Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 PM EST Fri Jan 17 2025

Valid 00Z Sat Jan 18 2025 - 00Z Tue Jan 21 2025

...Great Lakes...
Days 1-3...

Low pressure over the north-central Ontario along with surface
ridge over western Canadian Prairies combine to push Arctic cold 
south down the Great Plains and east across the Great Lakes through
Saturday night. The LES machine over Lake Superior begins this
evening for the U.P. on NWly flow, the western L.P. overnight.
Gulf-sourced moisture shifts in advance of the cold front will
surge over the eastern Great Lakes Saturday before tapering off
Saturday night. There's a bit of a lull Sunday over the eastern
lakes with LES kicking in there Monday. Day 1 snow probs for >4"
are 20-40% over the northern U.P. and 30-60% from the synoptic flow
south and east of Erie and east of Ontario. Day 2 snow probs for
>4" are 40-80% in the U.P. and portions of the western L.P., then
higher on Wly flow for Day 3 with 40-70% over the same U.P./L.P.
zones and east of Erie/Ontario.


...Colorado Rockies and High Plains...
Day 1...

The combination of both upper level diffluence ahead of a deep-
layer trough and the arrival of a bitterly cold Arctic air-mass
will
continue to produce some snow squalls into this evening along the
cold front from the north-central Rockies east across the Great
Plains.

Focused mountain snow is expected over the eastern slopes
of the CO Rockies and adjacent high plains including the I-25
corridor tonight into Saturday. The base of the main trough
descending from Canada has a vort lobe currently pushing south over
Idaho. This will shift SSE over the Four Corners through Saturday
which will aid lift over CO which has low level upslope flow post-
frontal with a 1050mb high moving into MT on Saturday. Snowfall
rates
reach 1"/hr per the 12Z HREF on the eastern slopes of the Front
Range this evening before the bands work their way south across the
Raton Mesa early Saturday. Day 1 snow probs for >4" are high
(60-90% over the Front Range down to Pikes Peak and 30-50% for the
I-25 corridor through the length of CO.

Be cautious of hazardous driving conditions in any further snow
squalls and on the eastern slopes of CO terrain.


...Central Appalachians, Mid-Atlantic and Northeast...
Days 2-3...

...Confidence has further increased from a non-ECMWF deterministic
solution general model blend for a disruptive winter storm from
the central Appalachians, central Mid-Atlantic through the
Northeast with locally heavy snowfall possible along and west of
the
I-95 corridor...

Guidance remains in good agreement on a two wave solution. The
initial wave on Saturday is a shortwave in advance of the main
polar sourced trough with the wave of concern the positively-tilted
trough axis that lifts from the Southeast Sunday through Sunday
night. AI-based guidance (ECMWF- AIFS and GFS GraphCast) have been
consistent with coastal low development off the Mid-Atlantic coast
Sunday lifting north past New England Sunday night. Ensembles
(ECMWF-EPS, GEFS, GEPS) continue to be snowier, and pretty much
all deterministics outside the ECMWF are as well. The 12Z CMC is
the farther west/most interior low track while the 12Z ECMWF is the
farthest east/offshore and has the lowest QPF of the
deterministics. The ECMWF-AIFS, however, now has five days of
consistent runs with a coastal low along the Mid- Atlantic coast.
This solution has drifted north and been more potent over the more
recent runs up through the 06Z. All to say that while most guidance
has considerable heavy snow (in the 6-10" range) from eastern PA up
through Maine, there remains some volatility and uncertainty in
the heavy snow swath location and intensity.

The heavier snow begins late Saturday night over eastern KY with
terrain and low level fgen aiding enhanced snow over the central
Appalachians through WV into PA and then northeast over the more
interior central Mid-Atlantic (generally NW from the I-95 corridor
of northern VA and north) with a max between eastern PA and
southern Maine. Day 2 snow probs for >6" are 40-70% over central
WV/far western MD into the Laurels of PA. Day 2.5 values for >6"
are 30-70% from eastern PA north of Philly through eastern Maine
with the highest probs from the Catskills through Mass and southern
NH. There is expected to be a tight rain/snow line around the DC
Metro and northeast east from I-95 through the NYC Metro and
Boston Metro. Only minor adjustments in this line makes for impact
changes to millions of people, so close attention should be taken
in the coming days.


...Gulf Coast...

Next week...
A reinforcing shortwave trough rounding the Four
Corners Monday brings about surface trough and eventual low
development off the lower TX Coast starting later Monday. Cold air
arrives into south Texas Sunday night, so overrunning flow may
cause an onset of wintry mix over south Texas prior to 00Z Tuesday.
Further info on this threat can be found in the medium range
discussion (PMDEPD) and in newly issue key messages linked below.


Jackson




...Winter Storm Key Messages are in effect and linked below...

Gulf Coast winter storm threat next week next week
https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

Northeast winter storm threat this weekend
https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png