Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
413 AM EDT Wed Mar 15 2023

Valid 12Z Wed Mar 15 2023 - 12Z Sat Mar 18 2023

...Four Corners to Northern Rockies...
Days 1-2...

Unsettled and active weather across the West associated with the
strong atmospheric river will begin to spread eastward across the
Four Corners region and across the central/northern Rockies over
the next couple of days. Current water vapor imagery shows two
shortwaves, a northern stream energy moving through southern Idaho
while the southern piece of energy is crossing through southern
California. As the southern stream shortwave slips toward the Four
Corners region tonight, this will bring a favorable setup for
forcing aided by left-exit region dynamics over the
southern/central Rockies. There should be enough residual moisture
in place to produce locally heavy precipitation today and tonight
with the greatest snowfall amounts likely across the western
Colorado Rockies (WPC snow probabilities for 6" are high) while
further north with the northern piece of energy, widespread 2-4"
amounts are expected across Montana and western Wyoming. Some
isolated 6-10" totals for the higher peaks in the terrain will be
possible. Heading into day 2, northeasterly upslope flow in wake
of a cold frontal passage into the Sangre De Cristo of
Colorado/New Mexico allow for an increase of probabilities for 8"
of snowfall, rising as high as 70-80% in these ranges.

...Upper Midwest...
Days 2-3...

Dual shortwave energy, one coming out of the Four Corners region
and the other out of the northern Rockies, will begin to phase
over the Plains and enhance the large scale forcing for ascent
across the central Plains to Upper Midwest later this week. At the
surface, in response to the lift, low pressure will organize in
the lee of the Rockies before racing off to the northeast from
central Kansas late tonight to lower Michigan by early Friday
morning. Rather impressive dynamics coming together with a surge
of strong low-level moisture transport aided by 50-60 kts at 850
mb will pull copious amounts of moisture up into the Upper Midwest
and wrap around the system while there is very strong
frontogenetical forcing setting up from Nebraska through
MN/northern WI. The 00Z guidance has trended stronger with a more
phased system compared to earlier cycles and confidence is
increasing for a stripe of heavier snow across parts of the
central Plains to Upper Midwest. The WPC snow probabilities have
increased considerably for 4 inches, now exceeding 50% from
northeast Nebraska through the Twin Cities and signals for 6" are
as high as 40 percent as well. Snow rates may exceed 1"/hr at
times as shown by the WPC snowband tool though that could be
complicated by the timing during the day, the higher sun angle
(making roadways warmer) and more marginal boundary layer
temperatures. While confidence in some of the higher snow totals
has increased, there remains some uncertainty in where that falls
given the likely narrow/localized nature of the band excepted.

By later in the day Thursday, the upper trough in the northern
Plains phases more with the shortwave in the southern Plains. An
increased and steady flow of Gulf moisture will work with the
favorable easterly flow off Lake Superior and this is likely to
produce heavy snow bands from the northern Minnesota Arrowhead,
northern Wisconsin, and the U.P. of Michigan. Intense snow rates
of 1-2"/hr are likely and already the WPC snow probabilities show
high signals for 8-12" in these areas and a decent signal (30-40
percent) for 18 inch totals. By the time the storm winds down,
localized 24 inch totals are possible.

From parts of the central Plains through the Upper Midwest and
U.P. of Michigan, travel could become difficult to dangerous given
the potential snow amounts and rates. A tightening pressure
gradient will also bring increasing winds such that blowing snow
and severely reduced visibilities will be possible. The latest
probabilistic WSSI (pWSSI) shows 40-60% chances of Moderate
Impacts (disruptions to daily life) from portions of Nebraska
northeastward to the U.P..

Taylor