Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
406 AM EST Thu Nov 30 2023

Valid 12Z Thu Nov 30 2023 - 12Z Sun Dec 03 2023

...Southwest & Southern/Central Rockies...
Days 1-3...

The western U.S. can expect an active and wintry pattern the
second half of the week thanks to an unusually strong North
Pacific jet extension delivering a seemingly endless supply of
Pacific moisture and multiple upper level disturbances. Focusing
on the Southwest and Southern/Central Rockies first, one upper
level trough over the Four Corners region will head for the
Southern Plains later today while another shortwave trough over
southern California heads towards the region Thursday night. These
waves embedded within the developing longwave trough will inject a
steady diet of 700mb moisture flux into the region, mean 250-500mb
troughing allows for sufficient vertical divergence at upper
levels. In addition, some mountain ranges in AZ, CO, and NM can
expect layer-averaged 300-850mb SW flow to foster strong
topographic ascent. This is a recipe for heavy snow over the
course of 2-3 days, particularly for elevations >6,000ft. Today
and into Thursday night, the heaviest snow is likely to transpire
in the CO/NM Rockies. Snow will pick up in intensity over the CO
Rockies and as far south has the San Juans and Sangre De Cristo in
northern NM on Friday. Finally, a third Pacific disturbance will
track east from UT to CO should keep periods of upslope flow into
the CO Rockies. WPC Probabilities over the next 72-hours show a
moderate-to-high risk (50-80%) for >8" from as far south as the
White Mountains of eastern AZ on north through the northern NM and
western CO ranges. The ranges sporting moderate risks (40-60%) for
snowfall >12" in the San Juans, West Elk, and Nacimiento
Mountains. The WSSI shows mostly Minor Impacts, with Moderate to
even some localized Major impacts in the higher elevations of the
northern NM ranges and the Whites Mountains of eastern AZ.

...Cascades through the Northern Rockies...
Days 1-3...

The robust North Pacific jet extension will be the catalyst for a
busy stretch of wintry weather from the Pacific Northwest to the
Northern Rockies and UT ranges through Friday. The first in a
series of upper level disturbances arrives Thursday morning but
contains meager moisture content and will be fairly progressive.
Snow from this system will advance farther inland through the
Columbia River basin and through eastern OR, northern NV, and
eventually into the Northern Rockies by early Friday morning.
Meanwhile, back in the Pacific Northwest, a more potent and
organized Pacific storm system will quickly reach the Olympics and
Cascades by Friday morning. This storm system will have higher
quantities of 850-700mb moisture at its disposal, while 850-300mb
mean winds will be stronger and oriented orthogonally to the
Olympics/Cascades. Snow levels will rise slightly on Friday,
generally average between 3,000-4,000ft (lower levels in the
farther north ranges). Similar to Thursday's event, this same
system will direct its plume of Pacific moisture inland from as
far north as the Lewis Range and Bitterroots to as far south as
northern NV, the Tetons of western WY, and the northern UT ranges.
As soon as the Friday system moves into the Northern Rockies
Friday night, the next Pacific storm is hot on its heels, bringing
yet another round of heavy snow to the Pacific Northwest Friday
night and into Saturday morning. Heavy snow from this storm will
once again make its way east through eastern OR/WA, northern NV,
the Northern Rockies, and as far south as the Tetons and Wasatch
Ranges on Saturday.

Looking at the 72-hour WPC probabilities, there is a high risk
(>70%) for snowfall totals >18" in the WA/OR Cascades and the
Olympics. Elevations >4,000ft have the best odds to see snowfall
amounts measured in feet, perhaps as much as 2-4 feet. Farther
inland, there are also moderate-to-high risks (50-80%) for
snowfall totals >12" in the Blue Mountains of northeast OR, the
Boise and Bitterroots of ID, the Tetons of western WY, and the
Wasatch east of Salt Lake City. The Independence and Ruby
Mountains of northern NV could also see snowfall totals topping
12" through Saturday. From an Impacts perspective, according to
the WSSI, the Olympics and WA/OR Cascades can expect Major Impacts
at elevations above 3,000ft, with Extreme Impacts in the tallest
and volcanic peaks of the Cascades. Snow Amount is the primary
driver, but there is also a combination of Snow Load and Blowing
Snow contributing to the Major and Extreme Impacts. These impacts
are including the major passes of WA and OR, likely leading to
treacherous or even impossible travel at times. Minor and Moderate
Impacts are more commonly found across the Northern Rockies with
elevations >6,000ft sporting Moderate Impacts through Saturday.

...Central Plains into the Midwest & Northern New England...
Days 1-3...

A 500mb shortwave trough tracking into the southern High Plains
Thursday afternoon will amplify and take on a negative tilt as it
approaches the TX Panhandle. Latest guidance shows the exceptional
upper level divergence aloft helps spawn a 700mb low over the OK
Panhandle that will track across southern Kansas Thursday night.
It is on the northern flank of the 700mb low where 850-700mb
moisture will wrap around the circulation and form a deformation
zone of precipitation. CAMs guidance are the most aggressive with
this band of precipitation producing moderate-to-heavy snowfall
thanks to strong dynamic cooling aloft. However, the deformation
zone will be progressive and SLRs are unlikely to be much higher
than 10:1 given the shallow DGZ above where the strongest vertical
velocities are likely to setup (DGZ between 500-600mb, most
saturated and strongest VVs between 600-800mb). This situation
bears watching as these setups can lead to rapid snow
accumulations, especially when snowing >1"/hr. The 00Z HREF did
depict as much as 40-50% probabilities for 1"/hr snowfall rates in
central and northeast KS Thursday night, peaking close to 60% over
southern IA early Friday morning.

To the north of a stationary front draped over the Middle
Mississippi Valley, precipitation will develop along and north of
the front while boundary layer temperatures remain sub-freezing.
The "overrunning" of >0C air aloft and <0C temperatures near the
surface is ideal for an icy wintry mix to take shape. The areas
most at-risk for treacherous ice accumulations are northeast KS,
northwest MO, and southern IA. WPC probabilities show a
low-to-moderate chance (20-40%) for ice accumulations >0.1"
between Thursday night and Friday afternoon. However, should the
dynamic cooling win out, the wintry mix could turn into a narrow
stripe of heavy snow. While this would put a dent in ice
accumulations, heavy snowfall rates would likely result in rapid
accumulations on all surfaces and cause significant reductions in
visibilities. Regardless of the precipitation type, residents in
the Midwest (especially from northeast KS to southern IA and
northern IL) are likely to contend with some form of hazardous
wintry weather for the Friday morning commute. The WSSI currently
shows these areas highlighted with Minor Impact potential,
indicating that a few inconveniences to daily life are expected
and that motorists should use caution while driving.

Lastly, this system is likely to bring some light snowfall
accumulations to central Michigan on Friday as WPC probabilities
show 20-40% odds for >2" of snowfall. Northern Maine may also pick
up some measurable snowfall on Saturday with WPC probabilities
depicting a low chances (10-20%) or >4" of snowfall in Maine's
North Woods.


***Key Messages for Pacific Northwest Winter Storm***

--Prolonged Winter Storm in the Northwest
A long-duration winter storm will begin today along and west of
the Cascades with rain for the valleys and snow at many mountain
passes. Heavier precipitation is expected to push into the region
tomorrow and continue into the weekend.

--Heaviest Snowfall & Worst Impacts in the Cascades
Through early Sunday, there is a high chance (>70%) of at least
12" of snow across the Cascade passes, including I-90. A high
chance for at least 12†of snow also exists in the Olympic
Mountains. Travel will be difficult and hazardous due to both
heavy and blowing snow.

--Mountain Snow to Extend into the Northern Rockies
As these Pacific storm systems track inland, heavy snow will
transpire across much of the Northern Rockies. Periods of heavy
snow are expected as far south as the northern Utah mountain
ranges.

--Potential for River Flooding this Weekend
Rising snow levels Sunday will change snow to rain over all
mountain passes through the Cascades. Several inches of rain will
bring the threat of river flooding late this weekend into next
week.


Mullinax