FOUS11 KWBC 100843
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 AM EST Mon Feb 10 2025

Valid 12Z Mon Feb 10 2025 - 12Z Thu Feb 13 2025

...Great Lakes... Days 1-2...

Large surface high pressure stretching from the northern Plains
through Mid-Atlantic creates enough of a pressure gradient across
the Great Lakes to promote continuing westerly 850mb wind up to 35
kts at times thanks to a couple passing shortwaves. The first 
shortwave is racing across the Great Lakes and New England this 
morning with a third impulse speeding across the region Tuesday 
morning as flat/progressive flow persists across the CONUS. Each of
these subsequent shortwaves will be accompanied by a weak cold 
front, driving enhanced CAA across the Great Lakes to support 
periods of lake effect snow (LES). On D1, 850mb temps fall to -15C 
or more, steepening low-level lapse rates sufficiently to lift 
inversion levels to nearly 10,000 ft, which with ascent maximizing 
into the lowering DGZ will cause bands of heavy snow with rates of 
1"/hr or more across the U.P. of MI, as well as SE of Lake Ontario 
and into the northern Finger Lakes region early D1 before flow 
shifts more westerly into the Tug Hill by tonight. WPC 
probabilities D1 are moderate (40-60%) for 6+ inches along the SE 
lake shore of Ontario and Tug Hill, as well as across the Keweenaw
Peninsula of the U.P. of MI.

...Mid-Mississippi Valley to the Mid-Atlantic... Days 2-3...

Confluent mid-level pattern will maintain progressive flow across 
the CONUS as the next in this parade of storms develops over the 
Central Plains and Ozarks Monday evening. This system will 
strengthen in response to vorticity energy shedding from the base 
of a longwave trough initially over Baja, combining with the RRQ of
a southern stream jet streak which begins to amplify and arc 
meridionally, leaving favorable diffluence atop the mid-level 
PVA/divergence. The downstream confluence of the northern and 
southern streams should result in suppression of this wave, keeping
it fast and generally flat from west to east, but will also result
in significant ascent and a prolonged period of precipitation as 
isentropic ascent from the Gulf surges northward and spokes of 
vorticity continue to lag back to the SW leaving a long duration of ascent.

The north-south trends with this system appear to have stabilized
as most upper features settle into place with only subtle changes 
in the forecast expected at this point. This aims the heaviest 
axis of overlapping QPF and subfreezing thermals from northeast KY,
through central VA into the Delmarva. 

Based on current guidance, this is beginning to look like a 
significant winter event containing snow and sleet and freezing 
rain, generally from the Ozarks almost due east into the Central 
Appalachians and the I-95 corridor from Richmond, VA to Baltimore, 
MD. The downstream confluence of the mid-level pattern will force 
this generally west-to-east axis of heavy precipitation, with 
290-295K isentropic ascent and accompanying WAA fueling the 
elevated PWs progged by NAEFS to exceed the 90th climatological 
percentile as far north as the TN VLY and southern Mid-Atlantic 
(NC/VA border). This will supply plentiful moisture for heavy 
precipitation, and the sharpening baroclinicity on the WAA leading 
to impressive 850-700mb fgen will help to concurrently enhance 
ascent. This suggests that precipitation rates will be heavy at 
times, although in areas with snow the DGZ appears elevated and the
best fgen is well below that, which when combined with modestly 
"warm" temperatures should keep SLRs low and this could be a heavy 
and wet snow for most of the area. The end of the latest 00z HREF
depicts the potential for snowfall rates up to 1"/hr in the
heaviest band. Current WPC probabilities are high (>70%) in 
northeast KY, the central Appalachians, central VA into southern MD
and the central Delmarva D2 for 4+ inches of snow. Medium probs 
(40-70%) expand northward through northern VA, MD, DE, and southern
NJ, with a very tight gradient expected along the northern and 
southern fringes. This places the highest potential for significant
snow between Richmond, VA and Washington, D.C.

Farther to the south, there is likely to be a transition zone from
rain to snow which will include axes of moderate to heavy freezing
rain, especially in the higher elevations around the Ozarks, as 
well as across portions of the Central Appalachians from eastern KY
through SW VA. There remains a lot of uncertainty as to the 
amounts of icing, especially noting that rates could be heavy at 
times which don't efficiently accrete, and with the general 
synoptic uncertainty continuing, but there is high confidence 
(>70%) for warning-level icing (above 0.25" of ice in SW VA, with 
locally as much as 0.6" of damaging ice possible through Thursday morning.

As vorticity lobes continue to shed eastward from the Baja trough,
and as the developing full latitude trough (merging northern and 
southern streams over the Intermountain West) lags the first 
impulse, a second system with additional precipitation will 
approach this same area from the SW late D3 /Wednesday aftn./ This 
will again produce an axis of all p-types from the Ozarks eastward 
to the Mid-Atlantic, and its possible in some areas there will not 
even be much of a break before this second impulse. The Wednesday 
system developing in the Central Plains will likely be more intense
with respect to the surface feature and crossing west of the
Appalachians, so more warm air should flood northward downstream, 
but the evolution of this second wave will be somewhat dependent on
the track of the first event. At this time, WPC probabilities for 
0.1" of freezing rain D3 generally just feature patch 10-30% from 
the Ozarks into northern KY.

...Central Rockies/Plains/Midwest/Great Lakes... Days 2-3...

As the longwave trough over the West passes the Rockies Tuesday 
into Wednesday, precipitation will expand across the central 
Plains. Snow over the Rockies will accumulate several inches 
Tuesday afternoon and overnight through early Wednesday over the 
Medicine Bow mountains with at least 6 inches above 10,000ft likely
(probs >50%). Positive tilt and broad SW flow will favor a 
progressive system D3, but the potential exists for moderate to 
heavy snow across much of Kansas Tuesday night into Wednesday. 
Upper jet will extend from TX northeastward to the Midwest with 
speed divergence over the Plains helping to promote ascent into a 
moistening column on WAA from the south in the lower levels. A 
large high pressure to the north will supply NE winds to the region
but the gradient is not expected to be strong enough to produce 
much blowing snow. By early Wednesday, snow could fall heavy at 
times over central KS as favorable FGEN aligns with sufficient lift
into a potentially very deep DGZ and/or isothermal layer, but 
these smaller scale features are hard to pinpoint this far out. In 
addition, SLRs may be close to 20:1 or higher in these favorable 
areas but otherwise waver around 15:1 given the colder air mass.
Dry air is always a concern along northern areas of the
precipitation shield, but the duel jet structure developing over
the central U.S. and a broad moist forecast in upper air relative 
humidity fields, the greatest uncertainty will likely be associated
with localized forcing and banding potential.

By Wednesday night, the system begins to further organize and pick
up forward speed as the surface low lifts through the Ohio and
Tennessee valleys. Snow is expected to expand from southern IA and
northern/central MO through much of IL, southern WI, northern IN,
and the L.P. of Michigan. A narrow corridor of mixed ptype is also
still expected somewhere from southwest MO through southern IL and
into southern IN and western OH. The event will quickly conclude 
by the end of D3 across the Midwest as CAA and dry air rushes into 
the region. However, increasing WAA will begin to spread snow, 
sleet, and freezing rain northward across the Lower Great Lakes and
Northeast by early Thursday.

WPC probabilities of at least 4 inches of snow for this large
system are >50% from far eastern CO across much of KS, southern 
NE, northern MO, southern IA, central/northern IL, southern WI, 
northern IN, and much of the L.P. of MI. This includes cities such 
as Kansas City, Des Moines, Chicago, and Milwaukee just to name a 
few. Within these probabilities, 20-50% chances for at least 8 
inches of snow follow along and just north of I-70 in KS and span
east-northeast across the Midwest towards lower Michigan. Maximum
snowfall amounts of 8 to 12 inches appear most likely. Southern 
side of the precipitation shield over OK and into the Ozarks will 
likely be more marginal, and a mix or sleet and freezing rain is 
likely due to the overrunning of the cold surface with warmer air 
aloft. There, WPC probabilities of at least 0.10" icing are low (10-30%).

...Pacific Coast... Day 3...

Although still far out in time with limited confidence and
extending into Day 4, it appears the West will begin to become 
more active again mid- to- late week as moisture begins to stream 
onshore from the Pacific. During D3, there remains noticeable 
spread as to where the best moisture plume will go (GEFS quicker 
and north compared to the ECENS) as it gets restricted to the SW on
the periphery of a large trough over the CONUS and beneath a 
short- wavelength ridge over the Pacific Northwest. At the same 
time, a deepening trough over the Pacific will begin to track 
eastward, leading to a push of moisture a bit farther NW and into
southern/central CA. This could result in some light to moderate 
snow reaching the Sierra before 00Z Thursday, but unquestionably 
more active weather with much heavier snow will develop beyond this
current forecast period. At this time, WPC probabilities are 
moderate (30-60%) for 6+ inches in the Sierra, generally above what
could be very low snow levels of 2000-2500 ft.

Snell/Weiss/Fracasso

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

https://www.wpc.ncep.noaa.gov/key_messages/La...

https://www.wpc.ncep.noaa.gov/key_messages/La...

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