FOUS11 KWBC 140709
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
309 AM EDT Mon Oct 14 2024

Valid 12Z Mon Oct 14 2024 - 12Z Thu Oct 17 2024

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

An anomalous upper level trough over the Northeast will lead to a
strengthening cyclone over northern New England this morning. The
850mb low positioned over Upstate New York will track north and
east through the Green and White mountains by this afternoon. To
the north and west of the 850mb low, a surge in both moisture and
NW winds will result in a stronger upslope component into the
Adirondacks, Green, and White mountains throughout the day. In
addition to the sufficient upper level ascent, an ari-mass that
is quite cold by mid-October's climatology will support sub-
freezing boundary layer temps for in the northern Appalachians. Locally
heavy snowfall rates are expected with 0.5-1"/hr rates possible in
the tallest peaks of the Adirondacks this morning and afternoon
according to the 00Z HREF on WPC's Snowband Probability Tracker. As
the storm tracks into Quebec this evening, snow will taper off and
be limited to just snow showers through Tuesday morning. Another
round of snow is likely to occur Tuesday late morning and through
the afternoon as a trough axis pivoting on the back side of the
storm system in eastern Quebec brings about an additional surge of
low- mid level moisture. In addition, steep lapse rates could
trigger potent snow showers, especially in the higher terrain of
northern New England above 2,000ft. Snow showers look to linger in
the northern Appalachians until Wednesday morning when the trough
axis moves east and off the New England coast.

Any locally heavy snowfall totals (>4") would likely be confined
to elevations above 2,000ft in the Adirondacks, Green, and White
mountains. WPC probabilistic guidance does depict some low-
moderate chances (20-40%) for >6" of snowfall in the tallest

3,000ft peaks of the Adirondacks. Below 2,000ft, snowfall amounts

will struggle to top 2" in the 1,000-2,000ft elevation layer, with
mainly a coating to 1" in the elevations below 1,000ft. Given this
snowfall is the first of the season for parts of the region,
motorists will be subjected to poor visibility and some snow
covered roads for the first time in months. The WSSI-P does show
low-moderate chances (20-40%) for Minor Impacts in parts of the
Adirondacks through Monday evening.

...Northwest...
Days 2-3...

A 500mb shortwave trough bringing with it a surge in 850-700mb
moisture and associated height falls will cause snow levels to
gradually drop to as low as 4,000ft by Wednesday. The air-mass is
not particularly cold for the time of year, so this setup favors
snowfall accumulation in the Olympics and Cascades to elevations

5,000ft. WPC probabilities do suggest low-moderate chances

(20-40%) for >4" of snowfall on Wednesday along the windward side
of the Cascade Range at elevations >4,000ft. As 500mb heights and
snow levels continue to fall, some of the higher level Washington
passes may start to see accumulating snow by Wednesday night. The
potential for accumulating snowfall reaches the Northern Rockies
(the Bitterroots, Absaroka, and Tetons most notably) by early
Thursday morning. WPC probabilistic guidance suggests the Lewis
Range of western MT could see locally higher totals surpassing 4"
at elevations >7,000ft, while the other mountain ranges mentioned
are most likely to see their best accumulating snowfall above
8,000ft on Thursday.


The probability of significant ice across the CONUS is less than
10 percent.

Mullinax


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FOUS11 KWBC 150737
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EDT Tue Oct 15 2024

Valid 12Z Tue Oct 15 2024 - 12Z Fri Oct 18 2024

...Northeast...
Day 1...

Following a brief tapering off of snowfall overnight, periods of
snow will pick back up over the northern Appalachians Tuesday and
linger into Tuesday night. This is due to an elongated upper level
trough pivoting around the western flank of a closed 500mb low
situated over the St. Lawrence Seaway that will traverse the
Northeast from west to east. In addition to the usual upslope flow
into the Adirondacks, Green, and White Mountains, the steep lapse
rates aloft will trigger potent snow showers that could cause brief
bursts of heavy snow at elevations >2,000ft. A few snow showers may
linger around northern New England Wednesday morning, but
accumulations would be minor and limited to the elevations >3,000ft
on Wednesday. WPC probabilities show low chances (10-30%) for
additional snowfall totals >4" along the higher elevations of the
Adirondacks, Green, and White Mountains through Wednesday morning.

...Pacific Northwest...
Days 2-3...

An upper level shortwave trough bringing a surge in 850-700mb
moisture and causing 500-700mb heights falls will cause snow levels
to gradually drop from 6,000ft Tuesday night to around 4,000ft
Wednesday evening over Washington's Olympic and Cascade Ranges. As
this initial 500mb vort max exits to the east Wednesday night, a
second trough will dive south through the region on Thursday with
snow levels bottoming out just below 4,000ft in some cases. Periods
of heavy snow are expected along the Oregon Cascades during the day
on Thursday. Snow should taped off some time Thursday night. WPC
PWPF showed high chances (50-70%) for snowfall totals >4" above
5,000ft in the Cascade Range through Thursday.

...Northern & Central Rockies...
Day 3...

Pacific moisture out ahead of the first upper level shortwave to
traverse the Pacific Northwest is forecast to produce periods of
snow in the higher elevations (>6,000ft in northern ID, >7,000ft in
the Bitterroots, Sawtooth, Absaroka, Tetons, Wind River, and Big
Horn Ranges) early in the day on Thursday, but it is with the
approach of the second trough where snowfall rates look to
increase. Stronger height falls Thursday evening will accompany an
air-mass that NAEFS shows at the 500mb and 700mb layers to be below
the 10th climatological percentile. Plus, NAEFS shows an anomalous
IVT that tops the 90th climatological percentile from the Great
Basin to Utah. The region will also be placed favorably beneath a
coupled jet structure that will help to maximize upper level
divergence from the Absaroka on south to the Wasatch Thursday night
and into Friday morning. Waves of low pressure will develop along
a strengthening frontal boundary that will track through the
Wasatch and into southern WY by Friday morning, prompting
precipitation that starts out as rain to make a quick changeover to
heavy snow.

WPC PWPF sports moderate-to-high chances (50-70%) for >6" of
snowfall in the Absaroka of southwest Montana with some moderate
chances (40-60%) for >12" in some of the Absaroka's tallest peaks.
Farther south, the Bighorns and Wind River Ranges sport low-
moderate chances (30-50%) of seeing >8" of snowfall through 12Z
Friday, while the Uinta how moderate-high chances (50-70%) for the
same snowfall totals through 12Z Friday. This is likely to be the
first winter storm of the season for these mountain ranges this
season and will result in impacts residents have not dealt with in
months (poor visibility, slick/snow covered roads). The WSSI-P does
sport moderate-high chances (50-70%) for Minor Impacts in the
Absaroka, Wind River, Big Horn, and Uinta ranges, as well as some
of the tallest peaks of the Wasatch through Friday morning.


The probability of significant ice across the CONUS is less than
10 percent.


Mullinax




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FOUS11 KWBC 160752
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Wed Oct 16 2024

Valid 12Z Wed Oct 16 2024 - 12Z Sat Oct 19 2024

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

An upper level shortwave trough ushering in a surge of 850-700mb
moisture and resulting in upper level height falls will cause snow
levels to gradually drop from 6,000ft early this morning to around
4,000ft by Wednesday evening over Washington's Olympic and Cascade
Ranges. As this initial 500mb vort max exits to the east Wednesday
night, a second trough will dive south through the region on
Thursday with snow levels bottoming out just below 4,000ft in some
cases. The heaviest snowfall is likely to occur along the
Washington Cascades Wednesday evening with moderate-to-heavy
snowfall possible along the tallest peaks of the Oregon Cascades
Thursday morning. WPC probabilities depict high chances (>70%) for

4" of snowfall in the Washington Cascades and peaks above 5,000ft.

The taller volcanic peaks of the Washington Cascades sport
moderate-high chances (50-70%) for snowfall >8" through Thursday.


...Northern & Central Rockies...
Days 2-3...

**First Significant Winter Storm of the Season to Impact the
Central Rockies**

The evolution of the first significant winter storm of the season
across portions of the Intermountain West begins as an upper level
trough enters the Pacific Northwest on Thursday. As Thursday
progresses, the upper trough will deepen as it tracks south through
the Great Basin Thursday night, then becoming a closed low along
the UT/AZ border by midday Friday. The low continues to slowly
track across northern Arizona Friday night but remain stalled over
the Four Corners region early Saturday morning.

NAEFS shows upper level evolution that is quite anomalous from a
variety of parameters. At 250mb, a jet streak over the northern
High Plains will be coupled with a jet streak at the base of the
amplifying upper trough, thus maximizing upper level divergence
over the Intermountain West on Thursday. As the upper trough cuts
off into an upper low by Friday, strong divergence beneath the left
exit region of the 250mb jet streak will be placed over Utah and
the Central Rockies. NAEFS shows 500-700mb heights and temperatures
that are routinely below the 10th climatological percentile,
particularly over the Great Basin and Utah late Thursday into
Friday. Perhaps most impressive are the IVTs which will eclipse
300 kg/m/s (above the 99th climatological percentile) in southern
UT Thursday night. Mean flow out of the SW will result in strong
upslope ascent into the Uinta and San Juans in particular.

In the Absaroka, Big Horns, and Wind River Ranges, WPC PWPF shows
high chances (>70%) for storm event snowfall totals >8" with at
least moderate chances (>50%) for snowfall >12" in the tallest
peaks. Farther south, the Uinta and San Juan sport high chances

70%) for snowfall totals surpassing 12". In fact, the Uinta sport

moderate chances (40-60%) for >18" of snowfall through Saturday
morning, with high chances (>70%) in the San Juan for >18" of
snow. The WSSI-P Moderate probabilities are keying in on the San
Juans as seeing the most impactful snow from this event, primarily
due to a combination of the algorithm's focus on snow totals,
snowfall rates, and snow load. Probabilities are showing moderate
chances (40-60%) for snowfall that would result in hazardous
driving conditions and potential closures and disruptions to
infrastructure in the San Juans, Wasatch, and Uinta. While Moderate
Impacts are deemed lower chance (10-30%) in the Absaroka, Wind
River, and Big Horn Ranges, potential impacts are elevated compared
to what the algorithm suggests given it is the first significant
winter storm of the season in these Ranges above 7,000ft. There are moderate-high chances (50-70%) for Minor Impacts in these Ranges,
as well as far south as parts of the Sangre De Cristo and far
northern mountains of Arizona.


The probability of significant ice across the CONUS is less than
10 percent.

Mullinax



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