Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 AM EST Tue Mar 07 2023

Valid 12Z Tue Mar 07 2023 - 12Z Fri Mar 10 2023

...West Coast...
Days 1-3...

A large, positively-tilted trough over western Canada and the
Pacific Northwest remains active this week with reinforcing waves
pinwheeling around the center of the gyre over the Canadian
Rockies. Low pressure off the WA coast will drift southeast to the
OR coast through tonight before opening Wednesday and ejecting
east over the northern Rockies Thursday. Pacific moisture (which
rounds a massive ridge through AK) streams inland south of this
low with a focus on far southern OR/northern CA terrain with high
probabilities for >8" over the Klamath, CA Cascades, and far
northern Sierra Nevada with snow levels low, generally around
1500ft. As the low opens and shifts inland Wednesday the heavy
snow pushes south a bit down the Sierra early Wednesday, but only
to Tahoe where there are moderate Day 2 probabilities for >8" with
continued snow levels around 2000ft under the shortwave trough.

Meanwhile, the next robust upper level low, currently over the
Northwest Territories of Canada will shift southwest down the east
side of the ridge through AK through Wednesday, deepening further
off the WA coast Wednesday night. However, by this time the ridge
into AK will be undercut by an Atmospheric River which the
offshore low will help direct through CA (while also drawing some
moisture up the West Coast). Snow levels surge to 9000ft over CA
late Thursday through the core axis of the AR where there will be
1.5" PW. Snow levels look to be closer to 7000ft out side of the
core axis and lower to the 3000-5000ft range over OR and 2000ft
over WA. Day 3 snow probabilities are moderately high for >8" over
the Klamath/Shasta/Siskiyou and southern Sierra Nevada with the
central/northern Sierra generally below the snow level with
considerable rain. For further information on this rain please see
the Medium Range (PMDEPD) and Excessive Rain (QPFERD) discussions.
Thursday starts relatively quiet before the storm.


...Northern Rockies/Plains & Upper Midwest...
Days 1-3...

Downstream of the large western trough remains active as impulses
eject east. The current wave, over the north-central High Plains
near the WY/SD border, continues a slow lift northeast over the
Dakotas today as it rides along a low level baroclinic zone from
the 1050mb surface high that moves into northern Saskatchewan.
Fgen driven snow continues to drift over the Dakotas today with
moderately high Day 1 probs for >4" additional along the SD/ND
border into central ND before weakening as it shifts northeast
across northern MN Wednesday.

On Wednesday morning, the closed low currently off WA ejects east
from the OR coast with moderate snow over the Wasatch and southern
WY ranges where there are moderate Day 2 probabilities for >4". A
lee-side inverted trough develops over the northern High Plains by
late Wednesday which adds to the fgen from the persistent 1050mb
high over the Canadian Shield causing light to moderate snow to
expand over the north-central Plains with moderate Day 2 snow
probs for >6" over the Black Hills.

As the shortwave trough crosses the Plains it takes on a negative
tilt and amplifies, resulting in heavier snow bands as they move
east over SD and over southern MN/northern IA into WI. Confidence
in this strengthening fgen zone has increased with Day 3
probabilities for 6" snow over 80% along the MN/IA border into
western WI. The flow ahead of this negatively tilted trough is
southerly with a likely narrow zone of changeover from rain to
snow which remains uncertain. The current probability suite
features 10/60 members from the 18Z GEFS which have some fair
outliers from rather potent solutions. Based on the 00Z consensus,
the QPF was lowered a bit from the previous forecast, though
confidence remains high for >6" over the Upper MS Valley. Key
Messages were begun for this portion of the system and are listed
below.


For days 1-3, the probability of icing greater than 0.1" is less
than 10 percent.


...Key Messages for the March 8-12 Winter Storm...

--Confidence remains high for a winter storm developing over the
Northern Plains and shifting over the Upper Midwest Thursday into
Friday. Bands of heavy snow, gusty winds, and hazardous travel
conditions are expected during this time. This system will track
east and may rapidly develop over or just off the northern
Mid-Atlantic during the weekend.

--Some uncertainty remains with the timing and track of the
system, which will determine where the heaviest snow and more
significant impacts will occur.

--Continue to monitor the forecast and plan ahead for potential
impacts to travel.


Jackson