Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
257 PM EST Thu Jan 25 2024

Valid 00Z Fri Jan 26 2024 - 00Z Mon Jan 29 2024

...Pacific Northwest...
Days 1-2...

An approaching frontal system escorting a conveyor belt of
500-850mb moisture will stream over the Pacific Northwest on
Friday and linger into the upcoming weekend. Snow levels will
initially start out around 4,000ft but as the warm front pushes
through Friday evening and strong 850mb warm air advection ensues,
snow levels will jump to as high as 7,000ft, essentially leaving
the tallest peaks of the Washington Cascades as the only elevated
areas that could support snow. The taller peaks north of I-90
(generally >6,000ft) sport low chances (20-40%) for snowfall
totals >4". The more noteworthy setup is the potential for icing
along the eastern slopes of the Cascades, the Lower Columbia
Basin, and surrounding the Blue Mountains of northeast Oregon.
This comes about due to a weak bubble of high pressure that allows
temperatures at and just above the surface to remain <32F. WPC
PWPF shows low chances (10-30%) for ice accumulations >0.1" in
these areas. The Probabilistic WSSI (WSSI-P) depicts
low-to-moderate (20-40%) chances for Minor Impacts along along the
eastern slopes of the Cascades in south-central Washington that
spills south into the Lower Columbia Basin. Hazardous travel
conditions are possible in these areas on Friday and into early
Saturday morning, especially where roads and walkaways are left
untreated.

...Southern Rockies & Central High Plains...
Days 1-2...

A 500mb trough diving through the Four Corners region this
afternoon will deepen as it tracks east, spawning a 700mb low over
over northeast New Mexico by early Friday morning. The upper low
track supports heavy snow in parts of the San Juans, the higher
peaks of northwestern New Mexico, and as far south as the Gila
Mountains of eastern Arizona. WPC PWPF shows low chances chances
(10-20%) for >4" of snow in the San Juans of southwest Colorado
with low chances (10-30%) in northwestern New Mexico and the Gila
Mountains of eastern Arizona. As a surface low forms south of
Lubbock, southerly 850mb winds will usher added moisture flux over
the northern flank of the low and favor a northeasterly low-level
upslope flow into the Sangre De Cristo and Raton Pass of southern
Colorado and northern New Mexico. WPC PWPF shows moderate-to-high
probabilities (50-80%) for >6" of snow in the Wet Mountains on
south to the Raton Mesa. The WSSI-P sports high chances (>70%) for
Minor Impacts along I-25 in southern Colorado and over the Raton
Pass going into northern Colorado. Expectation is for some
hazardous travel impacts in these areas tonight and into Friday
morning, especially along I-25 near the CO/NM border.

...Northeast...
Days 1 & 3...

A dying cold front approaching from the north will bring a brief
injection of cold Canadian air that becomes anchored by high
pressure over Quebec. Meanwhile, a potent upper level shortwave
trough over the Middle Mississippi Valley will escort an IVT over
the Northeast topping the 90th climatological percentile according
to NAEFS. Strong 850-700mb warm air advection and 290K isentropic
glide over the Northeast will give rise to precipitation once
again overrunning the sub-freezing boundary layer will result in
freezing rain over the Green and White Mountains. WPC PWPF shows
moderate-to-high chances (50-80%) for ice accumulations >0.1" in
portions of these mountains ranges with central New Hampshire most
likely to see those >0.1" amounts. Some low-chances (10-30%) exist
in portions of the Adirondacks and Berkshires as well. Farther
north, with a colder air-mass firmly in place thanks to a weak
area of high pressure to the north, this will allow for heavier
snowfall rates to transpire across northern Maine. Given the
strong divergence aloft and strengthening FGEN, hourly snowfall
rates around 1"/hr are possible. WPC PWPF depicts moderate
probabilities (40-60%) for >4" of snow in central Maine on Friday.
It is worth noting the WSSI-P shows central New Hampshire seeing
moderate risks (40-60%) for Minor Impacts in the Green and White
mountains due to Ice Accumulations. Hazardous travel conditions
are anticipated across most of northern New England with central
Vermont and New Hampshire most likely to see hazardous travel.
Snow will taper off Friday evening with high pressure building in
overhead for Saturday.

By Saturday evening, a potent upper level trough tracking through
the Mid-South will direct a stream of 850-700mb moisture flux
northward into the Mid-Atlantic while an 850mb low tracks into the
Ohio Valley. At 250mb, the right-entrance region of a 150kt jet
streak over southern Ontario will support strong upper level
ascent over the upper Ohio Valley and northern Mid-Atlantic
Saturday night and into Sunday. However, unlike last week, there
is little in the way of sub-freezing temps available in the
boundary layer. This event, at the present moment, looks more like
a late season snow event where elevation and a combination of
intense synoptic and mesoscale will play critical roles in which
areas see the heaviest snowfall. The areas most favored are to the
north of where the 850mb low is likely to track, which at the
moment based on ensemble guidance, would be from northern Ohio and
northern Pennsylvania to southern New York and interior southern
New England. Snow would likely start Sunday morning over portions
of northern Ohio and northern Pennsylvania before moving into the
Poconos and Catskills late Sunday. There remains a high degree of
uncertainty due to any subtle adjustments in the storm track
and/or the depth/axis of the upper trough causing notable shifts
in the heavy snowfall axis. Through Sunday evening, northern
Pennsylvania and the Southern Tier of New York sport 10-20%
chances for >4" of snow according to WPC PWPF, but this accounts
for only the start of this event. This winter storm is likely to
still produce wintry weather into Monday with the more elevated
terrain of the Northeast most favored for potential heavy snowfall
accumulations through Monday.

Mullinax