Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
259 PM EST Fri Mar 08 2024

Valid 00Z Sat Mar 09 2024 - 00Z Tue Mar 12 2024

...Southern Rockies...
Day 1...

A trifecta of meteorological factors are resulting in additional
snowfall in the Southern Rockies this afternoon and into tonight.
They include a lagging area of 500mb troughing, a surface frontal
boundary banked up along the slopes of the Front Range, and high
pressure over the Central Plains maintaining steady
easterly/upslope flow into the Southern Rockies. As upslope flow
at low levels and modest upper level diffluent flow persists this
evening, periods of snow are expected through midnight. As the
forcing aloft and easterly low level flow gradually subsides by
Saturday morning, snow will taper off and high pressure over the
Intermountain West will take control. WPC PWPF sports
moderate-to-high chances (50-80%) for >6" of snow in the tallest
peaks of the San Juans this afternoon and into tonight. The
Nacimiento Mountains sport moderate chances (40-60%) for >6" of
snowfall as well. The WSSI does suggest Minor to locally Moderate
Impacts possible in these mountain ranges through tonight, as well
as along the Sangre De Cristo in northern New Mexico where PWPF
shows low-to-moderate chances (20-40%) for >4" of snowfall.

...Northwest...
Days 1-3...

A series of upper level disturbances in the northeast Pacific will
produce rounds of heavy mountain snowfall from the Pacific
Northwest to the Northern Rockies through this weekend and into
the start of next week. The first is a sharp upper level trough
approaching Saturday morning that will induce plenty of
synoptically-forced vertical lift atop the atmosphere, supply a
healthy diet of 850-700mb moisture, and an approaching cold front
to aid in driving forced ascent at the low levels. One factor
initially reducing the higher end amounts is that 850-300mb mean
flow will be more out of the SSW, which is not as favorable for
enhanced upslope flow into the Cascades. Still, periods of snow
above 3,000ft in the Olympics and Cascades should lead to locally
moderate-to-heavy snowfall totals through Saturday night. Some
lighter snowfall totals are possible as far inland as the Blue
Mountains and northern Rockies of northern Idaho and far western
Montana. By Sunday morning, the next Pacific disturbance is
already fast approaching but this time in the form of a closed
500mb low and greater height falls throughout Sunday. In addition,
a potent 500mb vorticity max revolving around the base of the
trough will advect rich 850-700mb moisture into northern
California as well. The end result is heavier snowfall rates at
lower snow levels in the Olympics and Cascades Sunday morning,
then heavy snow looks to ensue over the northern California and
northern Sierra Nevada ranges late Sunday morning and into Sunday
evening. Snow levels along the northern California Coastal Range
will be as low as 3,000ft with heavier totals there, as well as
over the Salmon/Trinity mountains above 4,000ft. Pacific moisture
this time will have an easier time streaming into the heart of the
Northwest with the Blue Mountains, the Sawtooth/Boise of Idaho,
and into the Bitterroots also seeing heavy snowfall Sunday night
into Monday.

Speaking of Monday, as the upper low will have opened up into an
open and progressive 500mb trough over the heart of the
Intermountain West Monday morning. Meanwhile, a third Pacific
storm system will approach the Pacific Northwest bringing yet
another influx of 850-700mb moisture into the region. These two
features support additional rounds of heavy mountain snow in the
Olympics/Cascades of the Pacific Northwest, as well snow in the
Boise/Sawtooth, Bitterroots, and Tetons. Given the prolonged bouts
of snow from the northern Sierra Nevada and northern California
ranges to as far north to the Washington Cascades and Olympics,
72-hr WPC PWPF sports high chances (>70%) for >12" of snow in
these ranges through Monday afternoon. The Cascades, higher peaks
of the Washington Cascades, and Salmon Mountains of northern
California even sport high chances (>70%) for >18" of snowfall
through Monday afternoon as well. Confidence is high (>70%) for
snowfall totals >8" through Monday afternoon in the Blue
Mountains, Boise/Sawtooth, and northern Rockies of far northern
Idaho. Expect Moderate to locally Major impacts in these areas,
especially in the Olympics and Salmon Mountains where the
Probabilistic WSSI (WSSI-P) shows low-to-moderate chances for
Major Impacts (considerable disruptions to daily life) Sunday and
into early Monday morning.

...Great Lakes and Northeast...
Days 2-3...

By Saturday morning, low pressure in the Ohio Valley is set to
track northeast as a vigorous 500mb vorticity max over the
Mid-South directs the area of low pressure towards the
Mid-Atlantic. The upper trough will have no problem tapping into
rich Gulf of Mexico moisture as NAEFS shows IVT values >90th
climatological percentile values at 18Z Sat and 00Z Sun over the
Mid-Atlantic. As strong WAA ensues over the Northeast, the
antecedent air-mass in the Northeast is, well, saying "marginal"
may even be pushing it unless you are the Adirondacks, Green, and
White mountains where elevation and wet-bulb temperatures would
initially support snow Saturday evening. By 06Z Sunday, low
pressure is likely to strengthen over the Delaware Valley with
periods of snow expected in these aforementioned mountain ranges.
This initial front end thump of snow, while thermally in the
boundary layer initially is not impressive, is making up for it
through strong ascent via left-exit jet streak dynamics over head
(150 kts over the Deep South), strong 850-700mb FGEN, and the big
wildcard is the track of the surface low as a farther south track
may help keep snow ongoing in the White Mountains into early
Sunday morning. Recent guidance has continued to trend stronger
with the approaching upper trough over the Great Lakes and farther
south with the initial shortwave trough. It is within the realm of
possibility this trough comes in stronger and forces the storm
track farther south. That all being said, through 12Z Sunday, WPC
PWPF does shows moderate-to-high chances (40-60%) for snowfall >6"
in the Adirondacks and White Mountains at elevations at/above
2,000ft. Latest WPC Snowband Probability Tool does suggest the
potential for 1-2"/hr snowfall rates Saturday night and into
Sunday morning in these mountainous areas.

By Sunday morning, the aforementioned upper trough in the Great
Lakes will spawn a surface low of its own over Quebec while its
associated cold front ushers in strong 925-850mb CAA over the
Great Lakes. Latest NAM show delta-T sfc-850mb temps late morning
and early afternoon Sunday along the shores of Lake Erie and
Ontario that are as steep as 22-24F, supporting lake enhanced snow
bands that contain steepening sfc-850mb lapse rates. Latest trends
in ensemble guidance show a sharper and negatively tilted trough
over eastern MA, as well as a farther south track in the 500mb low
over the Great Lakes. This is helping to reduce 850mb temps over
interior New England and the central Appalachians. In New
England's case, this is also a byproduct of stronger vertical
velocities that would support a longer/heavier period of snow
Sunday with snow falling heavily on the backside of the low Sunday
afternoon and evening as well. By 00Z Monday, the surface low is
likely to be somewhere over central ME with heavy snow occurring
from the Chautauqua Ridge and Tug Hill Plateau to the Adirondacks,
Greens, and Whites. On the northern flank of the low, heavy snow
would be ongoing across northern and/or central Maine where steady
850mb WAA and a slug of 850mb moisture flux would support heavy
snow. Finally, by Monday morning, the storm should be located
somewhere close to Downeast Maine with heavy snow ongoing in
upslope favored areas of the Adirondacks, Green, and White
Mountains, as well as along the western and northern-most border
of ME/Canada. Snow may stick around through the daytime hours,
particularly in the Greens and Whites, where the best combination
of upslope flow and residual 850mb moisture will support
additional snowfall. WPC PWPF depicts low-to-moderate chances
(30-50%) for >6" of snowfall for the event along the Chautauqua
Ridge and downwind of Lake Ontario which includes the Tug Hill.
The Green Mountains heaviest snowfall is likely on Sunday when
strong upslope enhancement picks up. WPC PWPF shows
moderate-to-high chances (50-70%) for >6" of snowfall through
Monday morning. Ultimately, it will be the White Mountains and
northern ME that feature the highest probabilities for >8" of
snowfall for the event. The Whites sport high chances (>70%) or
snowfall totals >8" from the initial WAA thump of snow, while
portions of central ME have moderate-to-high chances (50-70%) for
>8" of snowfall as well.

In terms of hazards, the initial WAA thump of heavy snow in the
Adirondack, Green, and White Mountains will be a heavy/wet type of
snowfall, causing the WSSI-P to show in some cases at least 50%
chances for Moderate Impacts through Sunday due to Snow Load. In
addition, gusty winds on the western ans southwest flank of the
low Sunday afternoon could cause blowing snow that leads to
reductions in visibility from northern New York to as far south as
the Potomac Highlands. Lastly, the upper low on approach Sunday
morning in eastern PA will produce strong VVs via strong 500mb PVA
aloft. The strong height falls will also coincide with daytime
heating, resulting in steepening lapse rates across the central
Appalachians and northern Mid-Atlantic. This is a recipe for
potential snow squalls Sunday morning and afternoon. While recent
mild temperatures has allowed for most surface temperatures to
stay above freezing, even should little in the way of
accumulations occur, drastic reductions in visibilities could
still lead to hazardous driving conditions Sunday, particularly
when driving at high speeds.

Mullinax