Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
425 AM EST Tue Feb 28 2023

Valid 12Z Tue Feb 28 2023 - 12Z Fri Mar 03 2023

...Northeast...
Day 1...

Two waves of low pressure; one over the eastern Great Lakes and
one south of Long Island, will continue to produce periods of snow
over the Northeast today. This is due to a southeasterly fetch of
850mb moisture being funneled into the region while a negatively
tilted trough over Ontario provides added vertical ascent aloft.
The lingering high off the southeast Canadian Maritime and the
dual-low setup is also still supporting for a stubborn CAD signal
over New England. Coastal areas around the MA Capes on north along
the immediate New England coast line will still struggle to see
the heavier snowfall amounts, but not too far inland from the
coast, persistent snowfall rates around 0.5"/hr will occur west of
I-95 in southern New England with heavier rates up to 0.75-1.0"/hr
in the mountain ranges of northern New England. Additional
snowfall amounts of 4-6" of snowfall are likely in the
Adirondacks, Green, and White Mountains. The White Mountains and
south-central ME in particular have the best odds of seeing
snowfall totals >8" (WPC PWPF shows 30-40% probabilities there).
The Day 1 WSSI still depicts Moderate impacts for much of southern
New England, as far north as the Berkshires, and as far west as
northern NJ and both the Catskills and Adirondacks. Snow will
conclude across all these areas overnight Tuesday as the coastal
tracks south of Nova Scotia and the the best lift associated with
the trough over southeast Canada races northeast into northern
Quebec.

...Northern Plains to the Upper Midwest and northern New England...
Days 1-2...

The initial upper level disturbance that helped to produce heavy
snow in the Sierra Nevada on Monday raced east across the Rockies
and will enter the Northern Plains this morning. Strong vertical
ascent over the region will promote surface cyclogenesis over the
northern High Plains while a strengthening area of high pressure
over south-central Canada locks in sufficiently cold air to
support snow over the North-Central U.S.. Snow will break out over
the Dakotas Tuesday afternoon as a strengthening 850mb front sets
up overhead. Snow bands will wrap around the northern flank of the
emerging surface low, setting the stage for periods of heavy snow
across southern ND, far northern SD, and into central MN. Between
20Z Tues and 06Z Wed, the 00Z HREF showed X% probabilities of
>1"/hr snowfall across southern ND and into west-central MN.
Latest WPC PWPF showed 50-70% probabilities for >6" of snow in
these areas and the WSSI shows Moderate impact potential along
southern ND. The best mesoscale forcing will gradually weaken as
it approaches the Great Lakes but some lake enhancement off of
Lake Superior could still support 3-6" snowfall amounts on
Wednesday in the MN Arrowhead, far northern WI, and along the
northern coast of the U.P. of MI. This same disturbance will usher
in moisture to northern New England where additional snowfall of
>4" (WPC PWPF probabilities range between 20-40%) late Wednesday
into Thursday.

...West Coast, Four Corners region, and the southern and central
High Plains...
Days 1-3...

The longwave trough that has buried many of the mountain ranges of
the West Coast and Intermountain West with heavy snow will soon
finally progress east in the coming days. Before that happens,
another vigorous upper level feature diving south along the
Pacific Northwest coast will further amplify as it digs into
California Tuesday night. This setup will continue to direct rich
Pacific moisture at western Oregon on south through California
where more heavy snowfall is on tap. Snowfall rates in the Sierra
Nevada on Tuesday will be capable of falling at 2-3"/hr with
similar rates possible as far south as the Transverse Range
Tuesday night and into early Wednesday morning. Snowfall will once
again be measured in feet for elevations >3,000 feet in the Sierra
Nevada and above 5,000 feet in the Transverse Range. Some moisture
will spill into the Great Basin and the Intermountain West where
up to a foot of snow is possible in ranges such as the Tetons and
Wind River of WY, the Wasatch in UT, northern AZ, and the CO
Rockies. Some intense snow showers could take on the form of snow
squalls in parts of the Intermountain West this morning.
Meanwhile, as the upper trough digs through NV and into the Lower
CO River Valley on Wednesday, ample PVA advection and 700mb
moisture will stream out into the Four Corners region where
850-700mb winds will look to range between 35-60 kts. With falling
heights and colder temperatures arriving combined with the upslope
component into the Wasatch, the Mogollon Rim, the San Juans, and
the rest of the CO Rockies, periods of heavy snow are expected to
engulf much of the mountain ranges of the Four Corners region on
Wednesday. Latest WPC PWPF shows high probabilities of 70-80% for
24-hour snowfall totals >12" in southwest UT, the Mogollon Rim of
AZ, and the San Juans in southwest CO.

By Wednesday night, the upper low will be barreling through the
Four Corners region with cyclogenesis of a new wave of low
pressure occurring in lee of the southern Rockies early Thursday
morning. In addition, sufficiently cold temperatures will be
present in the central and southern High Plains. Along with the
surge of 700mb moisture from the Pacific aloft, 850mb winds out of
the east will upslope through western Kansas around the northern
periphery of the 700mb low. This setup will allow for a comma head
of precipitation to break out in the High Plains of northeast NM,
eastern CO, and western KS Thursday morning. Current WPC PWPF puts
odds of seeing >6" of snowfall at 10-30% generally, but with
strong mesoscale dynamics also at play and the available moisture
aloft, there is a plausible scenario for localized snowfall totals
to surpass 6" in these areas.

By Thursday night, the cyclone has become quite intense with NAEFS
depicting 500-700-850mb heights over the Red River Valley and MSLP
values over the ArkLaTex and eastern TX to be below the lowest
observed in CFSR climatology. Yet another 60 knot jet at 850mb
will deliver copious amounts of moisture along and ahead of the
warm front over the Mid-South and around the northern and western
flanks of the surface low. There remains a good amount of
uncertainty on precipitation type. With insufficient cold air on
the backside of the low, it will have to be primarily dynamic
cooling aloft that allows for snow to fall, and if so, would fall
quite heavy at times with strong wind gusts even more likely to
occur than snow. There is a dome of Canadian high pressure located
over Quebec by 12Z Friday which will help to lock in sub-freezing
air over the Great Lakes and Northeast. Boundary layer
temperatures will be sufficiently cold/dry enough for temperatures
to wet bulb below freezing initially, but the storm track further
into the Ohio Valley on Friday will undoubtedly lead to
transitioning precip types north of the warm front as well. One
thing is growing in confidence by Friday: a powerful storm system
is likely to bring impactful winter weather to parts of the
central Plains, Midwest, Great Lakes, and Northeast. Exact details
on amounts and where the best potential for heavy snow
accumulations remains lower in confidence for the time being.

Mullinax


...Key Messages for Winter Storms pushing through the Southwest...

--A series of winter storm systems will continue to impact the
West and Rockies through Wednesday with the final storm in the
series pushing over the Southwest U.S. Wednesday into Thursday.

--Mountains from southwestern Oregon through California will see
heavy snow rates of 2 to 3 inches per hour and additional snowfall
of 2 or more feet for the Sierra Nevada and southern California
ranges through Wednesday. Blizzard conditions will make for
dangerous to impossible travel.

--Heavy snow pushes across the high terrain of the Four Corners
and southern Rockies through Thursday where hazardous travel
conditions are likely. There is the potential for avalanches in
some mountain ranges.

--If you plan to travel through any mountain passes in the Western
states through Wednesday, be prepared for rapidly changing
conditions and have winter driving supplies.