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FOUS11 KWBC 041937
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
237 PM EST Sat Jan 4 2025

Valid 00Z Sun Jan 05 2025 - 00Z Wed Jan 08 2025

...Central Plains through the Mid-Atlantic...
Days 1-3...

...Major winter storm begins tonight across the Central Plains and
then shifts east to the Mid-Atlantic on Monday...

The first major storm of 2025 develops tonight as an amplifying
mid-level trough ejects from the Four Corners/Central Rockies and
closes off over Kansas by Sunday morning. This closed low will then
continue to deepen as it pivots eastward, reflected by NAEFS
500mb-700mb height anomalies falling to as low as the bottom 2.5th
percentile of the CFSR climatology. This closed feature will
continue to deepen gradually as it moves into the OH/TN Valleys
Monday, before slowly opening into a trough across the Mid-Atlantic
Tuesday morning. Through this evolution, a strengthening jet streak
will pivot around the base of the trough, reaching as high as
150kts, and overlapping effectively with the greatest height
falls/PVA to cause intense deep layer ascent. Beneath this synoptic
lift, a surface cyclone will develop in the lee of the Rockies and
then move nearly due east with subtle deepening until it exits
into the Atlantic on Tuesday.

As this surface low deepens and move east, impressive moisture will
be drawn northward from the Gulf of Mexico within a corridor of
intensifying isentropic ascent, especially within the 290-295K
surfaces. Here, mixing ratios climb to 4-6 g/kg, reflective of the
impressive moist driven by the warm advection, further evidenced
by PW anomalies surging to above the 97th percentile according to
NAEFS. The accompanying theta-e ridge is then progged to lift
cyclonically into a TROWAL surrounding this deepening low, and
pivot back into KS/MO on Sunday. This is likely to cause an
expanding area of precipitation, with the accompanying elevated
instability driving more intense precipitation rates through D1.
The pattern conceptually matches one that will support both an
intense laterally-translating band of heavy precipitation east and
downstream of the surface low, with a secondary, likely more
intense, pivoting deformation band to the NW of the low. Locations
that receive both of these bands, and stay all snow, will receive
the heaviest accumulations.

As far as snowfall amounts, they are likely to be quite impressive,
especially across the Central Plains and into the Ohio Valley.
Across the Plains, the lead WAA band will support snowfall rates of
1-2"/hr, and these types of bands can cause long duration of snow
in narrow corridors. While the guidance has come into much better
agreement with the placement of this band, there is still some
uncertainty as to the exact location, especially due to latitudinal
variations in the warm nose placement which will wreak havoc on
amounts due to mixing with sleet and freezing rain, as well as much
lower SLR (heavy-wet snow). As the low begins to push east, an
intense deformation axis will develop across central/eastern KS and
into NW MO. The guidance is quite aggressive with this axis, and it
appears probable that a pivoting band with convective snow rates of
2-3"/hr is likely as folded theta-e and -EPV overlap in cross-
sections suggesting upright (CI) convection potential. During this
time the column cools significantly, DGZ depth increases, and
winds intensify, so blizzard and near blizzard conditions are also
possible. It is this region that will likely experience the most
significant snowfall, reflected by WPC probabilities that are high
(70-90%) for 12+ inches from N-Central KS through far NW Missouri.
A broader swath of 6+ inches is likely (50-70%) encompassing this
region and spreading as far east as near Indianapolis on D1.

North of this developing system, some interaction of a northern
stream shortwave diving out of the High Plains will become absorbed
into the larger and more amplified developing system. This will
result in a swath of mostly light to moderate snow from SD into NE
on the periphery of the more impressive theta-e axis. While
snowfall across this region won`t be as intense or heavy as points
to the south/east, several inches of light fluffy snow is likely
within a cold and increasingly windy column. WPC probabilities are
moderate (30-50%) for 4+ inches from the SD/NE border and points
south across central NE.

Farther downstream, the system begins to move more quickly across
the OH VLY and into the Mid-Atlantic, which will limit the duration
of heavy snow moreso than in points west. Additionally, the
weakening upper low will cause the deformation axis to weaken
resulting in less intense snowfall rates. However, impressive rates
that will still exceed 1"/hr are expected within the band from
Illinois through Ohio, and then continue across portions of the
Mid-Atlantic where more uncertainty still exists. Additionally,
guidance has become more excited about a secondary shortwave
digging through the opening longwave trough, leading to an inverted
trough NW of the departing low Monday, with secondary low
development also possible east of the DelMarVa. This will, at the
very least, lengthen the duration of light to moderate snow from
OH to DE, with potentially a secondary axis of heavy snow along
this trough adding to snow totals. At this time, despite
latitudinal uncertainty, WPC probabilities are moderate to high for
6+ inches of snow, primarily on D2, from Indianapolis eastward to
the Washington, D.C. metro area. Locally as much as 12" of snow is
possible across the lower OH Valley, with lesser totals expected
across the Mid-Atlantic, although more uncertainty exists there.

And, not to be outdone, this system will also pose a significant to
damaging ice threat within the warm nose south of the heaviest
snow. The models have been trending a bit south/colder since
yesterday which pushes the ice axis a little farther south as well,
but the intense WAA over-topping the cold surface layer
accompanied by persistent E/NE dry-bulb advection is a classic
setup for damaging freezing rain. Although there is some
uncertainty into how efficiently the ice can accrete at times due
to what should be intense precipitation rates, the threat for
damaging ice has continued to increase from eastern KS through
southern MO and into central KY. Here, WPC probabilities for more
than 0.25" of ice are high (70-90%), with even some 10-30% chance
for greater than 0.5" of ice. This could result in downed limbs and
power lines leading to widespread power outages and impossible
travel. Farther east across the Central Appalachians and
surrounding foothills, WPC probabilities for 0.25" of ice peak
around 50%.

Finally, between the heavy snow and heavy icing, the forecast
profiles indicate a narrow stripe with cold layer depths greater
than 1000m, or above the 90th percentile for freezing rain in the
south-central US. This suggests sleet, especially in the presence
of a relatively weak warm nose aloft and the aforementioned
persistent dry advection at the surface. Heavy accumulations of
sleet are difficult to achieve, but in this setup it is possible
that 1-2" of sleet will accumulate causing tremendous driving
impacts. The greatest risk for the major sleet accumulations are
from near St. Louis southeast to extreme southern IN, but
fluctuations in placement are still possible.

This system has prompted the issuance of key messages which are
linked below.



...Western U.S....
Days 1-3...

A shortwave trough riding over a ridge axis centered off the
Pacific Northwest crosses Washington late tonight before being
absorbed into the trough over the Rockies that trails the low
developing over Kansas. Moisture streams in ahead of this trough,
making for moderate to heavy snow on the WA Cascades tonight with
snow levels rising from around 3500ft to 4000ft which means passes
will be impacted. Enough moisture reaches inland to bring moderate
snows to the Bitterroots and ranges around Glacier NP on Sunday
with snow levels rising from 2500ft to 4000ft. Day 1 snow
probabilities for >4" are moderate (around 50%) at Cascades pass
level and high (over 80%) for >8" in the higher North Cascades
terrain. Day 1 snow probabilities for >4" are 50-80% over the
greater Bitterroots and western Glacier NP ranges.

The next shortwave trough rides over the ridge Sunday night, diving
across the OR Coast and becoming very positively-tilted over the
the Desert Southwest on Tuesday. High initial snow levels around
7000ft over the OR Cascades only drop to around 6000ft under the
trough axis, so only the highest OR Cascades, well above pass
level will receive heavy snow. Snow levels will be closer to 5000ft
over the Great Basin and 4000ft over the northern Rockies. Day 2
snow probabilities for >4" are generally 30-60% over terrain in
eastern OR, much of central/southern ID, northeast NV, and central
MT.

As light to locally moderate precip shifts south with the trough
Monday night, snow levels in Utah will be around 5000ft. However,
on Tuesday there is a heavy snow potential on the eastern slopes of
the central/southern CO Rockies as strong high pressure (central
sfc pressure around 1040mb) shifts south down the Dakotas, aiding
an easterly low level flow to the moisture coming in from the
west. Day 3 snow probs for >4" are 20-50% for southern portions of
the Front Range and much of the Sangre de Cristos.


...Lake Effect Snow - Lake Ontario...
Day 1...

Low pressure centered over James Bay will weaken tonight into
Sunday as a rapidly developing low off Nova Scotia becomes the
dominant surface low in a broad gyre over southeast Canada. As a
result, the great pressure gradient between James Bay and a 1044mb
high over Manitoba will weaken and the strong WNW flow over central
Ontario will gradually diminish. Sufficient WNW flow will persist
over eastern Lake Superior, northern Lake Huron, southern Georgian
Bay, Lake Simcoe, and Lake Ontario to provide maximum moisture
fetch for LES single-banding to persist through midday Sunday
between the Tug Hill and Syracuse before drifting south and
weakening Sunday afternoon. Day 1 snow probabilities for >12" are
>80% on an axis from Oswego to Rome with the potential for a local
maximum exceeding 2 feet in Oswego County.


Weiss/Jackson




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

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

$$