605
FOUS11 KWBC 052136
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
436 PM EST Wed Feb 5 2025

Valid 00Z Thu Feb 06 2025 - 00Z Sun Feb 09 2025

...Pacific Coast through the Northern Rockies and Northern Plains...
Days 1-3...

...Active winter period across the west ends with a potent low
that tracks across California Thursday night, the northern Rockies
Friday, and the northern Plains Friday night...

The stormy pattern that has been ongoing across the West for
several days now will come to an end by this weekend. But until
then, periods of mountains and lowland snow will stick around the
Pacific Northwest this afternoon and evening thanks to an elongated
and positively tilted upper level trough over the northwest
Pacific. Farther south, a potent 500mb shortwave trough caught up
in the SWrly flow aloft will race towards California on Thursday.
NAEFS shows this system will have a healthy AR associated with it
and direct a >500 kg/m/s IVT (climatological percentiles topping
the 99th percentile) into the western U.S. Mandatory heights
associated with this trough are all below the 10th climatological
percentile as well, which should lead to a lowering of the snow
levels as it moves ashore on Friday. The heaviest snowfall in the
central and southern Sierra Nevada will generally reside above
6,000ft, while the northern Sierra Nevada and the
Shasta/Salmon/Siskiyou see heavy snowfall as low as 4,000ft in
some cases.

As the storm moves inland Friday morning, a cold Canadian air-mass
anchored by a 1030+mb high over the northern High Plains will be in
place, allowing for snow to the remain the primary precip type from
eastern Oregon and the Blue Mountains through the Sawtooth,
Bitterroots, Absaroka, Tetons, and Wind River Ranges. Snowfall
will be heavy in these ranges, including ranges farther south such
as the Bear River and Uinta ranges thanks to a highly anomalous IVT
>300 kg/m/s (>99.5 climatological percentile at 18Z Fri) and these
ranges placement beneath the left-exit region of a 90kt 500mb jet
streak. Snow will push east through central and eastern Montana
Friday afternoon, then into the Dakotas by Friday night. Snowfall
rates will increase along the ND/SD border Friday night and into
Saturday morning as 850-700mb frontogenesis (FGEN) ensues and WAA
aloft supports heavy snow.

WPC probabilities through Saturday morning show high chance
probabilities (>70%) for snowfall totals >24" above 7,000ft in the
central and southern Sierra Nevada, and above 6,000ft in the
northern Sierra Nevada and Shasta ranges. Similar high chance
probabilities for >12" of snowfall are present in the Siskiyou and
Salmon mountains, as well as the Tetons, Bear River, and Wind
River Ranges. The Blue, Sawtooth, and Absaroka above 6,000ft are
also forecast to receive as much as a foot of snow, while the High
Plains of central and eastern Montana have moderate-to-high chances
(50-70%) for snowfall totals >4". The WSSI shows Major Impacts
(dangerous to impossible travel, widespread closures and
disruptions) along the spine of the Sierra Nevada, in the Siskiyou,
Salmon, Shasta, and southwest OR coastal ranges. There are
expansive areas of Moderate Impacts (hazardous driving conditions)
from the northern OR coastal ranges and along the OR Cascades on
east through most of the aforementioned Northern Rockies Ranges.
Minor Impacts are also being depicted at the moment in central and
eastern Montana for late Friday into Saturday.


...Northern Plains through the Great Lakes and Ohio Valley...
Days 1-3...

Potent shortwave trough (700mb heights falling below the 1st
percentile of the CFSR climatology according to NAEFS) will advect
across the Dakotas and into Minnesota Thursday and Thursday night
before racing east and out of the area by Friday morning. Rapid
height falls and impressive PVA downstream of a strung out lobe of
vorticity rounding the base of this trough will combine with the
LFQ of a modestly poleward arcing jet streak to produce large scale
ascent into the region. At the same time, moisture will increase
as modestly veered flow leads to robust moist isentropic ascent at
285-290K which will result in an expanding area of precipitation.
This precip will occur entirely as snow from ND through MI as the
column is quite cold, and the WAA accompanying this isentropic
ascent will help deepen the DGZ impressively (SREF probabilities
for 100mb of DGZ depth 30-70%, highest along the Canadian border).
This will result in high SLR snow accumulating rapidly as snowfall
rates may exceed 1"/hr at times (30-50% chance according to the
HREF). The system is progressive, limiting total accumulations, but
WPC probabilities indicate a 50-70% chance of at least 4 inches in
NE MN, including the Arrowhead, and into the western U.P. of MI.

Farther downstream and to the east, the accompanying WAA will
overrun a retreating high pressure to the east, leading to an axis
of moderate mixed precipitation from the Corn Belt into the Ohio
Valley. The WAA will lift rapidly northward, but will drive a brief
period of robust fgen, in the 925-700mb region, leading to a period
of moderate to heavy sleet/freezing rain before precipitation
changes over to rain. WPC probabilities D1 are as high as 50-70%
for 0.1" of ice, highest across north/central Ohio.

After a brief respite on D2, the progressive flow across much of
the CONUS will surge another winter storm system eastward into the
Northern Plains to begin D3. This system is progged to be much more
impressive as the potent driving trough ejects from the Pacific
Northwest and into the Northern Plains by Saturday morning. The
overall mid-level pattern across the Northern Plains is progged to
remain generally flat, but rapid height falls and a potent
subtropical jet streak reaching 150kts will help push IVT exceeding the
90th percentile from NAEFS into the region. At the same time,
strengthening WAA downstream of a low pressure moving across the
Central Plains will draw higher moisture northward, and the overlap
of this WAA with the diffluent portion of the upper jet streak will
produce a stripe of impressive 850-600mb fgen, efficiently lifting
ascent into a deepening DGZ (SREF probabilities for 100mb of depth
exceeding 30%). This will result in a swath of snow from the
Dakotas through Wisconsin, with an embedded band of heavier snow
likely. There is still quite a bit of longitudinal (timing) and
latitudinal (placement) uncertainty amongst the global members, but
ensembles suggest high confidence for this event and impactful snow
is likely across the Dakotas and into Minnesota/Wisconsin. WPC
probabilities have increased dramatically today, and now support a
high risk (>70%) for more than 6 inches D3 near Aberdeen, SD,
surrounded by a larger expanse of >50% from far western SD through
western WI.

Then farther to the southeast, the WAA will lift a warm nose more
robustly immediately downstream of the surface low into the Upper
Midwest and Ohio Valley. This will cause a stripe of mixed
precipitation to develop before the end of the period from Illinois
through eastern Ohio, and WPC probabilities increase once again,
now featuring a 30-50% chance for at least 0.01" from near
Davenport, IA through Cleveland, OH.


...Mid-Atlantic and Northeast...
Days 1-3...

Generally flat and fast flow within the mid-levels will dominate
the eastern half of the CONUS, keeping systems progressive into the
weekend. This will allow for a series of winter storms to impact
the region.

The first of these will develop tonight as a modest shortwave
trough ejects from the Central Plains and lifts E/NE across the
Ohio Valley, reaching New England Thursday evening. A weak jet
streak will accompany this feature as it moves east, producing some
subtly enhanced deep layer lift, but most of the ascent will be
driven instead by impressive 850mb warm advection, which will
intensify especially over New England Thursday afternoon. This WAA
overrunning a retreating high pressure will drive intensifying
isentropic ascent, especially within the 290-300K layer, and
moisture will be copious as reflected by mixing ratios exceeding
6g/kg. This will allow PWs surging northward from the Gulf of
Mexico to exceed the 90th, and in some places the 97th percentile
of the CFSR climatology according to NAEFS ensemble tables. At the
same time, a wave of low pressure will develop offshore and move up
the coast from NC to New England on Thursday, additional enhancing
ascent across the region.

As WAA/isentropic ascent increases, precipitation will overspread
the area from SW to NE, with a mixture of rain, snow, freezing
rain, and sleet expected. The p-type challenges are significant
with this event.

Across the Mid-Atlantic states, into Upstate NY, and as far north
as southern New England, precipitation will feature a mix of all
p-types, and sorting out the predominant is challenging. The column
initially starts cold enough that everywhere north of the Mason-
Dixon line could start as a burst of snow within the WAA, with
sleet the initial p-type farther south. What happens thereafter
makes the difference with respect to impacts. Although the high
retreats quickly, a wedge of high pressure will remain embedded
south into the Mid-Atlantic, and initially surface wet-bulb
temperatures are well below 0C due to large T-Td depressions. This
suggests that as precip begins, the wedge will be reinforced and
hold firmer than guidance indicates, despite the overwhelming
influence of the WAA. The high-res models have warmed a little bit
today compared to previous runs, but the initial cold depth is
still well above the 75th% percentile for freezing rain for parts
of VA/MD/PA, suggesting a long period of sleet before WAA wins out
turning precip over to ZR. Of course, farther north into PA and
Upstate NY may experience more IP than ZR, while the higher terrain
of WV/MD/PA (Appalachians, Alleghenies, and Laurel Highlands) have
the best chance for damaging freezing rain accretions due to being
above the cold layer. Overall, the system is progressive, but
significant QPF falling into the cold layer will produce
significant icing for the Mid-Atlantic with hazardous commutes
likely Thursday morning. WPC probabilities are moderate (30-50%)
for 0.25" in the central Appalachians, surrounded by a much larger
area of at least 50% for 0.01" or more across much of the Mid-
Atlantic and into southern New England. This event continues to be
the source of key messages linked below (Keymessage_1).

Farther north into Upstate NY and New England, much of the
precipitation is expected to fall as snow. The intensifying WAA
will result in a band of heavy snow as the resultant 850-700mb fgen
drives intense ascent into the DGZ just above. This will cause a
burst of snow lifting SW to NE Thursday, first in the Poconos, then
all points northeast, including New York City and Boston. Snowfall
rates within this burst could reach 1-2"/hr as reflected by the WPC
prototype snowband tool and HREF probabilities, but rapid
translation of this band northeast will somewhat limit total
accumulations. Still, substantial impacts are likely, and WPC
probabilities are high (>70%) for more than 4" of snow in the
higher terrain from the Adirondacks into the Greens and Whites,
with lesser accumulations likely in the lower elevations.

As this first low pulls away into Canada, a second wave of low
pressure moving across Ontario will drive a potent cold front
eastward from the Great Lakes and through New England Friday
morning/aftn. This cold front will have a two-pronged effect on
winter weather. First, the passage of the cold front may incite
some convective snow showers or snow squalls from Upstate NY
through Maine as low-level fgen and some modest 0-2km instability
combine in an area of elevated RH. This is reflected by the SnSq
parameter exceeding 1 Friday morning in this area. While
accumulations within any squalls will be limited, brief intense
snowfall rates could cause hazardous travel.

Then behind this front, increasing CAA on W/NW flow will cause
modest lake effect snow (LES), especially downstream of Lake
Ontario where water temps are still marginally warm (+5) while Lake
Erie is completely ice covered at this point. With 850mb temps
falling to as low as -15C, this will steepen lapse rates
sufficiently to produce heavy snow into the Tug Hill and western
Adirondacks, although total accumulations are expected to be
somewhat modest as reflected by WPC probabilities that are 50-70%
for 4 inches D1 and D2.

Finally, as the active pattern continues, another shortwave trough
will dig out of the Central Plains Saturday, reaching the eastern
Great Lakes/Ohio Valley by the end of the forecast period. In a
pattern similar to that on Thursday, an accompanying surface wave
will develop across the Ohio Valley and then track progressively
eastward. The downstream WAA will again be impressive, and surge
another round of mixed precipitation into the Mid-Atlantic states.
There is still quite a bit of uncertainty as to both timing and
placement of the heaviest snow and icing amounts Saturday, but
current WPC probabilities indicate a moderate risk (30-50%) for at
least 0.01 inches from central VA through southern PA.


Weiss/Mullinax


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

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

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png

$$