Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
233 AM EST Wed Feb 28 2024

Valid 12Z Wed Feb 28 2024 - 12Z Sat Mar 02 2024

...Great Lakes...
Days 1-2...
A consolidating surface low, developing in response to an
amplified upstream shortwave trough, is forecast to lift northeast
across the upper Great Lakes this morning.  Favorable upper jet
forcing and mid level frontogenesis will support a band of heavy
snow developing northwest of the low, with some lake enhanced
rates of 1-2 in/hr lifting across the eastern U.P. of Michigan
this morning.  This will be followed by lake effect snow showers
continuing into early Thursday that will contribute to locally
heavy accumulations across the region.  WPC PWPF shows high
probabilities (greater than 70 percent) for accumulations of 4
inches or more, and moderate probabilities (greater than 40
percent) for accumulations of 8 inches or more along the eastern
U.P. through early Thursday.  Northwesterly to westerly winds and
strong cold air advection in the wake of the departing system will
contribute to lake effect snows in the lees of lakes Michigan,
Erie, and Ontario, as well as orographic snows into the northern
and central Appalachians.  Overall, accumulations are expected to
be light; however, there is some potential for locally heavy
amounts in some areas including northern Lower Michigan and
central New York.  

...West Coast & Northern Rockies...
Days 1-3...
**Significant multi-day winter storm to produce heavy snowfall
from the Cascades and Sierra Nevada to the Northern Rockies
through late week**

The forecast remains much of the same, with the models continuing
to a signal a multi-day event, that is likely to produce prolific
snow totals across portions of the Cascades and the Sierra Nevada.
 Precipitation has already begun, with warm advection
precipitation moving into western Washington and northwestern
Oregon this morning.  Supported by strengthening southwesterly
flow ahead of an approaching cold front, precipitation is forecast
to continue through the day, with rates increasing as the
strengthening winds direct deeper moisture inland.  Snow levels
which are already above 4000 ft in many locations will continue to
increase through the day until the cold front begins to press
southeastward across western Washington and then western Oregon
overnight -- bringing levels back below 2000 ft across much of the
region by early Thursday.  Meanwhile, snow levels will remain
above 4000 ft across much of the Intermountain West and the
northern Rockies.  The heaviest snows through early Thursday are
expected to remain mostly centered over the Olympics, the northern
Cascades, and the northern Rockies -- especially the northern
Idaho and the northwestern Montana ranges.  Snowfall rates
increasing to 2-3 inches/hour are expected to result in several
feet of snow across the higher elevations of the northern
Cascades.  The northern passes will be impacted as well, with WPC
PWPF showing high probabilities for accumulations of 8 inches or
more at both Stevens and Snoqualmie passes.  High probabilities
for 8 inches are indicated at Lookout Pass in the northern Rockies
as well.

On Thursday a strong cold front will push well inland through the
Northwest into the northern Rockies, while dropping south into
northern California.  Ample moisture spreading inland along with
strong forcing will support heavy snow moving east with the front,
impacting a greater portion of the northern Rockies, including the
central Idaho ranges on Thursday.  Meanwhile, strong ascent
afforded by favorable upper jet forcing and strong low level
frontogenesis will help support snowfall rates climbing in excess
of 3 inches/hour shifting south across portions of the Oregon
Cascades into the northwestern California ranges and the northern
Sierra Nevada on Thursday.  Snow levels will drop quickly behind
the front -- dropping below 2500 ft across much of western Oregon
by late Thursday and below 3000 ft over northwestern California by
early Friday.  By early Friday, heavy snow accumulations are
likely along the southern Cascades to the northwestern California
mountains and the northern Sierra Nevada.  Several feet can be
expected across the higher elevations, with heavy amounts
impacting the passes as well.  WPC PWPF shows high probabilities
for accumulations of a foot or more impacting the major southern
Cascade and Sierra Nevada passes.

An upper trough will remain in place and continue to amplify along
West Coast into the weekend.  Shortwave energy moving through the
broader-scale flow, along with coinciding plumes of deeper
moisture, will support periods of very heavy snow over the
southwestern Oregon and northwestern California coastal ranges,
the southern Cascades, and the Sierra Nevada.  Multiple feet are
likely along the Sierra Nevada, including the passes, from Friday
into early Saturday.  Meanwhile, relatively lighter but impactful
amounts are expected farther east into the central and southern
Idaho and western Wyoming ranges.  


***Key Messages for the Major Western Winter Storm***

--Winter Storm to Impact West Coast
A significant winter storm will primarily affect the Northwest
today and Thursday before significantly impacting the northern and
central California mountains Friday and Saturday.

--Heavy Mountain Snow Affecting Many Passes
Multiple feet of snow are likely (over 80% chance) for higher
elevations, especially above 5000 ft, including many Cascade and
Sierra passes. Extremely heavy snow rates surpassing 3 inches per
hour are possible in these mountain ranges.

--Blizzard Conditions Expected in Sierra Nevada
Strong winds will cause significant blowing and drifting snow,
with whiteout conditions, making travel impossible in the Sierra
Nevada. There is a high chance (over 70%) of substantial,
long-lasting disruptions to daily life in the higher elevations of
the Sierra Nevada Friday through Saturday, where blizzard
conditions and over 5 ft of snow are expected.

--Cold Air Will Lower Snow Levels This Weekend
As the storm moves south, snow levels will lower into some
northern California and Sierra foothill communities. Much colder
air is forecast by the weekend, with temperatures 10-20ĀF below
normal.


Pereira