Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
437 PM EDT Wed Mar 29 2023

Valid 00Z Thu Mar 30 2023 - 00Z Sun Apr 02 2023

...Northeast...
Days 1...
An amplifying shortwave will continue to sweep east across the
Great Lakes and southern Ontario this evening, before taking a
negative tilt over the St Lawrence Valley overnight.  This will
drive a sharp cold front across the Northeast this evening. 
Sufficient instability and strong ascent is expected to support a
convective rain showers changing over to brief but intense snow
showers, with embedded snow squalls.  The greatest threat for
heavy snow and whiteout conditions is expected to extend from
northern New York to western Pennsylvania early in the period,
before progressing east across northern New England overnight. 
Please see our Key Messages below for additional details. 

...The West/Rockies...
Days 1-2...
A deep closed low centered along the NorCal coast this afternoon
is forecast to gradually fill as it continues to sink south before
turning east overnight.  Some additional heavy snows are possible
along the southern Sierra and SoCal ranges before diminishing
tomorrow as the trough axis moves east of the region.  Meanwhile,
mountain snows are forecast to develop along a well-defined
low-to-mid level frontal zone as it moves from Nevada into Utah
and Arizona, with locally heavy amounts expected, especially along
the favored southwestern-facing slopes.  Farther to the north,
left-exit region upper jet forcing is forecast to add additional
support for heavy snows across the central to southern Idaho and
southwestern Montana ranges.  As snows spread into the Colorado
Rockies, snow showers are expected to continue across the northern
Rockies and Intermountain West into late Thu before diminishing
early Friday as the upper low/trough moves into the Plains. 

Overall, widespread heavy accumulations are not expected, however
some areas are likely to see a foot or more.  This includes the
Wasatch, where WPC probabilities for a foot or more are generally
above 50 percent. 

...Northern Plains and Upper Midwest...
Days 1-3...
As the trough in the West moves east, mid-level energy and
favorable upper jet forcing along the downstream ridge will help
support warm advection precipitation developing across the upper
Mississippi Valley into the northern Great Lakes Thu night into
early Friday.  Thermal profiles indicate a wintry mix is likely,
with a stripe of accumulating ice expected from eastern South
Dakota to northern Wisconsin and the UP of Michigan and some light
snow accumulations farther to the north.

By early Friday, the trough in the West is expected to move east
of the Rockies, with a closed low forecast to develop over South
Dakota-Nebraska.  Strong upper forcing and low-to-mid level
frontogenesis are forecast to support the development of heavy
snow on the northwest side of the associated surface low.  Model
differences remain, however the consensus for heavy accumulations
centered across South Dakota continued to grow with the 12Z runs. 
Strong forcing is expected to support precipitation changing to
snow within the comma head as it translates east across the upper
Mississippi Valley into the northern Great Lakes Friday night into
Saturday.  Probabilities for heavy snow continue to increase, with
the latest WPC run generally showing probabilities above 50
percent for accumulations of 6 inches or more along this corridor.
 Along this axis, accumulations of a foot or more can be expected.
 This will likely include portions of central South Dakota as the
system slows and intensifies over the region on Friday.  In
addition to heavy snow, this system is likely to bring strong
winds to the region as intensifies and moves east. Please see our
Key Messages below for additional details.  

...Pacific Northwest to the Northern Rockies...
Day 3...
Heavy mountain snows are expected to return to the region as an
amplifying shortwave trough originating over the Gulf of Alaska
dives southeast and moves across the region on Saturday.  Heavy
accumulations are likely for the Olympics and the Washington and
Oregon Cascades, with WPC probabilities indicating accumulations
of a foot or more likely (greater than 70 percent) for portions of
the area.  Generally, lighter but significant accumulations are
expected farther east for the Blue Mountains and parts of the
northern Rockies, including the Idaho and northwestern Wyoming
ranges, where amounts of 6 inches or more likely.

Pereira

...Key Messages for Northeast Snow Squalls...

--Snow showers and squalls are expected later this afternoon
through the evening and overnight hours along and behind a strong
cold front sweeping across the area.

--Where snow squalls occur, they will be accompanied by bursts of
heavy snow and strong gusty winds. Higher elevation areas more
likely to see stronger snow squalls.

--Any squalls could produce dangerous travel including whiteout
conditions, zero visibility, and quickly snow covered roads.

--If driving, remain alert for rapidly changing road conditions
and be prepared to react to any Snow Squall Warnings.

--Temperatures will rapidly fall below freezing this evening. Any
wet roads may quickly turn icy and lead to a flash freeze.


...Key Messages for Mar 31 - Apr 1 Winter Storm...

--A strong late winter storm is expected to track across the
central High Plains Friday and into the Great Lakes Saturday.

--While there remains some uncertainty in the exact placement of
the most impactful winter conditions, confidence is increasing
that the system will bring periods of heavy snow and strong winds
to the region.

--The combination of heavy snow and strong winds (gusts up to
50-55 mph possible) could create blizzard conditions, particularly
across portions of South Dakota and southwest Minnesota. Difficult
to near impossible travel conditions are possible.