Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 AM EST Thu Feb 29 2024

Valid 12Z Thu Feb 29 2024 - 12Z Sun Mar 03 2024

...Great Lakes...
Day 1...

Departing cyclone and strong cold front will support WNW flow
across the Great Lakes this morning but diminishing tonight as
heights rise and surface high pressure moves in. Thrice-cross-lake
flow (Superior-->Huron/Georgian Bay-->Ontario) will favor a mostly
single band into central NY near/north of Syracuse that should
slowly lift northward as the flow becomes more westerly this
evening. CAM guidance differs in location and amounts (<0.50" to
>0.75"), but several inches of snow are likely within the band,
where WPC probabilities for at least 4 inches are highest (>70%).
Max amounts may top 8-10 inches, with WPC probs around 50%.


...West Coast & Northern Rockies...
Days 1-3...

**Significant multi-day winter storm to produce very heavy
snowfall from the Cascades to the northern Rockies and especially
into the Sierra Nevada through late week**

Strong, building upper ridging into the Bering Strait will promote
digging troughing into the Pacific Northwest today into Friday via
multiple embedded vorticity streamers. Upper jet into WA/OR this
morning will dip down into NorCal this evening and increase as jet
streaks move through the region before lifting out through the
Great Basin into the western High Plains. This will carry the
surface cold front southward and eastward as well (now progressing
through western WA/OR) with its plume of moisture and IVT
preceding the front (generally +1 sigma anomalies) on SW flow.
Snow will continue to spread eastward into the northern Rockies
but especially through the Oregon Cascades and into the NorCal
ranges, then into the Sierra later this afternoon and evening.
Broad divergence on the LFQ of the upper jet will promote
widespread snow east of the Cascades where WPC probabilities for
at least 8 inches D1 are high (>70%) over the central ID ranges,
northwestern MT, and into western WY.

Into D2, upper jet will mostly meander over NorCal with a
continued supply of moisture into the OR Cascades and into the
Sierra as a trailing and stronger vort max moves in from the
Pacific. Snow levels will be on the higher side ahead of the
front, then fall as the front moves through but precipitation
forcing remains over the Sierra. With a nearly perfect
orthogonally-aligned flow and slow movement, QPF will maximize via
upslope into the windward terrain where snow will fall at
incredibly heavy rates per the guidance -- 2-5"/hr in the HREF for
several/many hours especially at elevations above 5000-6000ft
where several feet are likely just on D2 alone. Snow levels will
waver around 4000-5000ft on D2 over NorCal, where WPC
probabilities for at least 18 inches of snow are high (in many
cases near 100%) near and upwind of the Sierra crest. Snow will
continue over the Great Basin as well along the pivot point of the
mid-level, in response to the strong height falls into NorCal. WPC
probabilities for at least 12 inches of snow over western WY are
>50% over the Tetons and Wind River Range.

By D3, the longwave pattern will finally start to move inland,
with the trough axis coming ashore and the upper jet punching
inland to the central Rockies. This will favor another day of
prolific snowfall for the Sierra with maximized upslope, slowly
sinking southward to the southern Sierra with time. Another few
feet of snow are likely as snow levels continue to fall, bring
some snow to the foothills and elevations around 2000ft. WPC
probabilities for at least another 18 inches of snow remain high
(>70%) over the length of the Sierra on Saturday. With height
falls extending eastward to the Rockies, a large area of light to
modest snow will spread across the northern half of Nevada, across
the Wasatch, and into the CO Rockies. WPC probabilities for at
least 8 inches of snow are at least 50% in these areas, perhaps a
bit lower in CO which will be last for the moisture plume to
reach.

Impacts will be high in the Sierra where Blizzard Warnings will be
in effect -- major to extreme for the higher elevations -- which
will include the passes across the Sierra. Travel will be
dangerous and perhaps impossible with very heavy snow rates and
totals of several feet, along with blowing and drifting snow from
increasingly windy conditions. Tree damage and power outages are
quite possible.


The probability of significant icing of at least 0.25" is less
than 10 percent.


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

--Heavy Mountain Snow Affecting Many Passes
Multiple feet of snow are likely (over 80% chance) for higher
elevations, especially above 5000 ft, including the Sierra Nevada
passes like I-80. Extremely heavy snow rates surpassing 3 inches
per hour are likely.

--Blizzard Conditions Expected in Sierra Nevada
Strong winds will cause significant blowing/drifting snow and
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-Saturday, where blizzard conditions and
5+ ft of snow are expected.

--Widespread Damaging Wind in the Western U.S.
Wind gusts of 55+ mph are forecast across much of the West,
particularly across higher elevations and the Intermountain West
where 75+ mph gusts are possible. These winds would likely down
trees and power lines, resulting in power outages across affected
areas. 

--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 10-20F below
normal.

Fracasso