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FOUS11 KWBC 041953
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
253 PM EST Wed Dec 4 2024

Valid 12Z Wed Dec 04 2024 - 12Z Sat Dec 07 2024

...Michigan...
Days 1-2...

The Arctic front associated with a deepening area of low pressure
tracking to the north of the Great Lakes today is moving through
the Upper Midwest this morning and will soon be the catalyst for
yet another lake-effect snow (LES) event across much of the
Michigan U.P. and for both northern and western communities of
Michigan`s Mitten. At the onset today, a series of potent snow
showers along the Arctic front will race south and east as NWrly
winds begin to kick-start the lake-effect snow machine from the
Porcupine Mountains to the Hurons. These squalls will work their
way through the southern part of the state this afternoon and
evening with the favorable combination of strong frontogenetical
support at low levels and a robust 500mb jet streak (exceeding
observed 500mb winds for this time of year in the CFSR database 00Z
this evening) fostering a favorable setup for sustaining potent
snow squalls through this evening. Meanwhile, cold air advection
(CAA) over the warm waters of Lakes Superior and Michigan will
cause surface-850mb delta Ts to range between 34-40F. These steep
lapse rates combined with well saturated low-level profiles will
support multi-banded lake-effect streamers that produce 1-2"/hr
snowfall rates through tonight and into Thursday. Some hi-res CAMs
suggest the potential for a dominant single band streamer that
could produce heavy snow in southwest Michigan and into the
Michiana area bordering IN/MI.

LES should wind down Thursday night and taper off across much of
Michigan by Friday afternoon. WPC PWPF depicts high chances (>70%)
for snowfall totals >8" for much of the Michigan U.P. and in the
northwest part of the Mitten. Some localized areas could approach
18" in the hardest hit spots. There are low-to-moderate chances
(30-50%) for snowfall >8" where the single lake-effect band sets up
in southwest Michigan. The WSSI shows primarily Moderate Impacts
for most of these aforementioned regions, although some localized
Major Impacts (considerable impacts such as dangerous to impossible
driving conditions) are possible in parts of the Michigan U.P. and
northwest portion on Michigan`s Mitten.


...Eastern Great Lakes and Northeast...
Days 1-3...

The types of snowfall from the eastern Great Lakes through the
Northeast Thursday are WAA/Synoptic scale, lake-effect, snow
squalls, and upslope mountain snow. The LES and mountain snow are
forecast to persist into Saturday.

1. WAA/synoptic scale: This pattern is driven by a powerful mid-
upper level low over MI this evening before reopening into a
negatively-tilted trough over New England late Thursday. By 12Z
Thursday, NAEFS shows 700mb and 500mb heights that are near the
lowest observed heights at 12Z for this time of year in the CFSR
climatology over Pennsylvania, indicating how anomalously deep
this cold-core low becomes. Moisture will stream northeast and
boundary layer temperatures look to remain sufficiently cold
enough to support snow as WAA this evening, especially in the
higher elevations of the Berkshires, Green, and White Mountains.
Expect moderate snow to begin over the southern Adirondacks this
afternoon and then across the rest of the northern Appalachians
this evening. Heavy snow will then envelope much of northern Maine
overnight. By Thursday morning, the surface low will track east
closer to Caribou, Maine with additional snowfall expected along
an inverted trough on the backside of the storm. Cyclonic flow
will allow for upslope snow to persist through Thursday night in
the northern Appalachians before tapering off Friday morning. WPC
PWPF continues to show high chances (>70%) for snowfall amounts >8"
in the southern Adirondacks and Greens, then along the White
Mountains up through central and northern Maine. Localized amounts
topping 12" are likely in the higher portions of these mountain
ranges.


2. Lake-Effect Snow (LES): In wake of the exceptional LES event
that occurred downwind of Lakes Erie and Ontario over the past
week, yet another round of heavy LES is on tap starting tonight
and lasting through Thursday night, then another round Friday
night. The cold front passes Lakes Erie and Ontario late tonight
with surface-850mb winds quickly changing to the west behind the
front and then northwesterly Thursday afternoon. Expect mostly
multi- banded segments off Lake Ontario, with CAMs agreeing on
longer single banded solutions emanating from Lake Huron and over
Erie Co PA. This band could not only bring heavy snow to the hard
hit Chautauqua Ridge in far western NY, but potentially into
central PA as well. Lake-effect snow should linger through Thursday
night and begin to taper off later in the day on Friday as winds
weaken over the Lakes. However, winds backing to westerly flow
Friday night should allow some single band formation off both
lakes. Latest Day WPC PWPF shows high chances (>70%) for snowfall
above 18" in northwest Pennsylvania along the Chautauqua Ridge,
where there are also low chances (10-30%) for snowfall above 24"
through Friday night. Along the Tug Hill Plateau with more like 50%
for the southern Adirondacks which is after the WAA snow through
this evening. WPC PWPF shows moderate chances (40-60%)
probabilities for >24" in the highest terrain of the Tug Hill
through Thursday night. Regionally, these snowfall amounts combined
with whipping wind gusts topping 40 mph will likely result in
significant blowing/drifting of not just the snow from this event,
but also residual snowpack from heavy snow that accumulated over
the past 4-5 days.

3. Snow Squalls/Mountain Snow: This strong cold front, which
according to some cross-section guidance shows FGEN that extend as
far up in the atmosphere as 500mb, will coincide beneath the left-
exit region of a 100kt 500mb jet streak. By 00Z tonight, 500mb
winds over Iowa are topping the maximum observed winds for this
time of year in the CFSR climatology. As the Arctic front pushes
south and east, the thermal gradient will not be as strong as it
could`ve been had the frontal passage occurred during the daytime
hours. That said, the aforementioned deep 500-700mb height
anomalies and strong synoptic-scale forcing will provide sufficient
lift, along with ample 850-700mb FGEN to foster formidable snow
squalls this evening from southern Michigan and northern IN on east
through the Upper Ohio Valley and into the northern Mid- Atlantic
overnight. These squalls are likely to traverse the Delaware Valley
on south to potentially northern MD and northern VA in the early
Thursday morning. Boundary layer temperatures are more questionable
along the I-95 corridor, but fast moving squalls producing brief
bursts of snow should occur in parts of the Piedmont regions of
northern VA and northern MD. Day 1.5 PWPF for >0.1" has a curious
maximum over the northern VA Piedmont. Remember there are no safe
places on a highway during a snow squall as visibility drops
significantly and roads can become snow covered quickly, making it
hard to see on the road and slow down when necessary.

4. Upslope Mountain Snow: Farther south, strengthening low level
westerlies will flow orthogonally to the SW-NE spine of the Central
Appalachians. Strong vertical ascent not only via synoptic scale
and FGEN methods but also through orographic lift should prompt
bursts of heavy snow to continue after the frontal squalls late
tonight into Thursday with rapidly increasing SLRs. To make impacts
worse, 850mb winds over the Central Appalachians (including the
Laurel and Potomac Highlands) will be in excess of 50 kts, with the
strong winds leading to not only substantial blowing/drifting of
snow, but potential blizzard conditions that could even cause power
outages in some areas. WPC PWPF shows high chances (>70%) for >4"
of snowfall in the central Appalachians with parts of the Laurel
Highlands (far southwest PA, western Garrett County, MD, and south
of Garrett County in eastern WV) sporting moderate chances
(40-60%) for snowfall >8" through Friday evening.

...Washington Cascades...
Day 3...

The Pacific Northwest sees the return of some wintry weather on
Saturday as a Pacific storm system brings high elevation snow
(>4,000ft) to the region. Some moisture spilling over into the
Columbia Basin could also have boundary layer temps that are sub-
freezing on Saturday. Some minor ice accumulations are possible
within the Columbia Basin on Saturday, as are locally heavy
snowfall totals through Saturday night in the higher terrain
(>5,000ft) of the Cascades and Olympics through Saturday afternoon.
WPC PWPF shows low-to-moderate (30-50%) chances for >4" above
5,000ft through Saturday afternoon with potentially more high
elevation snowfall to come in the early onset of the medium range
period.


The probability of significant ice across the CONUS is less than
10 percent.


Mullinax




...Winter Storm Key Messages are in effect. Please see current
 Key Messages below...

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

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