FOUS11 KWBC 110852
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 AM EST Tue Feb 11 2025

Valid 12Z Tue Feb 11 2025 - 12Z Fri Feb 14 2025

...Ohio Valley to the Mid-Atlantic... Days 1-2...

Active pattern continues with two significant winter weather 
events progged to impact the area with different hazards through
Wednesday night.

The first is already taking shape across the Mid-Mississippi and
Ohio valleys this morning as precipitation begins to expand from
the Ozarks to the southern Appalachians, with snow and a wintry mix
expected to stretch from near southern IL to southwest VA by the
start of D1 (12z/Tues). This precipitation shield will be the 
result of an overlap of forcing and moisture expanding into the 
region from the west and south. A shortwave trough and accompanying
vorticity maxima is racing eastward across the Southern Plains this
morning while also weakening in response to confluent/flat flow 
across the eastern CONUS. Despite the modest amplitude of this 
feature, ascent will intensify through modest PVA and mid-level 
divergence overlapped with the right entrance region to a jet 
streak amplifying to the northeast. A weak surface wave may develop
and skirt almost due east ejecting off the Mid-Atlantic early
Wednesday, but in general precipitation will be driven by the 
overlap of the synoptic ascent and increasingly impressive moist 
isentropic upglide/WAA emerging from the Gulf.

The moisture surging northward will be significant as reflected by
mixing ratios within the robust 290-295K isentropic ascent 
reaching as high as 6-8 g/kg, drawing PWs as high as the 90th 
climatological percentile as far north as southern VA. This
moisture flowing northward overrunning the cold airmass in place
from the Ohio Valley to Mid-Atlantic will lead to an expanding 
swath of precipitation, with rounds of heavier rates likely 
embedded near where mid-level fgen is strongest. There remains some
uncertainty with respect to the northern edge of the heaviest QPF,
mainly due to dry air intrusion and if the strengthening WAA can
overcome it. This will lead to a sharp northern gradient along the
Mason-Dixon Line and into central NJ. Marginal surface temperatures
at onset during the day will also limit snowfall accumulation. However,
there is high confidence that a stripe of heavier snowfall rates
overcoming these marginal surface temperatures will spread east 
from Kentucky through Delaware. Snowfall rates of 1+"/hr are likely
at times, especially within the west-to- east oriented fgen band 
progged to start in eastern KY this morning and central VA by the
afternoon before weakening and gradually lifting northward into
southern MD and the Delmarva Peninsula. This is in conjunction 
with where 850-700mb fgen maximizes ascent and occurs in 
conjunction with CSS (EPV* < 0.25) to support CSI. Where this band 
develops, locally higher amounts of snow are possible, but in 
general WPC probabilities spanning D1 and early D2 are high (>70%) 
for more than 4" from eastern KY though the Central Appalachians 
eastward to southern MD just south of Washington, D.C. Locally 
higher totals are possible across the central Apps of WV and VA, 
where WPC probabilities for at least 8" mostly low (10-30%) outside
of the highest elevations. 

Farther to the south, a transition zone is still expected where a 
mixture of snow, sleet, and freezing rain will occur due to an
800-850mb warm nose aloft and a textbook winter CAD signature east
of the Apps in the surface pressure pattern that is forecast to 
linger through Wednesday. This area of mixed ptype is most likely 
from KY and into the Central Appalachians, especially along the 
Blue Ridge of NC/VA. While some sleet is possible, the predominant 
mixed p-type is likely to be freezing rain within this axis, and 
will almost certainly be significant (to locally damaging) in the 
vicinity of the Blue Ridge.

This first event winds down late Wednesday morning through the 
afternoon /D2/, but just as the column begins to dry aloft, renewed
WAA ahead of the next system approaches within this progressive 
pattern. While the DGZ dries out across the Mid- 
Atlantic/Appalachians, the low levels re-saturate, and it is 
possible in some areas there is never a break in wintry 
precipitation due to very light snow/snow grains/freezing drizzle 
Wednesday. Eventually the column re-saturates though as a more 
intense low pressure developing over the Southern Plains lifts 
northeast into the Ohio Valley with another round of mixed 
precipitation spreading into the region.

This second wave will be stronger, but also farther inland, so 
many areas will begin with snow or freezing rain and eventually 
transition to all rain. The heaviest precipitation will likely 
begin right around Wednesday evening in the Central Appalachians 
and then expand northeast into the Mid-Atlantic. High pressure 
wedging into the region will again force impressive isentropic 
overrunning, and locations from northern VA through PA will likely 
begin as a burst of moderate snowfall before quickly transitioning 
to sleet, freezing rain, and eventually all rain before waning 
Thursday evening. The heaviest snowfall is likely in the Poconos
and central PA terrain where WPC probabilities for 2+ inches are 
as high as 50-70%, with icing exceeding 0.1" likely (>70%) in the 
Laurel Highlands and parts of the Poconos.

WPC event total probabilities (12z/Tues to 12z Thurs) for more 
than 0.25" are above 70% in the vicinity of the Blue Ridge where 
locally more than 0.5" is also expected for some areas (50% 
chance). There also exists some non-zero (5-15%) probabilities for
at least 1" of ice in terrain of far northwest NC and adjacent VA.
This will cause severe impacts including power outages, scattered 
tree damage, and dangerous travel. Surrounding this, WPC 
probabilities for more than 0.1" of 10-30% encompass a much larger 
area including into parts of south- central VA and up the 
Appalachians to near the MD Panhandle and Laurel Highlands.

...Central Rockies/Plains/Midwest/Great Lakes... Days 1-3...

Troughing over the northern Great Basin this evening will dip into
the central Rockies tonight, with broad WSW flow across CO. This 
will favor an expansion of snowfall over the Medicine Bow mountains
and into the San Juans/Sangre de Cristos where upslope enhancement
will wring out several inches of snow area-wide. For the D1 
period, WPC probabilities for at least 6 inches of snow are >50% 
above 10,000ft or so. 

By late tonight into early Wednesday, troughing will pass the 
spine of the Rockies as low- level moisture increases over the 
foothills to the Plains via SE to E flow. This will combine with NE
flow at the surface to increase snow into the Front Range, though 
snow should generally be light overall. WPC probabilities for at 
least 4 inches are near 20-30% over parts of the I-25 corridor. 

The more intense snowfall will manifest farther east over Kansas
and southern Nebraska starting late tonight into Wednesday. There,
the potential exists for moderate to heavy snow across much of 
Kansas as the upper jet extends from TX northeastward to the 
Midwest. Combination of WAA, tightening thermal gradient 
(increasing FGEN), and favorable/deep DGZ/isothermal layer will 
promote locally heavy snow with >1"/hr rates (per WPC snowband 
tool). 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 even as low pressure deepens over the 
ArkLaTex. SLRs may eclipse 20:1 in the most favorable bands but 
otherwise hover close to 15:1 along the I-70 corridor from GLD to 
MCI. Further southeast across parts of central Oklahoma through
southeast Kansas and southern/central Missouri, overrunning will 
favor sleet and freezing rain. WPC probabilities for at least 0.1"
ice are low- moderate in this region (10-50%) and highest over
Missouri. For snowfall, PVA and FGEN will force much of the 
precipitation in advance of the trough with the axis of heavier 
snowfall along I-70 toward/into Kansas City, then extending 
northeastward. WPC probabilities for at least 4 inches of snow are 
high (>80%) over most of central/northern KS into southern NE with
some probs (30-50%) for at least 8 inches.

By Wednesday night, the system begins to further organize and pick
up forward speed as a relatively weak surface low (>1000mb) lifts 
through the Ohio and Tennessee valleys. The lack of rapid
deepening of the low pressure system due to the quickly lifting
longwave trough will prevent impacts associated with blowing and
drifting snow. 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 
forecast in a southwest to northeast band from southern MO through 
central IN and northwestern OH. This narrow corridor of mixed ptype
could remain relatively stationary for a period of time across
Indiana and northwest Ohio for measurable freezing rain and sleet.
The combination of strong PVA and lower level FGEN into a deep DGZ
should provide for an axis of heavier snow, but where that 
materializes is still bit fuzzy (including any lake enhancement 
off Lake Michigan on NE winds) as some uncertainty in the 700 mb
fgen field and high SLR environment could prompt multiple areas of enhancement.

Right now, the highest probabilities of at least 6 inches of snow 
maximize from northern MO through the Chicago metro and into 
central Lower MI, with >60% probs extending to the NW and SE by 
about 150 miles. This includes cities such as Kansas City, Chicago
and Milwaukee just to name a few. Within this region, 
probabilities of at least 8 inches are moderate (>40%) from 
northern MO to northern IL and across central Lower Michigan.

Like areas farther upstream (southern Plains), a mix or sleet and 
freezing rain is likely closer to the track of the surface low due 
to the overrunning of the cold surface with warmer air aloft over 
portions of central IN into northwestern OH. There, WPC 
probabilities of at least 0.10" icing have increased this forecast
cycle and are moderate (40-60%).

The positive-tilt to the pattern will help move the system along, 
with precipitation ending Thursday morning over Michigan, though NW
flow on the backside will stir up some lake effect snow for U.P. 
and L.P. of MI. Amounts should be light at the end of this forecast
period (12Z Thu - 12Z Fri).


 .... Continued ...
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