Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
457 PM EDT Sat Mar 28 2020

Valid 00Z Sun Mar 29 2020 - 00Z Wed Apr 01 2020


...Upper Mississippi Valley to Lake Superior...
Day 1...

A potent, vertically stacked low pressure system over the northern
Nebraska/Iowa border at 20Z (per water vapor imagery and surface
METARs) will lift northeast to Wisconsin through tonight before
shifting east to southern Ontario through Monday as it
fills/weakens. A particularly strong warm conveyor belt direct
from the western Gulf of Mexico will continue to wrap around the
system as it occludes and shifts northeast, allowing the existing
narrow swath of moderate to locally heavy precipitation in a
fairly narrow band oriented southwest to northeast to spread
across MN tonight. A turnover to snow within this dynamic TROWAL
band is expected tonight both as it lifts north and as the strong
late March daytime heating ends. Enough dynamic cooling is
expected to allow snow accumulations starting around the Buffalo
Ridge in southwest MN this evening, spreading northeast and
increasing the snow risk to the southwest side of Lake Superior
overnight. Briefly heavy rates of 1+"/hr are likely in the
deformation band with low probs for 4 or more inches limited to
northeast MN, but it should be noted that the global ensemble
members in the super ensemble struggled mightily with the dynamics
of this system and elevation should play a role. Therefore there
is a risk for 4 or more inches near the Buffalo Ridge and areas
northeast from there beyond the MN River Valley. The combination
of greatest cold, dynamic precip, and elevation come together just
southwest of Lake Superior. Day 1 snow probs for 8 or more inches
are moderate over far northwestern WI just south of Lake Superior
with low probs extending into the Porcupine Mtns of the UP and up
interior sections of the Lake Superior north shore.

Wrap around precip will be quite marginal with snow only expected
for the higher elevations of the UP into Sunday night.


...Northeast...
Days 1-3...

A warm front extending from the strong low pressure approaching
the Great Lakes tonight will lift northeastward (though struggle
on the east side of the Appalachians due to cold air damming)
through the Mid-Atlantic Sunday before stalling near southern New
England Sunday night as coastal low pressure develops on this wave
as the parent upper low shifts east across southern Ontario.
Isentropic ascent over this boundary will enhancing the coverage
and intensity of the precipitation. As precipitation expands into
New England, it will initially be snow across the White Mtns of NH
into Maine with some sleet and light freezing rain possible Sunday
night during wet bulb cooling over interior New England. The snow
may be heavy Sunday night in northern Maine in the cold column air
up there. Day 2 snow probs are moderately high for 6 or more
inches over northern Maine. On Monday, the surface low will linger
in the Gulf of Maine as jet level diffluence and height falls
persist. Snow will taper off in Maine Monday night as the system
shifts out to sea.


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

A pair of shortwave troughs, one approaching the Pac NW and the
other central CA will reach the coast late tonight and shift
inland and cross the Rockies through Monday. This is followed by a
potent cold core low that approaches Vancouver Island Sunday night
before amplifying east across BC through Monday as the parent low
persists over Vancouver Island. Moist advection and 700mb ascent
over the mountains will allow moderate to heavy snow above 3000 to
4000ft in WA/OR, 4000 to 5000ft across the Intermountain West and
snow levels as high as 6000ft in CO/UT Sunday night. The heaviest
snow is likely on Day 2.5 in the WA/OR Cascades where snow
probabilities are high for 12 or more inches. Otherwise, WPC
probabilities on Day 1 are moderate to high for 8 inches for the
highest Cascades, Olympics, and northern Sierra Nevada. On Day 2
this focus is in the WA/OR Cascades, Olympics and northern Rockies
of ID where probabilities are high for 8 or more inches. On Day 3
there are moderate to high probabilities for 8 or more inches for
the WA/OR Cascades, Olympics, and northern Rockies of
ID/MT/northwest WY. 3-day totals in the highest Cascades and
Olympics may exceed 4 feet.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson