Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 AM EDT Tue Mar 14 2023

Valid 12Z Tue Mar 14 2023 - 12Z Fri Mar 17 2023

...The West...
Days 1-3...

The significant atmospheric river will continue to impact parts of
the Western U.S. through tonight, initially over the northern
parts of California today followed by central/southern California
this evening through tonight. The greatest moisture transport is
expected after 12Z today progged by CW3E to be between 500-750
kg/m-s. This pronounced moisture will be driven by a combination
of confluent mid-level flow from the Pacific, subtle shortwaves
embedded within this flow moving onto the coast, and persistent,
albeit modest, upper level Pacific jet energy arcing northeast
into the Pacific Coast and Intermountain West. Snow levels will
continue to rise with the influx of warm air advection, reaching
as high as 6000 ft (north) to 8000 (ft) south today. By later in
the period, a cold front sweeps through the region and this will
lower snow levels back to 3000-4000 ft (north) to 5000-6000 ft
(south) into the day 2 period. However, the most intense
precipitation and associated snowfall should occur ahead of this
front during the period of higher snow levels. WPC snow
probabilities for 8 inches are high for the Sierra eastward across
parts of the Great Basin and into the Central/Northern Rockies.
Localized amounts across the Sierra could top several feet while
amounts of 12-24"+ are possible for the peaks of central ID toward
western WY.

Heading into the day 2 period, the split upper level troughing
passes through the Rockies combined with a coupled jet streak over
the Southwest U.S. should provide enhanced lift over much of the
Four Corners region into the Central/Northern Rockies. The
greatest accumulations are expected across western CO and western
WY where the WPC probabilities 6-8"+ are high. Finally, for day 3,
a frontal boundary is pushed southward and settles over the Four
Corners region while low pressure begins to organize across the TX
Panhandle. The setup favors upslope flow against the southern
Front Range and Sangre de Cristos where WPC probabilities are
already high for 6 inches with localized 1-2 ft possible in the
higher terrain.



...Central Plains to Upper Midwest...
Day 3...

Shortwave energy associated with the southern and northern stream
will push out into the Plains Thursday then attempt to phase and
amplify over the Upper Midwest late in the week. A couple waves of
low pressure organizing on the progressing frontal boundary are
forecast to develop/deepen through the Midwest and Great Lakes
while the deep southerly flow ahead of it should draw up moisture
before wrapping around the system. The pattern has the setup for a
classic winter storm though there remains considerable uncertainty
with the strength, track, and QPF with the system. Phasing or lack
of of the northern/southern streams due to timing/speed issues
continues with a pretty sizable spread among the 00Z guidance. How
that potential phasing occurs and the resulting surface low track
has shifted some this cycle, to the south/southeast though a more
amplified/phased system is still possible and could swing back to
the northwest. All told, from the central Plains to the Upper
Midwest, a stripe of heavier snow remains possible but is less
certain away from the northern parts of the Upper Midwest where
moisture, lift, and some lake enhancement from Lake Superior helps
increase confidence for heavier snowfall accumulations. The latest
WPC probabilities show 50-60% chances for 6 inches from the MN
Arrowhead through northern WI and much of western/central U.P. of
Michigan where a prolonged period of deformation band snow could
set up. Further to the southwest across central MN into the
Plains, there is considerably more uncertainty.

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

...Major nor'easter for the interior Northeast continues today...

Early morning analysis shows the area of low pressure deepening
off the coast of the Northeast with an inverted trough positioned
through NYC into central/upstate NY. Precipitation shield
encompasses all of eastern NY thorough southern New England. Big
picture, the forecast remains on track for a significant winter
storm affecting the interior Northeast. Closer to the coast where
there was continued uncertainty in the thermal profiles and
snowfall accumulations, a combination of lesser QPF and slightly
warmer temperatures has reduced the snowfall forecast some for
areas across southern New England into Boston and slightly lesser
amounts right along the coast in Maine. Away from the coast
however, the probabilities for additional snowfall today (after
12Z) remain high for 6-10" and localized amounts exceeding 10" are
still expected for the higher terrain areas. The WPC Snowband
Prototype Tool still shows a good signal for 2-3"/hr snow rates
early this morning, beginning to taper down to 1-2"/hr later this
morning through the afternoon. The best signal for a prolonged
pivoting snowband is across east-central NY through parts of
western MA and southern VT/NH. The storm will begin to wind down
later this evening and the low pulls away late tonight with a
gradual tapering of the snowfall southwest to northeast.


Taylor


...Key Messages for March 10-15 (Upper Midwest to Northeast)
Winter Storm...
--The greatest impacts from the major norâ€easter will continue
through this evening, producing heavy snowfall, strong winds, and
coastal flooding.

--Intense snow rates (1-2â€+/hr) and strong winds will continue
to produce dangerous to impossible travel across parts of the
region. The heavy-wet nature of the snow, combined with max wind
gusts of 55 mph, will result in scattered to widespread power
outages and tree damage.

--Additional snowfall accumulations of 6-12†are expected for
many areas of Upstate NY and New England. Locally higher
additional amounts of 12-18†remain possible, particularly in
the terrain.

--Widespread minor and localized moderate coastal flooding and
beach erosion are forecast tonight through Wednesday as the low
pressure deepens and stalls off the New England coast.


...Key Messages for Mid-March (Western U.S.) Winter Storms...
--Heavy Rain and Snow Tonight Into Wednesday
Considerable flooding impacts below 5000 feet elevation are
expected to shift south across much of the California coast,
Central Valley, and Sierra Nevada foothills.

--Heavy Rain and Snowmelt Expected
Heavy rain, combined with snowmelt in terrain below about 5000
feet to result in more widespread flooding from Tuesday into
Wednesday, particularly in low elevations and areas with shallow
and warming snowpack. Creeks and streams will continue to be
vulnerable to flooding from additional rain and snowmelt.

--Sierra Nevada Snow Load Impacts Worsen
Heavy rain absorbed into the particularly deep snowpack in the
Sierra Nevada along with heavy snow, measuring in feet above about
7500 feet, will further compound ongoing snow load impacts and
issues.