FOUS11 KWBC 090741
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
241 AM EST Mon Dec 9 2024

Valid 12Z Mon Dec 09 2024 - 12Z Thu Dec 12 2024

...CO Rockies/Sangre de Cristos...
Day 1...

Height falls moving out of the Great Basin this morning will
continue southeastward behind a cold front that has pushed into the
central Rockies. High pressure over the northern Rockies and
nosing down the High Plains will promote some modest upslope flow
into Colorado's Front Range during the day today. This upslope
component will extend as far south as the Sangre de Cristos in
northern New Mexico by tonight. Periods of snow across the region
are expected as the system moves quickly through the region and
heights build in by Tuesday. WPC probabilities are high (>70%) for
at least 4 inches of snow around the Palmer Divide southward to
Raton Pass. The highest and more remote elevations could see
localized amounts around 8-10" by the time the snow comes to an end
Tuesday morning. Lighter amounts are expected through the I-25
corridor but could be heavier around Raton Pass.

...Northeast...
Days 1-2...

Shortwave moving through the Ohio Valley this morning will help
bring in another surge of moisture coincident with an advancing
warm front. Sub-freezing air mass largely resides north of I-90,
along with some higher elevations through the Catskills, this
morning. As the warm front approaches I-80 this afternoon, an area
of low pressure will form over southeastern New England, helping to
maintain some northerly flow over northern New England. However,
with broad SW to S flow between 850-700mb, warm nose will help
support areas of freezing rain over eastern NYS into New England
today, then into Maine D2. Around a tenth of an inch of icing is
expected D1-2, but WPC probabilities for at least 0.25" are 10-20%
over western Maine (into the Kennebec Valley). Snowfall may be
limited with time due to the marginal thermal profile and lack of
cold enough temperatures in the production zone for dendrites as
the warm front attempts to push through. A few inches will be
possible toward precip onset, focusing over the Green/White
Mountains into Maine where WPC probabilities for at least 4 inches
are 40-80%.


Day 3...

By D3 (Wed-early Thu), upper pattern becomes much more amplified
upstream as a vigorous system lifts northeastward out of the
Southeast. Digging upper low into MN will raise heights over the
East Coast with milder temperatures above freezing ahead of the
front. On the backside of the front, colder air will filter in atop
a lagging precip shield, supporting a rain-to-snow transition
initially over the eastern TN Valley and central/southern
Appalachians first (Wed afternoon), then eventually into NYS Wed
evening and overnight into early Thu. WPC probabilities for at
least 4 inches of snow through 12Z Thu are low to moderate
(10-40/40-70%) over the central Appalachians (eastern WV) and into
parts of central NYS into the Adirondacks which will be quicker to
change over from rain to snow. Additionally, as the low deepens
quite smartly into Canada, cyclonic flow will wrap across Lakes
Erie/Ontario as 850mb temps crash to -10 to -15C on westerly flow.
This will support single banded lake snows by early Thursday,
continuing beyond this forecast period.


...Upper Great Lakes...
Days 2-3...

Upstream anticyclonic wave breaking over northwestern Canada will
support a digging and deep upper low moving into the Great Lakes on
Wednesday. An arctic front will switch winds to northwesterly as
much colder air is dragged into the region (850mb temps -15 to -25C)
which will kick-start the lake-effect snow (LES) machine into
high gear toward the end of D3 and continuing beyond this forecast
period. WPC probabilities of at least 4 inches of snow are high
(>70%) over much of the northern shore of the Michigan U.P. and
along the western coast of Michigan's Mitten. For at least 8
inches of snow, the highest probabilities lie over northwestern
Lower Michigan. Snowfall rates of 1-2"/hr are likely in more intense bands.


Fracasso


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