Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
445 AM EDT Sat Mar 19 2022

Valid 12Z Sat Mar 19 2022 - 12Z Tue Mar 22 2022

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

An occluded low will shift over the eastern Great Lakes/St.
Lawrence Valley today before crossing northern New England
tonight. Ahead of this low, a warm front and associated WAA will
surge northward on S/SW flow out of the Gulf of Mexico to expand a
precipitation shield across the Northeast this morning. Snow in
higher terrain and far northern areas will give way to mixed
precipitation, generally for the terrain of northern NH and
northern/central ME. Freezing rain will occur as the warm nose
lifts atop sub-freezing surface air locked into the terrain. Day 1
WPC probabilities for over 0.1" of freezing rain are 30% to 60%
for northern interior Maine.

...The West to the High Plains...
Days 1-3...

An amplifying shortwave trough will cross the Pacific Northwest to
central California coasts today before digging south to the Baja
tonight and closing into a low over AZ/NM Sunday night. This low
then shifts east over the southern Rockies and onto the southern
High Plains Monday before turning north up the Plains Monday
night. Uncertainty with the strength and timing of the initial
wave into southern CA early Sunday persists with the resulting low
(in particular the surface lee-side low over the High Plains)
having a varied track in guidance. The faster and more potent GFS
is the farthest north with considerable snow on the eastern slopes
of the CO Front Range while the slower and weaker ECMWF is farther
south and east with rather little snow outside the higher terrain
in CO. In between are the CMC and UKMET which were a bulk of the
input for QPF along with ensemble means and the NBM.

The amplification of this trough/low will lead to intensifying jet
streaks both upstream and downstream of the primary longwave axis,
leading to enhanced upper diffluence and lift. Ample Pacific
moisture is with the cold front currently approaching the PacNW
coast and increasing warm air advection ahead of a surface cold
front combined with rising IVT embedded within the upper jet
streak will drive PWs to +1.5 standard deviations above the climo
mean by Sunday across the Great Basin with Gulf moisture lifting
up the Plains ahead of the trough. The combination of impressive
upper diffluence and height falls/mid-level divergence will drive
lee cyclogenesis Monday morning across eastern NM, with this
surface low deepening while it shifts east then north over the
Plains Monday night.

Tonight through Sunday night, the mountain snowfall will be driven
by the cold frontal passage and jet level dynamics with moderate
Day 1 snow WPC probabilities for over 6" in the Cascades, central
Sierra Nevada, and the Salmon River Mtns of ID. For Sunday, the
trough elongates and shifts inland, with moderate probabilities
for snow over 6" inches across the WY ranges.

Then late Sunday night into Monday night, there is increasing
confidence for widespread heavier snow for the central and
southern Rockies from the Laramie Range southward through the
Sangre De Cristos before spreading onto the High Plains. The
intensifying lee surface low will help wrap moisture westward into
the terrain and High Plains and enhance ascent through upslope
flow. With less confidence in a solution as extreme as the 00Z
GFS, Day 3 WPC snow probabilities for over 6" have dropped to
moderately high high along the Front Range, Palmer Divide, Raton
Mesa, and Sangre De Cristos as well as adjacent High Plains of
eastern CO and northeast NM. There is still the potential for a
foot locally with locally 12 inches of snow in the higher terrain.


Jackson