Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
411 PM EDT Wed Mar 22 2023

Valid 00Z Thu Mar 23 2023 - 00Z Sun Mar 26 2023

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

Another pronounced shortwave trough will drop quickly out of the
Gulf of Alaska toward the Pacific Northwest Thursday, bringing
additional height falls and PVA across portions of WA/OR into the
Northern Rockies. Favorable upper diffluence aided by the left
front quadrant of a developing jet streak will help bring
widespread precipitation across the terrain, especially across the
WA/OR Cascades but also southward into the northern CA ranges and
northern reaches of the Sierra. Moisture spilling eastward will
also bring widespread precipitation to the Great Basin and
Intermountain West. By far, the greatest synoptic setup for heavy
snow will be for the Olympics and WA/OR Cascades where a prolonged
period of orthogonal flow combined with lowering snow levels will
drive some significant snowfall through the weekend.


The latest WPC probabilities are high for 6 inches or more from
the northern Sierra through the OR/WA Cascades and also some of
the higher peaks of the northwest CA, southwest OR coastal ranges
and WA Olympics. High chances (greater than 70 percent) for 6
inches also exist extend eastward through the San Gabriels
eastward across the higher terrain of the Wasatch, San Juans, CO
Rockies, and as far north as the Wind Rivers. By Day 2 into Day 3,
most certainly the highest snowfall amounts will be found across
the OR/WA Cascades where the 3-day  totals could exceed 2-3 feet
with localized 4 feet for the peaks.

Additionally, as snow levels lower to just a few hundred feet,
significant accumulations are likely at the important passes, with
WPC probabilities indicating a high chance for at least 6 inches
as Snoqualmie, White, Santiam, and Willamette passes. With snow
levels very low Friday into Saturday, even some light snow is
possible into the lowlands and valleys around Portland, OR.


...Plains through the Upper Midwest...
Days 1 & 3...

The main longwave trough will remain positioned over S. California
and the Desert Southwest today into tonight, however a lead weak
impulse/shortwave trough is expected to shed out into the central
Plains late tonight into Thursday. This vorticity lobe will remain
sheared and modest in amplitude as it moves into more confluent
flow to the east, but impressive LFQ diffluence atop it owing to a
strengthening subtropical jet streak should still result in robust
synoptic ascent across the Plains and into the Upper Midwest. A
fairly tight baroclinic zone will develop with high pressure
centered over North Dakota and an area of low pressure organizing
over Kansas. The overlap of strong upper divergence, lower level
frontogenesis and modest moisture in place should spread a streak
of precipitation across portions of northern Nebraska tonight
where the latest WPC probabilities of 4 inches are around 20-30
percent.

Southern stream shortwave trough energy ejecting out of the Four
Corners region into the S. Plains will take on a negative tilt
Friday into Friday night. Strong right entrance region jet
dynamics over the Midwest will help drive a deepening surface low
from the Mid-MS River Valley toward the Upper Midwest and Great
Lakes into Saturday morning. Guidance is beginning cluster
together somewhat on the surface low track and intensity such that
a stripe of heavy snow is likely on the northwest side of the low
track across parts of the Upper Midwest into the Great Lakes. One
of the main uncertainties is the thermal profiles and the degree
of dynamic cooling that can take place. Given the increasing
signal for strong fgen and the time of day (overnight/early
morning), thinking is that enough forcing should allow for a
changeover to wet, heavy snow from portions of northern IL into
southern/southeast Wisconsin and further downstream across the
northern areas of the L.P.. The other uncertainty is the storm
track with the 12Z guidance still showing some spatial differences
northwest to southeast. Overall though, the NBM and WPC
probabilities for 4" have increased and now exceed 30 percent
across parts of southern Wisconsin and above 50-60 percent for
northern areas of the L.P., though there is certainly guidance
that shows potential for localized higher amounts that will need
to be fine tuned in the later forecast updates given the remaining
uncertainty in the placement of the heaviest snowfall.


...Northeast...
Days 1 and 3...

A potent shortwave trough moving south of Hudson Bay will leave
downstream divergence across New England on Thursday and this will
combine with increasing upper diffluence within the LFQ of a
potent jet streak over the Great Lakes to drive potent deep layer
ascent into the Northeast. At the same time, a warm front
extending from a triple point near the Great Lakes will lift
northeastward, driving enhanced warm and moist advection to fuel
PW anomalies that may reach as high as +2 standard deviations by
late Thursday. The overlap of this lift and moisture will yield an
expanding area of precipitation from southwest to northeast, but a
marginal thermal structure and a northward advancing warm nose
will keep most of this as rain. However, across far northern New
England and especially northern Maine, the column will remain cold
enough for periods of moderate to heavy snow, and WPC
probabilities for more than 4 inches of snowfall are as high as
50% in the far northern part of the state.

On Friday, another strong shortwave trough begins to move out of
the Four Corners region into the S. Plains and takes on a negative
tilt across the Mid-MS River Valley. Coupled jet streak will
provide impressive forcing for ascent such that a deepening area
of low pressure is forecast to develop into the Midwest. Moisture
will spread eastward across portions of the Northeast late during
day 3 through Sunday and with residual cold air in place thanks to
departing high pressure, some snow and/or mixed precipitation is
likely, especially for the higher elevations of the Adirondacks
and the Greens/Whites Mountains where WPC probabilities for 4" are
in the slight chance range. Some mixed precipitation is possible
further south, though at this time any freezing rain accumulations
looks to be fairly light.


Weiss/Taylor