Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
438 AM EDT Tue Mar 12 2019

Valid 12Z Tue Mar 12 2019 - 12Z Fri Mar 15 2019

...Central Rockies...Central Plains...Western Great Lakes...

...Significant Blizzard likely Wednesday into Thursday...

A closed southern stream low will track from Baja California to AZ
today before opening as it interacts with a northern stream trough
currently approaching the Pacific NW. The southern stream trough
quickly becomes negatively tilted over the southern Plains tonight
with rapid surface low development over eastern CO through
Wednesday morning before the occluded/vertically stacked low
ejects northeast across the central Plains to the Great Lakes
through Thursday. Interaction with another northern stream trough
over the Great Lakes shifts the low into CA Thursday night.

The strong synoptic forcing tonight into Wednesday will be aided
by intense mesoscale forcing within a pivoting 700mb deformation
band, and prolonged and highly sloped frontogenesis. Additionally,
strong WAA on a 50 to 70 kt southerly LLJ ahead of the low and
from an open Gulf of Mexico will drive PWATs to +3/+4 standard
deviations above the climo mean, which will then wrap around the
low in a robust TROWAL further intensifying lift. The strong
dynamics in a very moist environment have the potential to produce
extremely heavy snowfall, and the risk exists for thunder-snow
where enough instability collocates with theta-e lapse rates below
0C. The 00Z NAM/GFS/ECMWF and the ensemble means remain in good
agreement with the placement of the low and the heaviest snow band
NW of the track through Wednesday. The heaviest snow is expected
is a swath extending northeast from southeast WY/over the NE
Panhandle to central SD as well as the northeast slope of the
Black Hills, where Day 2 WPC probabilities for 18 inches or more
are above 50 percent. A large area of moderate to high
probabilities surrounds this swath from central CO into ND.

Wednesday night, the occluded/vertically stacked low pushes
northeast across eastern NE. A dry slot looks to circumnavigate
the low by this time with the potential for a compact inner TROWAL
to develop on the immediate north side of the low. Thermally
marginal conditions for snow are expected in this area, but the
intense dynamics could make for a potent/windy swath of snow over
northeast NE/southeast SD even if snowfall itself is not more than
a couple inches.

On Thursday, the low will reach the Great Lakes while weakening.
The primary snow swath weakens as it pushes over northern MN with
the secondary weakening as well over southern MN/northern WI. Day
3 probabilities for four inches are low in the upper Midwest
except for far northern MN where the primary swath moves along
it's axis of orientation.

A stripe of moderate freezing rain is likely southeast of the
heavy snow swath Wednesday as cold air wraps behind the low in
response to a cold front dropping southeast ahead of high pressure
sprawling over the northwest CONUS. As surface temperatures cool
first, robust forcing into the warm nose will produce a period of
freezing rain, and WPC Day 2 probabilities are moderately high for
a tenth inch northeast from northern NE to the common border of
SD/ND/MN. Heavy rain rates, and a quick transition to snow will
likely preclude accretions from reaching 0.25 inches, though Day 2
probabilities are moderate near the NE/SD border in this swath.


...Four Corners...

Days 1-3...
A deep closed low moves from Baja California to AZ today before a
northern stream trough coming from the Pacific NW interacts with
the low across the Great Basin tonight. Influx of tropical Pacific
moisture across the Four Corners states continues today before
getting shunted east tonight. However, forcing from the trough
will allow snow to continue over terrain (snow levels decrease
through Wednesday) into Day 3.

Three day probabilities for an additional 18 inches is high over
the White Mountains of AZ and the San Juans of CO with coverage
for 8 inches high for all high mountain ranges/rims in the four
corners states.


...Pacific Northwest to the Northern Rockies...
Day 1...

A shortwave and leading weak atmospheric river will continue to
shift across the Pacific NW today and the Great Basin tonight.
Snow levels will generally range from 1000 to 2000 ft. This
progressive system has moderate Day 1 probabilities for 8 inches
over the OR Cascades/Blue Mountains/northern ID/western MT Rockies.


Jackson