Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 PM EST Mon Feb 10 2025
Valid 12Z Mon Feb 10 2025 - 12Z Thu Feb 13 2025
...Great Lakes...
Day 1...
A strong cold front will exit into New England Tuesday morning,
leaving strong CAA across the Great Lakes in its wake. 850mb temps
plummeting to -15C to -20C, coldest over Lake Superior, will
support a renewed period of lake effect snow (LES) in the favored
W/NW snow belts across the U.P. of MI and into the Tug Hill
Plateau. A very cold column over Lake Superior will drive the DGZ
down to the surface which will allow for a maximum intersection of
lake-induced ascent and the snow growth region. Farther east across
Lake Ontario, the overlap won't be quite as efficient, but some
higher moisture due to an upstream connection will additionally
enhance the LES potential. Snowfall rates of 1"/hr or more are
likely Monday night and Tuesday before subtle height rises bring an
end to LES before D2. WPC probabilities for 6+ inches are above
70% in the Tug Hill Plateau and portions of the eastern U.P. near
Whitefish Pt on D1.
...Mid-Mississippi Valley to the Mid-Atlantic...
Days 1-3...
Active pattern continues with two significant winter weather
events progged to impact the area with different hazards through
the week.
The first begins early Tuesday morning as precipitation begins to
expand across the Ozarks, Mid-Mississippi Valley, and Tennessee
Valley. This precipitation 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 will
eject from the Four Corners this evening and race eastward into the
Southern Plains by Tuesday morning as it weakens 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 accompanying this system, 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 of Mexico.
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 VA/KY. This will an
expanding swath of precipitation, with rounds of heavier rates
likely embedded. The guidance has generally started to stabilize
the footprint and thermal structure, but some wavering is still
possible especially the upstream shortwave intensity and speed
continues to fluctuate. However, there is high confidence that a
stripe of heavy snow will spread east from Kentucky through
Delaware. Snowfall rates of 1+"/hr are likely at times, especially
within a west-to-east oriented fgen band progged to setup from
eastern KY into DE, 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 D2 are high (>70%)
for more than 4" from eastern KY though the Central Appalachians
eastward near Washington, D.C. and into southern DE. Locally more
than 8 inches is possible where the most intense band tracks.
Farther to the south, a transition zone is still expected where a
mixture of snow, sleet, and freezing rain will occur. This is most
likely from the Ozarks, through 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. WPC probabilities for more than 0.25" are above 70% in the
vicinity of the Blue Ridge where locally more than 0.5" is possible
(30% chance). 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 the Ozarks, parts of Kentucky, and the
Central Appalachians near the MD Panhandle and Laurel Highlands.
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 spread 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 the end of D2 in the Central Appalachians and
then expand northeast into the Mid-Atlantic around 00Z/Thursday or
slightly afterwards. 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 where WPC probabilities for 2+
inches are as high as 50%, with icing exceeding 0.1" possible
(>30%) in the Laurel Highlands and parts of the Poconos.
...Central Rockies/Plains/Midwest/Great Lakes...
Days 1-3...
Troughing over the PacNW/BC this evening will dip into the Great
Basin and into the central Rockies tomorrow evening, 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 D1.5 period, WPC probabilities for at least 6
inches of snow are >50% above 10,000ft or so.
By Wednesday morning, 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
30% over parts of the I-25 corridor.
The more intense snowfall will manifest farther east over Kansas
starting late Tuesday 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. Trend in the recent guidance has
been a bit to the N/NW especially east of the Plains, which may
bring more ice into southeastern KS than what was seen 24 hours
ago. There, overrunning will favor sleet and freezing rain from SW
to NE from OKC to southeastern KS into southern MO. WPC
probabilities for at least 0.1" ice are low-moderate in this region
(10-50%). 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 (>70%) 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 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
forecast in a southwest to northeast band from southern MO through
central IN and northwestern OH. Here, the NW trend in the 12Z
guidance was more pronounced (esp the 12Z NAM), and WPC made a
modest adjustment to the NW given other signals from other
models/ensembles (which were not as dramatic or not shown at all).
SLRs should increase to the NW deeper into the colder column, but
pared back the NBM which could be a bit too high. Nevertheless,
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 bit fuzzy (including any lake enhancement off Lake
Michigan on NE winds).
Right now, the highest probabilities of at least 4 inches of snow
maximize from northern MO through the Chicago metro and into
central Lower MI, with >50% probs extending to the NW and SE by
about 100 miles. This includes cities such as Kansas City, Des
Moines, Chicago, Milwaukee, and Detroit, 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 are low (10-40%).
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 - 00Z Fri).
...Pacific Coast...
Days 2-3...
A large trough will amplify across the Intermountain West,
upstream of which 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 D2 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 D2.
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. There continues to be a lot of
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 12+ inches across much of the Sierra, generally above
4000 ft, and above 70% (above 3000 ft) in the
Shasta/Trinity/Klamath region. This could be quite impactful to
many area passes.
...Northeast...
Days 1 & 3...
On Tuesday, 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 have
become more 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
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. This
may end up more as convective snow showers than true squalls, but
briefly intense snow rates and restricted visibility could cause
dangerous travel Tuesday afternoon/evening.
Then during D3 /00Z Thursday to 00Z Friday/ more 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 late D3 everywhere but ME/NH.
There is still considerable spread in the speed of this system
forecast by the deterministic global members, but at least moderate
snowfall accumulations are likely, especially in higher terrain,
as reflected by WPC probabilities that are above 50% for 4+ inches
from the Adirondacks across much of northern New England.
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