Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
336 AM EST Fri Jan 20 2023

Valid 12Z Fri Jan 20 2023 - 12Z Mon Jan 23 2023

...Great Lakes & Northeast...
Day 1 & Day 3...

An upper trough over the Northeast will eject east offshore and
track south of Nova Scotia by Friday evening. At the surface, a
double-barrel low structure; with the primary coastal low off the
Massachusetts Cape and a second surface low sits below a
vertically stacked upper low over the Adirondacks, will be the
culprit for keeping snow in the forecast today. On the northern
flank of the coastal low, 850mb moisture transport will decline
precipitously throughout the day as the coastal low tracks east.
Despite the loss of an Atlantic moisture fetch, there still
remains a large area of >90% RH values in the 850-300mb layer
across the Northeast Friday morning and into the afternoon hours
thanks to the upper trough being slower to move east. Cyclonic
flow and temperatures turning up sub-freezing throughout the
atmospheric column will allow for snow to be the primary
precipitation type for just about everywhere, with the lone
exception being the New England coastline. Latest WPC PWPF depicts
20-40% probabilities for snowfall totals >6" in southern Vermont
and Downeast Maine today. The WSSI does show some areas of
Moderate impacts across southern Maine, meaning there will likely
be hazardous driving conditions and disruptions to daily life in
these areas today. Meanwhile, cyclonic flow on the western flank
of the upper low will result in some lake effect snow downwind of
Lake Erie, as well as some upslope snow into the central
Appalachians. WPC PWPF shows probabilities for >4" of snowfall
ranging between 10-30% in these areas today.

High pressure builds in over the region on Saturday, providing a
brief break before the next winter storm makes its approach from
the Gulf Coast Sunday afternoon. This next storm system is due to
a sharpening trough axis tracking across the Mid-South Sunday
evening that will produce robust vertical motion and WAA over the
Southeast and Mid-Atlantic. Combined with the diffluent
right-entrance region of a 175 kt 250mb jet over Maine located off
the East Coast, this will result in a strengthening wave of low
pressure tracking up the Eastern Seaboard Sunday night. Thermal
profiles along the I-95 corridor remain generally too mild for
snow, but in the central and northern Appalachians, snow will be
the primary precipitation type. There does remain quite a spread
in where the heaviest snowfall is forecast to end up heading into
Sunday night and through Monday. However, the Winter Storm
Outlook, utilizing WPC's super ensemble, shows a large area of
>30% probabilities for snowfall amounts to exceed warning
criteria. These areas of >30% probabilities of exceeding warning
criteria stretch from northern Pennsylvania to northern New
England, where there is a large swath of 50-80% probabilities for
Days 3-4. Residents of northern New England will want to continue
monitoring this next winter storm throughout the weekend as this
next winter storm is likely to produce additional hazardous travel
and detrimental impacts to infrastructure Sunday night into Monday.

...Central High Plains...
Days 1-2...

An upper low tracking out of northern Arizona this morning will
make its way into the southern Rockies this afternoon.
Strengthening diffluent flow in the 250-500mb layer will support a
deepening wave of low pressure over eastern New Mexico. As the low
deepens, the low level jet around 850mb will pick up in intensity,
delivering a conveyor belt of 850-700mb moisture flux northward
and wrapping around the mid-level circulation center. This
scenario is classic for a warm conveyor belt to lead to the
development of a TROWAL on the north and west flank of the 700mb
low, where periods of heavy snow are expected. The areas likely to
fall beneath this TROWAL will be southeast Colorado, far northwest
New Mexico, western Kansas, and into the Oklahoma and far northern
Texas Panhandles. 00Z HREF shows southeast Colorado and western
Kansas likely to be located beneath the TROWAL the longest, giving
them multiple hours with hourly snowfall rates as high as
1-1.5"/hr according to WPC's Snowband Probability Tracker. The
latest WPC PWPF shows a large portion of southeast Colorado with
anywhere from 60-80% probabilities for >6" of snowfall from Friday
evening to Saturday evening. Some of these values do bleed into
far northwest New Mexico and into western Kansas. The ECMWF EFI
showed a high confidence area of >0.8 and shift of tails between
1-2, which gives higher confidence in a significant snowfall event
for the area. There are still some subtle details that make the
forecast unclear. While the probabilities largely support a 6-10"
even in southeast Colorado, the intense dynamics and 850mb FGEN at
play could lead to even longer periods of heavy snow rates. The
pivot point of the TROWAL could extend as far south as the TX/OK
Panhandles as well.

The WSSI does show a large footprint of Moderate impacts from
northeast New Mexico and southeast Colorado to western Kansas.
There is also a small area for Major impacts along the CO/NM
border. All this to say these areas can expect disruptions to
daily life, including hazardous travel conditions with gusty winds
up to 40 mph possible. Whiteout conditions and blowing snow are
likely in the most intense bands.

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

A potent upper level trough from the northeast Pacific will dig
southeast into the Pacific Northwest on Saturday. The trough will
be accompanied by a 150kt 250mb jet with the nose of the jet
reaching western Washington by late morning. Much of Washington
and the northern Rockies will be positioned beneath the diffluent
left-exit region of the jet streak, which combined with an
injection of Pacific moisture and crashing heights aloft will
support periods of snow, especially above 5,000ft. Latest WPC PWPF
does show 40-60% chances for >8" of snowfall in the Washington
Cascades on Saturday, with lower probabilities (30-50% chances)
for >8" of snow in the Idaho Bitterroots and into portions of
western Montana. The trough will continue to plunge south through
the Rockies on Sunday with crashing snow levels and the same
injection of Pacific moisture engulfing the northern and central
Rockies. The trough will be progressive and thus should keep a
limit on seeing a widespread footprint of snowfall totals >12".
That said, some ranges do feature some modest chances (10-30%
chance via latest WPC PWPF) for snowfall totals >6". This includes
the Bighorns of northern Wyoming, and the heart of the Colorado
Rockies. There is some growing consensus that a wave of low
pressure will form in the lee of the central Rockies sometime late
Sunday night into Monday, but there remains disagreement in
ensemble guidance on where the low forms and how long it takes to
form. It is worth monitoring in future forecast cycles as it may
bring yet another round of heavy snow to the central and southern
High Plains, as well as the central and southern Rockies early
next week.

Key Messages for the Northeast Winter Storm:

-- A winter storm will produce heavy snow and some freezing rain
across parts of the Northeast through Friday.

-- Heavy snow rates of up to 1 inch per hour will create dangerous
travel tonight into Friday morning across portions of New England,
with lighter periods of snow continuing through much of Friday.
More than 6 inches of snow are expected from Vermont through
southern Maine.

-- Freezing rain will likely produce ice accumulations of up to
1/10 inch, especially in the Catskills, Berkshires and southern
Greens, creating slippery roads and hazardous travel.

Mullinax