Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 PM EDT Wed Mar 13 2019

Valid 00Z Thu Mar 14 2019 - 00Z Sun Mar 17 2019

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

...Significant Blizzard continues into Thursday...

Days 1-2...

Deep low pressure will be over Central KS to begin the period and
then will begin to weaken as it lifts northeast into the Great
Lakes Thursday night and its parent upper trough fills. Most of
tonight and Thursday will persist the significant blizzard across
the Central and Northern Plains, while weakening will bring a
reduction in both snowfall amounts and intensity into Day 2.

Initially, forcing at both the synoptic and meso scales is
intense, and a swath of heavy snow and blizzard conditions will
continue from Nebraska into Minnesota. Moist ascent will occur as
a strong southerly LLJ transports anomalously high moisture
northward from the open Gulf of Mexico, and into the cold air
across the Plains. This will then be lifted within the TROWAL
cyclonically into the deformation zone on the NW side of the low,
coincident with a stripe of enhanced mid-level frontogenesis to
produce extremely heavy snowfall, and HREF probabilities are over
90% for 1"/hr tonight before falling quickly into ND on Thursday.
During the overnight hours, the potential exists for thunder snow
as well, especially across SD where intense lift into the deep DGZ
correlates with theta-e lapse rates less than 0C, and this further
supports the intense snow rates. With the relative slow motion of
the surface low, an extended period of these snow rates will
contribute to heavy snow accumulations, and WPC probabilities
suggest a small potential for an additional 12 inches of snow
beginning at 00Z/14 for day 1 from SW SD into extreme NW MN.

Additionally, a model signal continues for a secondary band of
intense snowfall across central/eastern NE Thursday morning in the
immediate vicinity of the low center. A band of precipitation is
expected 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 a
secondary warm conveyor belt wrapping west/south around the
center. Although the temperature profile is marginal, and will be
critical, intense lift and strong dynamic cooling should allow for
rapid changeover of precip type from rain to snow as the band
moves eastward. Antecedent rainfall and warm temperatures should
hinder accumulation, but intense rates would overcome this where
elevated instability has the potential to produce thunder snow,
and probabilities have increased to over 50% for 1"/hr, and WPC
probabilities feature a high risk for 4 inches in a small area of
eastern NE.

Thursday night, the low will reach Lake Superior while
elongating/weakening. The primary snow swath weakens as it pushes
over northern MN with the secondary weakening more slowly over
southern MN/northern WI. Probabilities for four inches are low in
the upper Midwest except for the western UP where the secondary
swath moves along its axis of orientation.

A stripe of moderate freezing rain is likely tonight into Thursday
southeast of the heavy snow swath from NE across SD into western
MN 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,
which may persist longest across eastern SD along the secondary
TROWAL axis. It is in eastern SD where probabilities are highest
for 0.1 inches of accretion.


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

A northern stream trough digging from the Pac NW will rotate
through the region around the base of the significant close low
through Thursday night. Modest height falls and weak jet level
diffluence within the LFQ of an associated jet max will produce
snow in a slowly drying column. Snow levels will fall through
Thursday, from initially 4-5 kft, to 2-3 kft. Snow will be
heaviest during this period along the Sangre De Cristo Mountains
where WPC probabilities are moderate for 8 or more inches during
the 2 day period. Lighter snows are likely in the high terrain of
the Wasatch, mostly on day 1, as the forcing shifts eastward
reducing the duration of snowfall across Utah.


Weiss