FOUS11 KWBC 290749
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
249 AM EST Wed Jan 29 2025

Valid 12Z Wed Jan 29 2025 - 12Z Sat Feb 01 2025

...Great Lakes, Northeast, Mid-Atlantic... Days 1 & 3...

An Alberta Clipper type low will be racing east across New England
and into the Gulf of Maine to start D1, driving an arctic cold
front southward in its wake. This evolution will result in three 
areas of moderate snowfall D1.

First, the guidance continues to indicate that the environment 
will be favorable to support widespread and impressive snow squalls
today, especially across the interior Northeast and as far east as
I-95 from near Philadelphia, PA through Portland, ME. While 
snowfall within any snow squalls will result in minimal 
accumulations (less than 1" most areas), snowfall rates in excess
of 1"/hr as reflected by HREF probabilities in areas with
impressive overlap of 0-2km fgen and CAPE will create dangerous
driving due to highly variable visibilities and snow covered roads.
The strongest squalls are likely along and just behind the front,
and many areas will likely experience squalls through sunset on
Wednesday, and Key Messages (linked below) are in effect for this event.

Behind the front, strong CAA across the Lakes will help support
another round of lake effect snow (LES) in the favored W/NW snow
belts, especially across the eastern U.P. of MI, and east of Lakes
Erie and Ontario. The heaviest accumulations are likely across the
Tug Hill and western Adirondacks (somewhat aided by upslope flow as
well), with lighter accumulations elsewhere. Overall, LES D1 
should be modest, but WPC probabilities feature a high risk 
(70-90%) for 4+ inches of snow in the Tug Hill, western 
Adirondacks, portions of the Greens, and the higher elevations of the Whites.

Finally, Downeast Maine will also likely experience a narrow
corridor of moderate to heavy snow as secondary low pressure
developing offshore beneath the LFQ of an upper jet streak angles
moist isentropic ascent onshore. The axis of moderate snow is
likely to be narrow, but a ribbon of snow accumulating to several
inches is likely, as reflected by WPC probabilities for 4+ inches
as high as 50% along the immediate coast.

Thursday appears quiet across the region, but this changes quickly
late Thursday night/Friday morning as a larger scale storm system
approaches from the SW. This system will emerge from the Central
Plains as a strong closed low, opening slowly as it approaches the
Mid-Atlantic Friday evening, and reaching the Atlantic coast by
Saturday morning. Impressive downstream divergence initially will
somewhat weak as the low fills, but will still overlap directed
moisture as a theta-e ridge surges northward from the Gulf of
Mexico leading to PWs that reach above the 90th climatological
percentile according to NAEFS. The antecedent airmass is modestly
cold for wintry precipitation, so much of the accompanying
precipitation will likely be rain across the Mid-Atlantic, but the
higher terrain of PA (around the Poconos) and into Upstate
NY/southern New England will likely receive some mixed
precipitation of sleet/freezing rain, with snow farther north.
There will also likely be some gradual cooling of the column from
north to south late in the forecast period in response to dynamic
effects and ageostrophic drainage as the low pulls away, resulting
in at least a brief period of heavy snow. WPC probabilities D3 for
heavy snow exceeding 4 inches are confined to the Adirondacks and
northern New England, where they are generally 10-50%.

There remains quite a lot of uncertainty into the placement of the
mixed precip zone, and how much moisture will lift northward, but 
current WPC probabilities for freezing rain are 10-30% for more
than 0.1" of ice near the Poconos, with a broad swath of 30-70% for
0.01" of ice encompassing much of the interior northern Mid-
Atlantic and southern New England.

...Southwest through the Four Corners... Days 1-2...

Anomalous closed low will continue to advect slowly northeast
across the Four Corners today before moving into the Central Plains
on Thursday. This feature will maintain amplitude, reflected by
500mb heights fall towards the 2.5 percentile according to NAEFS,
suggesting ascent will persist downstream as it moves. The most
impressive lift is likely immediately east of the upper low where 
height falls and divergence combined with the diffluent LFQ of a 
strengthening and poleward arcing jet streak, which is also where 
the greatest moisture is expected as a PW plume surges and lifts 
cyclonically into NM/CO. With a generally cold air mass in place, 
and steep lapse rates beneath the upper low persisting, rounds of 
heavy snow are expected to continue, especially in the higher 
terrain above 5000 ft from the Mogollon Rim to the San Juans, 
Sangre de Cristos, and even as far east as the Raton Mesa and 
Palmer Divide. However, the moisture is expected to get cutoff and
pivot east into the Plains during D2, bringing an end to the 
snowfall in the region. 

Before that happens, heavy snow will accumulate in the higher 
terrain of the Four Corners reflected by WPC probabilities D1 of 
50-90% in the White Mountains of AZ, as well as the San Juans and 
Sangre de Cristos. During D2, the focus shifts east, with heavy 
snow likely confined to the Sangre de Cristos once again.

Farther east into the High Plains including the I-25 urban 
corridor, wraparound snow aided by upslope could result in 
impactful accumulations across these areas. Even just by D2 there 
is uncertainty in the amount of available moisture due to extremely
different camps in the guidance, but WPC probabilities currently 
suggest a 50-90% chance of at least 2 inches both D1 and D2, 
greatest in the higher elevations in the vicinity of the Raton Mesa
and Palmer Divide.

...Pacific and Interior Northwest... Days 2-3...

A more active period begins in the West Thursday as an atmospheric
river (AR) begins to push onshore the coasts of WA/OR before
spilling inland on Friday. IVT within this AR has a high
probability (>60%) of exceeding 500 kg/m/s according to both ECENS
and GEFS probabilities, with this AR being surged onshore in 
response to a deepening trough across the Pacific moving eastward.
The overlap of confluent mid-level flow with a surging Pacific jet
streak reaching the coast will drive the moisture onshore, 
reflected by IVT eclipsing the 90th percentile Friday, with a core 
above the 99th percentile in the northern Great Basin overnight 
into Saturday morning. This increased moisture will manifest as an 
expanding area of precipitation, but with WAA accompanying the AR, 
snow levels will climb steadily to as high as 4000-5000 ft on 
Friday before collapsing to as low as 1000 ft by the end of the 
forecast period, lowest in WA state, behind a cold front.

The result of this will be increasing snowfall, primarily in the WA
Cascades and Olympics D2, expanding along the Cascades and into
northern CA, while concurrently reaching as far east as the
Northern Rockies and Salmon River/Sawtooth Ranges D3. SLRs will
likely be quite low ahead of the cold front, but should rise
steadily as snow levels crash late in the forecast period. This is
reflected by high WSSI-P probabilities for snow load late D2 into
D3, creating impacts at higher elevations. WPC probabilities are 
moderate (30-50%) for 6+ inches D2 in the higher elevations of WA. 
By D3 these probabilities increase and expand, with a high risk 
(>90%) for more than 6 inches along the Cascades of WA and OR, the 
Olympics, Northern Rockies, Salmon River, and Sawtooth Ranges.

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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