Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
412 PM EST Wed Feb 12 2025

Valid 00Z Thu Feb 13 2025 - 00Z Sun Feb 16 2025


...Central Appalachians...
Day 1...

...Next winter storm will further enhance freezing rain concerns
over the central Appalachians into tonight. Key Messages linked
below...

A shortwave impulse is over AR/MO this afternoon which is on the
leading edge of a sprawling, positively-tilted trough axis over the
Rockies. This is bringing Gulf moisture up across the Southeast
into the Ohio Valley and up the Mid-Atlantic. Precip has once again
overspread southern VA, but given the strong warm nose, this is
rain over the coastal plains and well into the Piedmont. However,
afternoon temperatures are at or below freezing from the Blue Ridge
foothills and west over Virginia to the Allegheny Plateau in
central WV, then south along the crest of the Appalachians into NC
and north through the Laurels of PA. This cold will only slowly
erode tonight with continued freezing rain, adding to the existing
ice impacts. WPC Day 1 ice probabilities for >0.25" additional ice
after 00Z are around 10% for the Allegheny Highlands of VA with
40-60% probabilities for >0.1" from southwest VA north through the
Blue Ridge/Shenandoah Valley, Potomac Highlands, Laurels, and up
through the Allegheny National Forecast in northwest PA.


...Great Lakes and Northeast...
Days 1-2...

...Winter Storm continues over the Midwest and Great Lakes tonight,
spreading over Northeast Thursday. Prolonged heavy lake-effect
snow downwind of Lake Ontario Thursday night into Friday...

Shortwave trough over MO will lift over the Great Lakes tonight
before amplifying as it tracks over the Northeast Thursday. A sign
of this amplification is a re-strengthening of SWly jet over the
eastern Great Lakes tonight and over New England Thursday with a
return of 150kt flow by 12Z Thursday. Decent banding lingers from
Des
Moines through Milwaukee to across MI north of Grand Rapids on the
north side of a dry slot in the developing wave. Ideal DGZ depth is
making for good snow rates in this band which will continue rest of
this evening west of Lake Michigan and into the overnight over the
L.P. of MI. This axis extrapolated ENE over the Northeast overnight
and Thursday tracks very near the northern NY border with heavy
snow generally over the Thousand Islands NY and across northern
Maine. Warm air advection makes for a wintry mix over much of the
rest of the Northeast/New England tonight/Thursday with snow
changing to some sleet and freezing rain.

Day 1 snow probs for >6" are 20-50% over eastern MI north of
Detroit, in the 30-50% range for the Adirondacks, Greens, Whites,
and northern Maine as well as long the northern NY border. Freezing
rain probs for >0.1" ice is 20-40% for the Poconos, Catskills,
Berkshires, southern Greens and Whites.

Following the passage of the surface low across the eastern Great
Lakes Thursday morning, 1035mb+ high pressure building into the
Mid- Mississippi Valley will prompt strong WNWly flow over open
Great Lakes (Erie is ice covered). After a brief period of
favorable conditions for snow showers forming off lakes Superior
and Michigan Thursday morning, focus turns to Lake Ontario
Thursday afternoon through Friday as -15 to -20C 850mb
temperatures cross. Day 2 WPC probabilities for >8" are 40-70%
from the southeast lakeshore through southern Oswego county, NY to
a line over the north side of Syracuse. Snowfall totals locally
over a foot are possible.


...Sierra Nevada, Pacific Northwest through the Rockies...
Days 1-3...

...Atmospheric River with heavy Sierra Nevada snow affects
California tonight into Friday. Key Messages are linked below...

Upper low centered out near 140W west of the OR/CA border is
directing an atmospheric river (AR) that reaches the northern CA
coast this evening and shifts south down the coast to southern CA
through Thursday night as the low opens into a slow moving trough.

The atmospheric river has IVT around 500 kg/m/s making for a wave
of moderate to topographically enhanced heavy precip tonight
through Thursday night over the Sierra Nevada. Snow levels over
the Sierra Nevada are initially 4000ft at onset this evening,
quickly rise to 7000ft by Thursday morning, then drop back to
6000ft under the trough axis Thursday night. Both Days 1 and 2 snow
probs for >18" are high along the length of the Sierra Nevada with
4-7ft 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 4500ft through Thursday with
lower precip rates than in the core of the AR over CA. Day 1.5
snow probs for >12" are 50-80% for the higher Shasta/Siskiyou/
Trinity Mtns and Day 2 snow probs for >8" are 50-80% for the OR
Cascades.

The lingering cold air brings light lowland snow and ice potential
for southern WA through western OR. An appreciable freezing rain
threat is over northwestern OR Thursday/Thursday night mainly
along the Coastal Ranges. Day 1.5 ice probs for >0.1" are 40-80%
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
2 snow probs for >8" are above 80% for the Tetons through the
Wasatch
and Tushar Mtns (UT) and across the western slopes of the CO
Rockies with around 50% probs still for the Sawtooths of ID, and
Kaibab Plateau in AZ and northern NM ranges. Snow rates greatly
lower across the west Friday as the trough axis drifts over the
Rockies, but broad light to locally moderate snow persists over
much of the terrain. Most of the Rockies have over 30% probs for an
additional 6" on Day 3.


...Upper Midwest and Great Lakes...
Day 3...

As the upper trough crosses the southern Rockies Friday night into
Saturday, a strengthening southwesterly oriented jet streak across
the Plains places the Upper Midwest in the favorable left-exit
region over an inverted surface trough. Potent axis of 700 mb WAA
may allow for additional lift beginning across the Upper Midwest
Friday before sliding eastward into Lower Michigan by Saturday
morning. Current WPC probabilities for >4" are around 20% over
eastern Neb and western IA with >6" probs of 30-60% mainly over
southern WI and much of the L.P. of MI. Dynamic banding is forecast
with this re-developing wave and guidance has variability in
placement, so further details on heavy snow banding will come into
better agreement in subsequent shifts.


Jackson




...Winter Storm Key Messages are in effect for both the Northeast
and West as linked below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png