Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
421 PM EST Tue Feb 11 2025
Valid 00Z Wed Feb 12 2025 - 00Z Sat Feb 15 2025
...Mid-Atlantic...
Day 1...
...Winter Storm Ongoing Across the Mid-Atlantic with Impacts from
Significant Icing and Heavy Snow. Key Messages linked below...
A winter storm ongoing across the Virginias has already produced
multiple inches of snow from southern West Virginia through
central
Virginia, with icing reports along the Blue Ridge in North
Carolina.
Local storm reports show the highest snowfall amounts as high as
12 inches in the Allegheny Highlands of WV/VA. Meanwhile radar
shows the heaviest snowfall rates in a swath across north-central
Virginia into southern Maryland where snowfall rates over an inch
per hour are also being reported.
This storm is being caused by intensifying warm advection moving
over a stubbornly cold air mass caused by a 1030 mb area of high
pressure centered off of Nantucket. Thus, the predominant flow is
easterly. This flow off the Atlantic will become increasingly warm
with time as the flow turns more southeasterly. Any warming is
being countered by the aforementioned heavy snow rates, leading to
dynamic cooling by the sheer volume of the heavy falling snow.
Meanwhile in the mid-levels (850 mb) the flow will be south to
southwesterly throughout the event, though increasing in intensity
through this evening. In the upper levels, all of the lift will be
driven by the right entrance region (RER) of a 150+ kt jet over New
England. It's important to note that other than the jet, there
will be very little if any additional upper level support from any
shortwaves. Therefore, most of the support will be from warm
advection of a warm air mass over the colder air mass that remains
persistent near the surface.
Weak 700 mb waves will track along the plume of moisture which
extends from east Texas to the Delmarva. These will lead to
localized increases in intensity of the precipitation through
tonight. The rain-snow line is unlikely to move much from its
current position across much of southern Virginia through tonight.
A weak surface low/trough will from late tonight (around 06Z) off
the Virginia coast which will pull the precipitation plume east
into the Atlantic and largely end the wintry side of the event from
west to east. There is some disagreement in the high resolution
models as to when this will happen however, as a secondary weaker
wave may keep light frozen precipitation ongoing over northern VA
and the Delmarva as late as sunrise Wednesday morning. Ultimately
the snow event will end once drier air and a lack of mid-level
disturbances can overcome an otherwise saturated air mass that
stands ready to produce additional light precipitation at a
moment's notice.
While dynamically this snowfall event will be the weaker of the
two
storms (the other described below over the central Plains to the
Great Lakes), it may be the longer-duration of the two for
portions
of the Virginias, so the highest snowfall accumulations between
the
two storms could very well be similar.
To the south along the rain-snow line, there is expected to be a
long-duration ice event due to freezing rain for portions of the
Appalachians from central West Virginia through the Blue Ridge and
Shenandoah Valley from Roanoke, VA south into far northwestern
North
Carolina. As mentioned above, strong warm advection in the mid-
levels will override colder air near the surface, unlike further
north where the warm air will fail to warm temperatures above
freezing in the midlevels, further south the warmer air will
succeed
at both eroding the cold air enough to warm temperatures in the
mid-
levels above freezing, the cold air will remain stout at the
surface, even as a shallow layer. This will set up the potential
for
prolonged icing. Most of the precipitation will occur with this
first storm that is ongoing now. Some areas around Roanoke have
started the storm as snow but will transition to freezing rain as
the warm air aloft in this area has some success at impinging
northward tonight. Icing for many may exceed a quarter inch, and
could approach a half inch through Wednesday morning in the areas
where both the surface cold air and precipitation persist the
longest.
Late tonight into Wednesday, most of the central Mid-Atlantic will
be dry or light drizzle with pockets of freezing rain lingering
over the VA Piedmont and on ridges which would further add to ice
impacts.
WPC probabilities are moderate (50-60%) for 1/4 inch of ice along
I-
81 south of Roanoke south to extreme northwestern North Carolina,
though amounts drop to very low (5-10%) for 1/2 inch of ice. On
the
snow side, probabilities are moderate to high (60-70%) for at
least
6 inches of snow north and west of Roanoke, some of those have
already been realized). Farther east, those probabilities are low
to moderate (30-40%) from DC south along I-95 through
Fredericksburg, VA.
...CO Rockies/Central Plains/Midwest/Great Lakes...
Days 1/2...
...Winter Storm expected starting over Kansas/Nebraska spreads
northeast across Lower Michigan through Wednesday Night...
Troughing over the northern Great Basin 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. Snow probs after 00Z for >6"
are 40-60% above about 8000ft in portions of the Sawatch and San
Juan Mtns.
Downstream, a winter storm is developing over the Central Plains
while the one over the Mid-Atlantic one persists into tonight. For
forcing, there will be some notable differences between the two
storms. While there will certainly be some warm advection bringing
Gulf moisture north into the central Plains and Midwest, this storm
will be much more dynamic in the upper levels. A broad longwave
trough over the northern Plains will begin to shift eastward as a
potent shortwave trough rounds the southern periphery of the
trough. This will enhance the downstream jet, which will intensify
the forcing needed for surface cyclogenesis from the southern
Plains and eventually into the Ohio Valley. Thus, the expected
swath of snowfall associated with this low will be associated with
a well- developed surface low in much more of a "classic" form.
With the intensifying and fast-moving low providing the primary
forcing for the snowfall, expect heavy snow to develop across
central Kansas tonight, and track northeastward across far
northwestern Missouri, central and eastern Iowa, far northern
Illinois, southern Wisconsin, and across much of the Lower
Peninsula of Michigan through Wednesday night. The low will be
plenty strong enough to produce bands of heavy snow embedded within
the comma- head region as it picks up Gulf moisture to its
south...but due to the fast moving nature of the low, the duration
of any heavy snow, especially outside of any banding will be
shorter than the current Mid-Atlantic storm.
On Wednesday, as the storm moves into the left exit region of a
southwest-to-northeast oriented jet, the snow will spread into the
Midwest and upper Lakes. With lake-enhancement on the
northeasterly
winds, there may be locally heavier snow and higher accumulations
southwest of Lake Michigan, which may include the Chicago and
Milwaukee metros. Of course, this will be highly dependent on
where
any embedded bands associated with the storm can line up with the
lake, resulting in the localized enhancement. Blowing and drifting
of snow in the strong winds is likely right along the lake shore.
Additional enhancements are possible along the Lake Huron
shoreline
into Wednesday night for the same reason...a rather unusual
direction for Lake Huron enhanced snowfall.
On the southern side of the comma-head region of the low, enough
warm air is expected to move in around the center of the low to
result in an area of mixing/freezing rain just south of the swath
of
snow from central Oklahoma tonight northeast across much of
southern
Missouri, including the St. Louis area, then extending northeast
across central Indiana, including the Indianapolis area, and into
far northwestern Ohio, including the Toledo area. Each of these
areas, and of course those in between, have the potential to see
some light to moderate icing, up to 1/4 inch. Once again the
timing
of any icing should be shorter than areas further east, which
should
limit any icing accumulations and thus, the magnitude of any
impacts.
WPC probabilities for ice are moderate (40-60%) for 1/10 inch of
accumulation from northeast of Indianapolis northeast through
Toledo, OH. Probabilities are low (5-30%) for 1/10 inch of ice
from
south of St. Louis northeast through Indianapolis. For snowfall,
probabilities for 6 inches or more are moderate (40-60%) for much
of
northern Kansas through south-central Iowa, lower (20-40%) through
eastern Iowa and northwestern IL, then increase back to the
moderate category (40-70%) along the lakeshore in Wisconsin due to
lake-enhancement from Milwaukee south to northern Chicago, as well
as for much of eastern Michigan north of Detroit.
...Sierra Nevada and much of the West...
Days 1-3...
Day 1...
An expansive, positively-tilted trough lingers over the west
into Wednesday with northwest flow persisting over CA. A shortwave
impulse rides along the central CA coast tonight into Wednesday
producing areas of light to locally moderate precip with snow
levels around 3000ft in the southern Sierra Nevada and around
4000ft in the SoCal transverse and peninsular ranges. Day 1 Snow
probs for >4" are 30-50% in the southern Sierra Nevada and the
highest SoCal ranges such as near Big Bear.
Days 2-3...
A stronger low shifts toward the OR/CA border Wednesday night,
opening into a trough as it drifts east over the Great Basin
through Friday. A strengthening and increasingly northerly jet
shifts toward the West Coast Thursday night. An atmospheric river
with IVT around 500 kg/m/s shifts over the CA coast Wednesday night
through Thursday. A wave of moderate to topographically enhanced
heavy precip shifts east with the accompanying warm front Wednesday
night through Thursday. Snow levels over the Sierra Nevada are
initially 3000ft Wednesday evening, but quickly rise to 6000ft by
Thursday morning. Both Days 2 and 3 snow probs for >18" are high
along the length of the Sierra Nevada with 4-6ft forecast for the
High Sierra through Friday.
Farther north into the Shasta/Trinity region and Oregon,
pronounced cold air will be slow to retreat, with snow levels
generally rising only from 2000ft to 4000ft through Thursday with
lower precip rates than in the core of the AR over CA. Day 2.5
snow probs for >12" are 50-80% for the Shasta/Siskiyou/Trinity
Mtns and Day 3 snow probs for >8" are 40-70% for the OR Cascades.
The lingering cold air brings light lowland snow potential for
much of the PacNW with an appreciable threat for freezing rain
mainly Thursday over northwest OR. Day 2.5 ice probs for >0.1"
are 30-60% for mainly the Coast Ranges west of the Willamette
Valley.
Moisture quickly spreads east throughout the Intermountain West
Thursday through Friday ahead of the slow moving trough axis. Day 3
snow probs for >8" are above 70% for the Wasatch and Tushar Mtns in
Utah as well as for western slopes of the CO Rockies and around
50% for the western Sawtooths and Tetons into southeast ID.
...Northeast...
Day 2...
The system coming from Michigan Thursday morning will bring
widespread precipitation from SW to NE over the Northeast.
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
Wednesday night through Thursday with a wintry mix expected across
the Northeast with the far northern NY border east of Lake Ontario
and northern Maine getting the most snow. Day 2 snow probs for >4"
are limited to the Tug Hill, Adirondacks, Greens, Whites, and
interior Maine with 40% for >8" over the Thousand Islands area and
60% over northern Maine. Day 2 ice probs for > 0.1" are 20-30% in
the Poconos, Catskills, and Berkshires into the Litchfield Hills.
Jackson/Wegman
...Winter Storm Key Messages are in effect...
Central Plains through Midwest Winter Storm tonight into Thursday:
https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png
Mid-Atlantic Winter Storm continuing tonight for snow/Wednesday
night for ice:
https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png
Key messages are also in effect for the Atmospheric River into
California Wednesday night into Friday and can be found on Weather
Prediction Center social media.