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).
...Sierra Nevada into much of the West...
Days 1-3...
As a large trough amplifies across the Intermountain West,
upstream shortwaves will begin to dig along the Pacific coast as
vorticity impulses shed east from an amplifying closed low over the
Pacific. While this low won't really become organized until late
D3, energy pivoting onshore beginning late D1 will spread an axis
of moisture into central and southern CA, with snow in the terrain
above generally 3000-4000 ft. This impulse is weak and will be
directed almost due SW by the downstream trough, but WPC
probabilities are moderate (30-50%) for more than 6 inches across
primarily the southern Sierra D1.5.
After this first impulse, the closed low offshore begins to deepen
and pivot eastward. Height anomalies drop steadily offshore,
reaching as low as 1st percentile within the CFSR climatology by
00Z Friday near the CA/OR border, and the accompanying and
impressive downstream divergence will begin to displace the trough
with a brief period of shortwave ridging. Increasing moisture
advection onshore, driven by confluent mid-level flow south of this
closed low topped by a collocated strengthening jet streak will
surge IVT as high as 500 kg/m/s into the coast late Thursday. This
will manifest as expanding and intensifying precipitation, with the
accompanying warm front/WAA lifting snow levels rapidly to as much
as 6000 ft in the Sierra. However, farther north into the
Shasta/Trinity region and into Oregon, pronounced cold air will be
slow to retreat, leading to some lowland snow potential as well as
overrunning/freezing rain. Moisture will then quickly spread
throughout the Intermountain West through the end of the period
(12z/Fri) along with the progressing, but weakening upper trough.
There continues to be some uncertainty in timing and placement,
but there is high confidence that this event will eventually result
in heavy wintry precipitation across the West. Current WPC
probabilities are high (>90%) for 18+ inches across much of the
Sierra, generally above 4000 ft, and above 70% for 12+ inches
(above
3000 ft) in the Shasta/Trinity/Klamath region. This could be quite
impactful to many area passes. Further east into the Intermountain
West and central Rockies, high probabilities (>70%) for at least 8+
inches of snow exists across the Wasatch into the Tushar Mts of
Utah, as well as the San Juan Mts of Colorado.
...Northeast...
Days 1 & 3...
Today, a strong cold front will race from the Great Lakes through
New England, bringing with it an increased risk for convective snow
showers and snow squalls. The high-res CAMs remain aggressive with
their depiction of simulated reflectivity along this front Tuesday
aftn, suggesting a greater threat for snow squalls. The greatest
risk appears to be from northern Upstate NY across northern New
England which is where the best overlap of 0-2km fgen and RH
overlap, and although instability is modest, the SnSq parameter
does reach +2 across this region on the 00z GFS. This may end up
more as convective snow showers than true squalls, but briefly
intense snow rates and restricted visibility could cause dangerous
travel this afternoon/evening.
Then during the end of D2 and into D3 (centered around 00Z Thursday
to 00z Friday) more widespread significant precipitation will
overspread the region from SW to NE. This precipitation will be
associated with a robust low pressure lifting across the Ohio
Valley, driven by a modest shortwave on the downstream edge of a
deepening trough over the middle of the CONUS, overlapped with the
RRQ of a 150 kt poleward arcing jet streak moving across the
eastern Great Lakes. Downstream of this system, moist advection
will maximize in response to low- level WAA on S/SE flow
overrunning a retreating high pressure that will try to wedge back
to the west into New England. This will result in an expansion of
wintry precipitation, likely starting as snow everywhere before
gradually transitioning to sleet and freezing rain, especially
south of I-90, before winding down Thursday night.
There is growing confidence with the speed of the system, with at
least moderate snowfall accumulations likely, especially in higher
terrain and northern Maine, as reflected by WPC probabilities that
are above 50% for 4+ inches from the Adirondacks across much of
northern New England, with >80% probs in northern Maine.
Additionally, light to moderate icing is possible as reflected by
WPC probabilities of 10-30% for 0.1" in the Catskills, Berkshires,
and Litchfield Hills.
Weiss/Fracasso/Snell
...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