Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
317 AM EST Sat Dec 17 2022

Valid 12Z Sat Dec 17 2022 - 12Z Tue Dec 20 2022

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

The long duration, coast-to-coast winter storm that impacted much
of the Lower 48 over the last 9 days is finally in its endgame.
Today, the coast storm responsible for another day of heavy
snowfall will track northeast through the Gulf of Maine. A potent
850mb low will continue to direct a steady stream of moisture
embedded within anomalous easterly flow (50-60 kt winds, or <1st
climatological percentile over Maine through Saturday morning).
This fosters a favorable setup for upslope enhancement as strong
low level easterly flow is forced vertically, resulting in heavier
snowfall rates. Note that there are also likely to be some terrain
shadowing from downsloping winds into valleys, so some valleys
could see noticeably lesser snowfall totals than their more
elevated neighbors. Snowfall rates within the heaviest bands may
fall at >1"/hr. Latest WPC PWPF shows 70-90% probabilities for >6"
over much of northern Maine and into northern New Hampshire for
Saturday. The WSSI continues to show much of interior Maine seeing
"Moderate" to even "Major" impacts today, which suggests hazardous
travel conditions and considerable disruptions to daily life are
anticipated.  By Sunday, a nearby inverted trough will continue to
keep snow in the forecast over far northern Maine. WPC PWPF does
depict 50-70% probabilities for >4" of snowfall on Sunday in far
northern Maine.

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

While snow gradually tapers off this weekend in New England, lake
effect snow bands will be in full force downwind of Lakes Erie and
Ontario. This is due to the prolonged, cyclonic SW flow across the
Great Lakes from the remnant low pressure system that was the
Northern Plains and Upper Midwest blizzard earlier in the week.
Across western Michigan, multi-banded and scattered lake effect
snow showers will traverse western and northern Michigan.
Meanwhile, more intense single band lake effect snows will ensue
over far northwest PA and western NY, which includes the Buffalo
metro area. Sufficient instability and low level converging winds
off the mainland should foster snowfall rates of 2"/hr within this
band. By Saturday night and into Sunday, low level winds will
become westerly, forcing the band near Buffalo to drift south
while another intense lake effect snow band over the Tug Hill and
just south of Watertown. Similar snowfall rates are possible,
while the increase in elevation likely means slightly heavier
hourly snowfall rates. By Monday, W to WNW winds will lead to less
intense lake effect snows off Lake Erie, and force the snow band
off Lake Ontario to drift farther ESE. Through the weekend these
areas, from the immediate Buffalo metro and its southern suburban
communities on north to the Tug Hill, feature daily WPC PWPF
probabilities of 30-50% for snowfall totals >12". Given these
bands longevity, it is quite possible to see some areas pick up 2
to 3 feet of snow this weekend and through Monday. WPC WSSI
depicts "Extreme" impact potential in the southern Buffalo metro
area and in the Tug Hill, while "Major" impacts could unfold from
Erie and far western NY to lower elevated areas surrounding the
Tug Hill. Expect dangerous, to if not impossible, travel
conditions in areas sitting beneath these intense lake effect snow
bands.

...Pacific Northwest & Northern Rockies...
Days 2-3...

A deepening upper low over southwest Canada will help to direct an
Arctic front south through the Pacific Northeast and the Northern
Rockies/Plains Sunday and through Monday. Meanwhile, a
strengthening 250mb jet streak along the British Columbia coast
will place its divergent left-exit region over Washington,
northern Idaho, and western Montana. This combined with upsloping
easterlies at the surface in western Montana will result in
periods of heavy snow in the Northern Rockies. In western
Washington, mean 850-300mb winds out of the W-WNW favors upslope
flow in to the Cascades, where with abnormally colder than normal
temperatures, means snow levels will also be noticeably lower.
Latest WPC PWPF shows moderate chances (40-60%) of snowfall totals
>8" in the Cascades and in the highest peaks of the Lewis Range
and Purcell Mountains on Sunday. As the jet streak moves a little
farther south and east on Monday, and a slight introduction of
700mb moisture flux arrives from the Pacific, periods of snow will
shift south down the Cascade Range and into the Bitterroots.
Probabilities for >8" of snowfall are not quite as high as Sunday,
but there is still a large portion of these ranges with 40-60%
odds of seeing >6" of snow. The Cascades are currently forecast to
see the most impactful snowfall totals with WPC's WSSI depicting
some "Moderate" impact zones for both Sunday and Monday. All of
these mountain ranges mentioned in this section have at minimum
"Minor" impacts, which implies there could be treacherous travel
conditions and motorists should use caution in these affected
areas on Sunday and Monday. It is also worth noting some light
snow accumulations are possible along the I-5 corridor in western
Washington on Monday, as the WPC PWPF does highlight some 20-40%
probabilities for >1" of snow.

Key Messages for December 9-18 Winter Storm:

-Norâ€easter moves into the Gulf of Maine today with periods of
heavy snow continuing across northern New England.  Lake-effect
snow bands to set up downwind of Lakes Erie and Ontario today
through Sunday.

-Heavy snowfall rates of 1-2 inches/hour are forecast across much
of interior Maine this morning and into the evening hours. Storm
totals there are forecast to range between 1 to 2 feet. Dangerous
travel conditions and scattered power outages are expected in this
heavy, wet snow.

-Heavy lake-effect snow will develop this morning across parts of
western New York south from Buffalo, and the Tug Hill Plateau.
Major impacts due to heavy lake-effect snow will begin today and
continue through Sunday across these areas.

Mullinax