FOUS11 KWBC 270841
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 AM EST Wed Nov 27 2024

Valid 12Z Wed Nov 27 2024 - 12Z Sat Nov 30 2024

...Midwest through Northeast...=20
Days 1-2...

The base of a positively-tilted northern stream trough is over
northern NV early this morning and shift east to the central Plains
today, over the Midwest tonight, then turn up the Northeastern
Seaboard Thursday. Lee-side surface low over the southern High=20
Plains will shift to the Mid-South today, then rapidly develop over
the Mid-Atlantic tonight all the way through the New England coast
Thursday night. Although it will remain progressive, the=20
accompanying ascent will intensify through Thursday as a downstream
SWly jet streak intensifies to 130kt and becomes coupled over the
Mid-Atlantic to the trailing southern stream jet. This synoptic=20
ascent impinging upon a surface baroclinic gradient (enhanced by=20
downstream WAA) will drive cyclogenesis with the surface low
reaching 990mb over the Gulf of Maine Thursday evening.=20

With the 00Z suite tonight, confidence has increased with a bit
more southerly track than prior runs with the 00Z ECM in better
alignment with the consensus (previously it was farther north/west).
Increasingly potent snow banding begins this evening over central
IL/IN with cyclogenesis and nocturnal effects aiding moderate to
briefly heavy snow development late tonight over north-central=20
OH/northern PA. The greatest threat looks to develop Thursday=20
morning with a laterally-quasi-stationary band moving from=20=20
northern PA and central Upstate NY and then Thursday afternoon over
the Greens and Whites of VT/NH into Maine. Persistent WAA into the
system results in some increasingly sloped fgen, especially in the
700-500mb layer, which will intersect the DGZ, furthering support=20
for heavy snow banding. These types of bands can produce sharp=20
snowfall gradients, and the intensity can dynamically cool the=20
column sufficiently to overcome near-freezing surface temps to=20
cause impactful, low SLR snowfall in spite of terrain.

Day 2 PWPF (12Z Thur to 12Z Fri) snow probs for >6" are 10-30% over
the Catskills and southern Adirondacks and around 50% for the
Greens and Whites and well into interior Maine. However, these
probabilities rely heavily on the 12Z ECENS which were farther
north than the 00Z deterministic consensus and use p-type methods
that are highly biased toward terrain and cannot handle dynamic
cooling in bands, so the threat is better realized in the winter
storm watches which are raised for eastern NY and central New
England.=20

The extent of banding in eastern Maine remains an area of
uncertainty with some 60hr CAMs (like the RRFS) indicating decent
banding nearly to the coast. This will need to continue to be
monitored today as more CAMs come into view.=20


..Great Lakes...
Days 2/3...

***Long lasting Lake Effect Snow event Friday into next week***

A closed mid-level low lingering over James Bay Thursday into
Sunday in the wake of the aforementioned coastal low over the=20
Northeast Thursday will result in amplification of the eastern=20
CONUS trough. This creates persistent cyclonic WNW flow across the
Great Lakes with enhanced CAA and some saturation in the DGZ above
the warm lakes making for a potent/prolonged LES event. Lake-=20
induced instability, progged to approach 1000 J/kg, will aid=20
intense snow rates. Day 2 probs for >6" are limited to the U.P.=20
with mainly WNWly flow off Lake Superior. Day 3 probs for >6"=20
expand to the northern L.P. and become categorical with single=20
banding expecting east of Lakes Erie and Ontario. This flow then
persists through Saturday night before flow veers more northerly,
shifting the snow bands south.=20

The arctic-sourced cold and subsequent LES off the Great Lakes=20
through the weekend is the subject of the Key Messages which are=20
linked below.=20


...Southern Rockies...=20
Day 1...

Additional heavy snow falls over southern CO and northern NM
Rockies today until the shortwave trough axis crosses the southern
Rockies this afternoon. Day 1 snow probs for >8" additional after=20
12Z are 20-30% for portions of the San Juans and Sangre de Cristos.


Lee-side surface cyclogenesis over the southern High Plains will
aid snowfall east of the Continental Divide today. Day 1 snow probs
for >2" are 47-70% for the I-25 corridor from the Palmer Divide
down to the Raton Mesa.=20


The probability of significant ice across the CONUS is less than
10 percent.=20


Jackson



...Winter Storm Key Messages are in effect for northern Plains and
Great Lakes...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=
stKeyMessage_1.png__;!!DZ3fjg!9WHTInuTodfZQ4pcXGYxGj82raKd7mTxSM8tFx365sh9Q=
JJzAihsaSrvPOLedb-777ptmrhKI8yf6bg1hSsnEgLh7CM$=20

$$

=3D =3D =3D
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