Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
322 PM EST Wed Jan 04 2023

Valid 00Z Thu Jan 05 2023 - 00Z Sun Jan 08 2023

...Western U.S...
Days 1-3...

A powerful and dangerous Pacific storm system is set to deliver a
myriad of hazards to the West Coast, but it will be California
that receives the brunt of its impacts. This includes heavy
rainfall and resulting flooding, damaging winds, pounding surf,
coastal erosion, and copious amounts of mountain snowfall.
Focusing on the snowfall aspect (with some wind impacts also
included), NAEFS shows another impressive atmospheric river
associated with this storm system ushering in a slug of >90th
climatological percentile PWs to California this afternoon and
through Thursday. This moisture is being transported through an
IVT that peaks >750 kg/m/s, which ranks about 4-6 standard
deviations above normal for this time of year. California on north
to southern Oregon will also be located beneath the divergent
left-exit region of a 140 kt 250mb jet streak, fostering a
favorable environment for synoptic-scale ascent atop the
troposphere. Strong SW flow leading to upslope flow along the
Coastal Range, the Shasta, Sierra Nevada, and Transverse Range
will also result in enhancing precipitation rates in these areas.
This is indicated by the 12Z HREF which sports high chances
(70-90%) for 3"/hr snowfall rates from the Trinity and Shasta
mountains on south along the Sierra Nevada tonight and into
Thursday. There are moderate chances (40-60%) for similar rates
along the San Gabriel and San Bernadino mountains Thursday
morning. These kind of snowfall rates coincide with whipping wind
gusts over 40 mph, and could even approach 70 mph in the highest
terrain of the northern Sierra Nevada.

The impacts from this storm will be extensive across much of the
Golden State. When it comes to winter weather, the WSSI through
Thursday depicts "Extreme" impacts from the Trinity/Shasta on
south through the Sierra Nevada. Thursday also features "Major to
Extreme" impacts in the highest elevations of the Transverse
Range. In these areas, the shear volume of snow and its heavy, wet
consistency are what is driving these more extensive impacts. Snow
will be measured in feet in these ranges with the Sierra Nevada
and Shasta seeing the best odds of snowfall ranging between 2-4
feet, locally up to 5-6 feet in some spots. The snowfall rates
mentioned above (3"/hr likely, locally heavier possible) alone
would cause dangerous travel conditions and pose a threat to
infrastructure. That said, wind gusts in excess of 50 mph, perhaps
even approaching 80 mph in some cases in the tallest peaks, will
only add to the dangerous conditions with whiteout and potentially
blizzard conditions. Expect widespread closures and disruptions to
infrastructure in the worst hit areas of the Shasta, Sierra
Nevada, and Transverse Range.

As the southern portion of the upper low breaks free and tracks
into the Southwest Thursday afternoon, a stream of PVA and 700mb
moisture flux accompanies the shortwave trough, resulting in
periods of mountain snow in ranges such as the Wasatch and western
portion of the Colorado Rockies. WPC PWPF does suggest some
moderate probabilities (40-60%) for >6" of snowfall in these areas
late Thursday into Friday afternoon. A very brief bubble of high
pressure will situate itself over California Friday morning before
the next Pacific upper trough approaches the Northwest. This
directs coastal/valley rain and mountain snowfall farther north
from the Shasta to the Cascade Range by Friday afternoon and into
Friday night. Eventually, a new wave of low pressure will form off
the California coast by Saturday morning. It is this wave of
developing low pressure that will escort the next atmospheric
river into northern and central California on Saturday. For the 24
hour period between 00Z Fri - 00Z Sat, the Shasta and northern
Cascades feature 50-70% probabilities of receiving >12" of
snowfall. In addition, NAEFS showed >90% climatological percentile
values for IVT values (400-500 kg/m/s just offshore Saturday
morning) and 850mb winds over coastal/central California
throughout the day. The combination of heavy snowfall rates and
gusty winds will once again prove to cause travel problems in the
higher elevations of northern California late in the day on
Saturday.

...Upper Great Lakes & New England...
Days 1-2...

The stationary low over the Upper Mississippi Valley begins to
slowly move east tonight and eventually reach the Great Lakes by
Thursday afternoon. Cyclonic, easterly flow on the northern flank
of the 850mb low continues across Michigan's U.P. and into far
northern Wisconsin. This funneling of moisture in an environment
featuring sub-freezing temperatures supports periods of snowfall
this evening, which could fall heavily at times. As the next wave
of low pressure over Lake Huron matures, the original occluded low
over the Midwest will continue to weaken. The cold conveyor belt
of easterly 850mb flow is then expected to weaken and result in
diminishing snowfall rates Thursday morning. However, the upper
low will still be around the Great Lakes Thursday and lingering
cyclonic flow will help to stirrup additional snow showers
throughout the region. Latest WPC PWPF shows 30-50% probabilities
for snowfall totals >4" over the northern shores of Michigan's
U.P. this evening through Thursday afternoon.

Meanwhile, farther east, a wave of low pressure developing along
the occluded front across the eastern Great Lakes will coincide
with a wedge of cold air in place thanks to Canadian high pressure
over Quebec. As the low passes through, moist southwesterly flow
aloft will overrun the cold sub-freezing airmass in place
producing a favorable setup for freezing rain. Recent trends in
guidance have trended slightly warmer in parts of central New
England, making the North Country of New York and much of the
White Mountains of New Hampshire most susceptible to accumulating
ice. Latest WPC PWPF does indicate 30-50% chances for >0.25" of
ice along the northern New York/Canada border, between this
evening and Thursday evening, with the White mountains also having
10-20% probabilities. There is also a swath of 30-50%
probabilities from southern New Hampshire to coastal Maine for ice
accumulations >0.1". These areas can expect the most treacherous
travel conditions with the potential for power outages and tree
damages, particularly where ice accumulation surpasses 0.25". It
is also worth mentioning that wind gusts could approach 35 mph at
times, which combined with the weight from the ice on trees and
power lines, could exacerbate the power outage and tree damage
potential.


...Key Messages for Early January Atmospheric Rivers...

--Strong Atmospheric River Arrives Wednesday: A powerful
atmospheric river is arriving in California today and will
continue into Thursday.

--Dangerous Travel Conditions in the Mountains: Extremely heavy
snow rates above 3 inches per hour are nearly certain above 5000
feet, which will lead to dangerous, and at times impossible,
travel in the mountains of northern and central California.

--Heavy Rain, Flooding Likely at Lower Elevations: Considerable
flooding impacts are likely, especially Wednesday Night to
Thursday. Rain rates over 1 inch per hour may lead to rapid water
rises and mud and rock slides. Coastal areas of California and the
Sacramento Valley are most at risk.

--Downed Trees and Power Outages Possible: Widespread wind gusts
in excess of 50 MPH and saturated soils may make trees more
susceptible to blowing down, with power outages also possible.

--Additional Atmospheric Rivers into Next Week: The next
atmospheric river will arrive Saturday, and another one on Monday.
The cumulative effect of many rounds of heavy precipitation may
lead to increasing impacts.


Mullinax