FOUS11 KWBC 200857
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
457 AM EDT Wed Mar 20 2024

Valid 12Z Wed Mar 20 2024 - 12Z Sat Mar 23 2024

...New England...
Days 1-2...

A potent shortwave trough rounds an upper low over the northern
Ontario/Quebec border this morning, tracking over the Great Lakes
today, taking on a negative tilt as it crosses New England
tonight. This will drive strengthening height falls and downstream
PVA, collocated with the left exit of a modest pivoting jet streak
within the pinched flow southeast of the amplifying trough. This
will result in an intensifying surface low out of eastern New
England this evening. The 00Z consensus is for the track to be a
bit farther east which brings the cold enough atmospheric column
for snow farther east, expanding the area of heavy snow over Maine.
Day 1.5 PWPF for >6" is 40-80% over all but coastal and far
southern Maine as well as the White Mountains and northern NH and
the Northeast Kingdom of VT.

Additionally, the associated strong cold front will race across
Upstate NY through this afternoon. Along this front, CAMs remain
in good agreement that waves of snow showers will occur, especially
from northern PA/central NY into western New England this
afternoon into this evening. The snow squall parameter lights up
across this area, driven by 200-400 SBCAPE overlapped with 0-2km
theta-e lapse rates falling well below 0C/km and high low- level
RH. This suggests that at least scattered snow squalls will occur,
with briefly heavy snow rates and gusty winds resulting in
hazardous travel.


...Northern Rockies through the Upper Midwest and Great Lakes...
Days 2-3...

A stationary front persists over the northern High Plains through
Thursday while intensifying cyclonic flow begins to envelop the
northern Rockies through northern Plains south of an amplifying
closed low over Manitoba. A notable impulse from off the PacNW
today will shift east in zonal flow before getting caught in this
cyclonic flow over northwest MT early Thursday which then drives
surface cyclogenesis along the stalled front and a focused/
progressive wave that shifts from western Nebraska late Thursday to
the southern Great Lakes by late Friday. Ascent will be aided by
the right entrance region of a modest/backing jet near the
US/Canadian border and PVA from the mid-level impulse. This ascent
atop the baroclinic gradient/surface low will drive a west to east
oriented band of precipitation beneath the jet streak that will be
met with Gulf-sourced moisture streaming up the Plains over the
Dakotas late Thursday. An expanding precipitation shield northeast
of the developing low is expected as the low approaches the Upper
Midwest Thursday night. The low then shears/weakens into an
inverted trough over the Midwest Friday, but synoptic dynamics
remain strong as the cyclonic flow centered over southern Hudson
Bay increases with plenty of Gulf moisture available. More marginal
thermal conditions accompany the trough over the eastern Great
Lakes Friday night, but the expansive precip shield extends into
the cold enough thermals of the northern Great Lakes.

The heaviest snow is expected around Glacier NP where increasing
surface ridging out of the Canadian Prairies drives flow with an
eastern component upslope into this area with the impulse passage
early Thursday further aiding development/ascent.

Day 2 PWPF for >6" is 80-100% in the Lewis Range of Glacier NP with
stripes of 20-40% values from banding over southern ND into west-
central MN. Day 2.5 PWPF for >6" is when values are most expansive
with 30-70% values over much of WI and the northern half of the
L.P. of MI. The focus thereafter is farther north of the trough
axis and generally over southern Ontario, but the snow bands reach
the eastern Great Lakes late Friday with Day 3 PWPF for >6" 20-50%
over the Tug Hill and Adirondacks and north over the rest of NY
from there.

Back over the northern Rockies, the Day 3 PWPF for >6" are 20-50%
for all of the Northwest MT ranges as the baroclinic zone in the
immediate lee persists with mid-level Pacific moisture increasing
as the ridge axis just west pivots south as the Hudson Bay low
continues to amplify, keeping snow going over the northern Rockies.


...California...
Day 3...

Upper low currently on the northern BC coast shifts southwest
through Thursday, phasing with other shortwave troughs to form a
deeper closed low off the OR coast downstream of a potent omega
high over eastern AK. This low then makes a wobbly approach to the
OR coast Friday as an occluded low that then stalls near the coast
until a reinforcing trough later in the weekend. A subtropical
moisture feed is directed in from the southwest, working its way
south over the entire state of CA late Friday through Saturday.
Snow levels in this feed look to be 6000-7000ft though levels drop
to 4000-5000ft over the Shasta/Siskiyou and Sierra Nevada Saturday
under height falls where Day 3 PWPF for >8" are 50-80%. Wintry
precip likely continues over these areas through Sunday under upper
troughing.


Weiss/Jackson


***Key Messages for Northern Plains Winter Storm March 23 - March 26***

-- Winter storm becoming likely:
Confidence is increasing that a large storm system will produce a
swath of heavy snow, as well as freezing rain and sleet, across
portions of the Northern Plains and Upper Midwest into early next
week

-- Widespread heavy snow:
Heavy snow will likely overspread eastern Montana late Saturday,
then expand into the Northern Plains and Upper Midwest by Sunday
night. Significant accumulations are likely. Snow should exit the
area during Tuesday.

-- Significant impacts due to snow, ice, and wind:
A combination of heavy snow and gusty winds, as well as freezing
rain south of the heavy snow, could result in hazardous travel and
impacts to infrastructure.

-- Forecast changes anticipated:
Uncertainty remains with the timing and location of the storm
track, which will affect where the most significant impacts will
occur. Keep up to date with the latest forecast as this storm
evolves.


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