Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
311 PM EDT Fri Mar 29 2024

Valid 00Z Sat Mar 30 2024 - 00Z Tue Apr 02 2024

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

Active period for much of the western 1/3 of the Lower 48 starting
today, as an upper low off the coast of Northern California pushes
inland early Saturday. The larger pattern will see building upper
ridging into southern Alaska, promoting anticylonic wave breaking
into western North America though maintaining a positively-tilted
trough in the Southwest US. However, the trough will contain
multiple streams and take on a somewhat disjointed progression,
ultimately slowing its exodus from the Desert Southwest even by the
end of the period (Mon evening).

First part of the event (Sat) will feature the closed low moving
southeastward paralleling the central CA coast, with broad
downstream southwesterly flow helping to stream moisture into the
Sierra, Great Basin, and into the UT/western WY ranges. IVT values
of 250-300 kg/m/s will intersect SoCal with some extension to the
southern Sierra with PW anomalies of +2 to +3 into southern NV.
Snow levels will start around 5000-600ft ahead of the cold front in
CA, dipping somewhat behind it tomorrow evening. Farther inland,
snow levels will be higher -- amount 7000ft -- favoring the higher
Uintas in UT. WPC probabilities for at least 12 inches of snow in
the Sierra are highest (>50%) above 5000ft.

Into D2, the upper low will split into two entities -- the
offshore low still progressing down the coast offshore while the
lead vorticity stream forming its own center over Nevada. With the
moisture plume decidedly east of the Sierra, crossing over the
northern Baja into AZ, snowfall will be widespread over the Great
Basin to the Rockies, including over the Mogollon Rim. Snow levels
will slowly fall, but the trough axis will be extended SW to NE so
heights will be slower to fall east of the Sierra. Regardless,
orographic upslope will favor SW- facing terrain and many NV ranges
into the Uintas. WPC probabilities for at least 6 inches of snow
are high (>70%) generally above about 7000-8000ft. For D3 (Monday),
additional northern stream vorticity will slip down from the PacNW
into the backside of the trough over SoCal, slowing the
advancement of height falls through the Four Corners. This will
favor continued moisture into the Mogollon Rim and into the San
Juans in CO, where an additional several inches are likely. WPC
probabilities for at least 6 inches are highest over these regions.


...Maine...
Day 1...

A strong coastal low lifting into Nova Scotia will gradually depart
to the northeast Saturday, bringing a slow end to preciptation
across Maine. However, an impressive TROWAL pivoting cyclonically
back into Maine and overlapping with the advecting deformation axis
will likely drive heavy snowfall through Saturday morning,
especially across eastern and northern Maine. The strongest forcing
will occur early D1, but even as ascent begins to wane, the
pronounced CAA behind the low will result in a deepening DGZ and
cooling column to offset some loss of omega and allow rapid
accumulation to persist thanks to the increasing SLR. Snowfall
rates across eastern Maine could exceed 1"/hr, which when combined
with gusty north winds will result in significant impacts the first
half of D1 before rapid improvement occurs. WPC probabilities for
more than 4 inches of snow reach as high as 80% in far northern
Maine.


...Upper Midwest...
Day 1....

Wave of low pressure moving eastward across the Midwest and into
the Great Lakes Saturday will remain progressive within generally
flattened flow and beneath a low-amplitude shortwave. Despite the
fast motion and modest intensity, downstream moist advection will
persist on 295-300K moist isentropic ascent which will pivot
northward into a cold column to expand wintry precipitation from MN
into the U.P. of MI. The total duration of moderate to heavy snow
is likely to bejust 6-12 hours, but a favorable crossover of ascent
ino the DGZ within the best WAA should still result in an axis of
moderate accumulations before the system exits to the east Saturday
night. WPC probabilities for more than 4 inches of snow are highest
along the Arrowhead and into the Iron Ranges of MN where they reach
30-40%.


...Northern/Central Plains...
Day 3...

An amplified but positively tilted trough across the Intermountain
West will begin to pivot eastward as it evolves into a closed low
centered near the Central High Plains by late Monday. This will
drive pronounced height falls eastward, which will combine with
downstream mid-level divergence, the LFQ of a strengthening
subtropical jet streak, and persitent troughing in the lee of the
Rockies to drive cyclogenesis east of the Front Range Sunday
evening. While there remains some longitudinal spread in the
developing low placement, it is likely to deepen considerably once
it initiaties in response to the impressive synoptic lift, and move
east/southeast towards the Southern Plains during Monday.

This deepening low and associated strong synoptic ascent will work
across an environment with plentiful moisture as SW flow around the
longwave trough surges IVT as high as +4 sigma according to NAEFS
into
the Four Corners, which will combine with increasing moist
isentropic lift downstream of the low from the Gulf of Mexico to
produce an expanding shield of precipitation. The column north of
this low is likely to be initially marginal for snowfall outside of
the higher terrain, but as the low deepens and cold air drops south
beneath a Canadian high pressure, wintry precipitation in the form
of heavy snow is likely to expand across the High Plains from WY
into the Dakotas and Nebraska. With uncertainty high at this time
range, the placement of the heaviest snow axis remains in question,
but where it does occur, the setup will support possible banding
and heavy snow rates. Although changes are likely, current WPC
probabilities for more than 4 inches of snow are 20-40%, highest
across northern NE/central SD, but reach above 50% in the Black
Hills.


Weiss/Fracasso