Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
319 PM EDT Wed Mar 16 2022

Valid 00Z Thu Mar 17 2022 - 00Z Sun Mar 20 2022

...Central Rockies...
Day 1...

Upper trough will dig along the CO/UT border Day 1 (Thu
morning/afternoon) with a southern vort max moving eastward
through AZ/NM. These will merge into a disjointed and briefly
closed low at 500mb over the Plains by Day 2 as upper divergence
aids in surface pressure falls and cyclogenesis out of the TX
Panhandle through OK. This low will lift northeastward into the
lower Great Lakes and the Northeast by the end of Day 3. For the
Day 1 period, upslope flow into the Front Range and eastern slopes
of the CO Rockies via deep and moist northeasterly flow
(precipitable water values up to +2 sigma) will support modest
snowfall over the Front Range southward across the Palmer Divide
and into the Sangre de Cristo Mountains as the 700mb low closes
off early Thu. Totals may exceed a foot at higher elevations that
remain all snow, as snow levels above 6000ft this evening as
precip begins will fall sharply overnight. WPC probabilities for
at least 4 inches of snow were low to moderate over eastern
Colorado but increase to moderate to high near Denver and along
parts of the I-25 corridor especially into Colorado Springs.


...Midwest to Great Lakes and northern New England...
Days 2-3...

System organizing over the southern Plains will lift northeastward
as the upper trough and jet move into the Great Lakes and
Northeast/southern Canada Days 2-3. Northwest side of the precip
field will lie astride marginally-supportive temperatures as cold
air is drawn southward into the region. Deformation band
indicating by some of the models would support periods of snow
that could accumulate on at least grassy surfaces and especially
overnight. CAM guidance and higher-res mesoscale guidance were not
uniform in this depiction over KS late Thu into early Fri, but
overall support has increased for at least some snowfall and
perhaps a couple inches should the ingredients come together. Will
need to monitor the next set of guidance for further details but
these marginal spring events are tough to predict. Farther
northeast into Day 3, global models indicate potential for another
area of deformation snow over parts of IA into WI as the surface
low continues northeastward and the upper low weakens into an open
trough. Nevertheless, large spread in snowfall exists here with an
upper bound well above 4 inches (probability distribution skewed
well to the right to higher values but lower likelihood). WPC
probabilities for at least 4 inches of snow are below 10 percent,
but raw ensemble output suggests this may be underdone. Again,
will need to monitor future guidance cycles for consistency and
trends.


...Pacific Northwest...
Days 1-3...

Weakening shortwave moving into WA/OR Day 1 into early Day 2 will
bring generally light snow to elevations above ~4000ft over the
Washington Cascades into northern Idaho and northwestern Montana.
By Day 3, a stronger trough and attendant cold front will bring a
leading surge of modest moisture (precipitable water values up to
+1 sigma) to the Northwest, including into northern California.
Snow levels will fall below 4000ft behind the front as
precipitation starts to wane over WA/OR. Snow levels over the
Sierra will start near 5000-6000ft at the end of Day 3 when snow
starts to accumulate a few inches over higher elevations,
continuing beyond this forecast period into Day 4.


For Days 1-3, the probability for receiving at least 0.10" of
freezing rain is less than 10 percent across the lower 48.

Fracasso