfall rates will lessen into early 
Friday (as SLRs rise a bit) and end completely by Friday night as 
upper ridging builds in.

The lingering cold air brings light lowland snow and icing 
potential for southern WA through western OR. Freezing rain threat 
will advance northward on the edge of the precip shield this 
morning from western OR (Coastal Ranges) northward across the 
Columbia River into southwestern WA. WPC probs for >0.1" are 
moderate (40-80%) for mainly the Coast Ranges west of the 
Willamette Valley. 

Moisture quickly spreads east throughout the Intermountain West 
through Friday ahead of the slow-moving trough axis. Moisture 
transport into the Great Basin and Wasatch/Rockies will be notable 
per the ECMWF EFI and NAEFS IVT percentiles >99th percentile. WPC 
probs for >8" are above 80% for the Tetons through the Wasatch and 
Tushar Mtns (UT) and across the western slopes of the CO Rockies 
with >60% probs for the Sawtooths of ID, and Kaibab Plateau in AZ 
and northern NM ranges. Snow rates peak early Fri from west to east
as the trough axis drifts over the Rockies, but broad light to 
locally moderate snow persists over much of the terrain into early 
Sat (end of D2). By Sat/D3, the last piece of the broad longwave 
trough will slip through the central Rockies, yield another several
inches of snow centered over western CO where WPC probs for at 
least 6 inches are moderate (40-70%).

Finally, a sprawling North Pacific system will enter the PacNW at 
the end of D3 with generally light snow for the WA Cascades.


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

Leading edge of the western trough Friday afternoon will manifest 
itself at the poleward exit region of a 140kt upper jet across the 
Plains and near the equatorward entrance region of the northern 
stream jet across southern Ontario. This will promote broad lift 
over the Upper Midwest with accompanying WAA atop an inverted 
surface trough. Quick westward flow aloft will keep the system 
moving through the region, but potential exists for a quick-hitting
few inches of snow with room for some localized bands of heavier 
snow across WI. WPC probs for >4" snow are >50% over much of 
central and southern WI to the IL border and into Lower MI. 

Farther east into D3, snow will expand into the Northeast on 
continued WAA ahead of a deepening upper trough and strengthening 
upper jet arced from the Lower MS Valley to the St. Lawrence River 
Valley. Light to moderate snow is forecast across southern Ontario 
eastward into NY and New England where WPC probs for >4" snow are 
moderate (40-70%), especially above 1500ft or so. Across the 
interior Mid-Atlantic, cold surface temperatures from the departing
high pressure will be overrun with WAA precip, resulting in areas 
of freezing rain from the southern Appalachians northward through 
the Laurel Highlands into the Catskills. WPC probs for at least 
0.1" icing are moderate (40-70%) with low-end (10-30%) probs for at
least 0.25" icing. This will depend on how strong the southerly 
flow will be to scour out the cold surface temperatures vs precip 
timing/onset and intensity. 

Fracasso/Jackson

...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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