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

Valid 00Z Wed Mar 13 2019 - 00Z Sat Mar 16 2019

...Central Rockies...Central Plains...Western Great Lakes...
Days 1-3...

...Significant Blizzard likely tonight into Thursday...

Potent southern stream trough will be over Arizona to start day 1
/tonight/ and move eastward into the TX Panhandle and then
northeast into Nebraska while deepening as it phases with northern
stream energy from the Pacific NW. This deepening will be
accompanied by good jet dynamics, all of which will spawn lee
cyclogenesis followed by rapid intensification across NE on
Wednesday. The low will become vertically stacked as it occludes
over NE, with subsequent weakening and more rapid ejection
northeast into the Great Lakes forecast Thursday night into Friday.

Forcing at both the synoptic and meso scales will be intense, and
a swath of heavy snow and blizzard conditions are likely. The
strong synoptic lift will be driven by height falls and jet level
diffluence, which will be enhanced by mesoscale ascent due to
strongly sloped frontogenesis and a pivoting area of 700mb
deformation NW of the surface low. 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, with HREF probabilities exceeding 90% for 1"/hr. The
risk exists for thunder-snow as well where enough instability
collocates with theta-e lapse rates below 0C. The NAM/GFS/ECMWF
and the ensemble means remain in reasonable 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 WPC probabilities for 18 inches or more are above 50 percent.

Additionally, a subtle NW shift in the guidance has allowed better
moisture return into ND which will collocate with a band of
700-500 fgen and the TROWAL within the DGZ to produce higher
probabilities for 12 inches there. High probabilities for 6 inches
surround this area from central CO to far northern MN.

A significant change to the guidance has occurred in that more
snow is forecast into eastern NE and SE SD as the surface low
pulls away Wednesday night. An intense band of snowfall is
becoming more likely as a stripe of 925-700mb fgen pivots eastward
coincident with a strong 700mb deformation axis, continued PVA
around the stacked low, and subtly enhanced jet-level diffluence
as a jet max rotates cyclonically around the low. This will be
accompanied by secondary WCB wrapping west/south around the
center. This suggests a band of heavy snow is likely to collapse
E/SE despite marginally conducive thermal profiles for snowfall.
After warm temperatures and rainfall, accumulation may be
hindered, but heavy rates should overcome this where elevated
instability has the potential to produce thunder snow. Guidance
has come into better agreement with a band of heavy snow producing
6 or more inches, but this may be overdone due to unfavorable
antecedent conditions, and WPC probabilities are less than 60% for
4 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 have increased for 0.25
inches, and are high for 0.1 inches northeast from northern NE to
the common border of SD/ND/MN. Heavy rain rates, and a quick
transition to snow will likely preclude heavier accretions.


...Four Corners...
Days 1-2...

A deep trough lifting out of Arizona tonight will tilt negatively
and close off as it interacts with northern stream energy from the
Pac NW. An influx of tropical Pacific moisture will spread
eastward on Pacific Jet energy across the Four Corners states.
Steep lapse rates beneath the upper low and increasing diffluence
in the LFQ of the approaching jet beneath the Pac NW impulse will
produce ascent and snowfall across the terrain of the Great Basin
and points SE. Snow levels will be around 3000 feet in Nevada, to
as high as 6000 ft in New Mexico, but snow will be significant
above these levels. WPC probabilities are high for 12 inches in
the San Juans and Sangre De Cristos, as well as the White
Mountains of Arizona and the Rockies in Colorado. Lesser, but
still significant snows, are likely from the Mogollon Rim to the
Wasatch and Sacramentos. By day 2 the forcing pulls away to the
northeast, and snowfall will wind down from SW to NE.


Weiss