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

Valid 12Z Wed Mar 13 2019 - 12Z Sat Mar 16 2019

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

...Significant Blizzard into Thursday...

Days 1-2...
Partial phasing of a northern stream trough over the Great Basin
with the southern stream moving north from NM overnight will allow
the low to re-close this morning as rapid lee cyclogenesis
persists over CO. The low will quickly become vertically stacked
today as it occludes over KS reaching maximum intensity this
afternoon before it ejects northeast over eastern NE tonight and
IA on Thursday before reopening as another northern stream trough
interacts with the system across the northern Great Lakes Thursday
night.

Forcing at both the synoptic and meso scales is intense, and a
swath of heavy snow and blizzard conditions are expected to
develop in an expansive TROWAL extending northeast from northern
CO across southeast WY/the NE Panhandle and across SD. 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. Strong WAA on a 50 to 70 kt
southerly LLJ ahead of the low and from an open Gulf of Mexico
(with tropical Pacific moisture as well) will drive PWATs to +3/+4
standard deviations above the climo mean, which will then wrap
into the robust TROWAL further intensifying lift. The strong
dynamics will cool the column and result in a quick changeover
over from rain to wintry mix to snow. Rates in this very moist
environment have the potential to produce extremely heavy
snowfall, with HREF probabilities exceeding 50% in many areas
along the TROWAL corridor for 2"/hr. Thunder is already observed
in the leading band of precip over northwest NE and the risk for
thunder-snow persists where enough instability collocates with
theta-e lapse rates below 0C. The 00Z GFS/ECMWF and the ensemble
means remain in excellent agreement with the placement of the low
and the heaviest snow band NW of the track through Wednesday. The
heaviest snow is expected in the Front Range of CO, terrain in
southeast WY and then along a swath extending northeast from the
NE Panhandle to central SD as well as the northeast slope of the
Black Hills, where Day 1 WPC probabilities for 18 inches or more
are above 50 percent.

Despite the short-lived intensification and gradual weakening
thereafter, high probabilities for 6 inches surround the heaviest
snow area from central CO to eastern ND and the Day 1.5
probabilities are 70 to 80 percent for 6 inches over far northwest
MN.

A secondary TROWAL on the fully occluded low develops over central
NE to southeast SD with a dry slot between the primary TROWAL and
the secondary one. A band of snow 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. Heavy snow is
possible despite marginal thermal profiles. After warm
temperatures and rainfall, accumulation should be hindered, but
heavy rates would overcome this where elevated instability has the
potential to produce thunder snow. Day 1.5 WPC guidance remains
moderate over north-central NE.

On Thursday, 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. Day 2.5 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 today 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, and WPC Day 1
probabilities for 0.25 inches are moderate from the NE Sandhills
to the Coteau des Prairies of northeastern SD. Heavy rain rates,
and a quick transition to snow will likely preclude heavier
accretions.

There is an ice threat tonight into Thursday along the secondary
TROWAL axis where the warm trough aloft maintains freezing rain
over eastern SD. The 00Z NAM highlights this threat, though the
NAM is generally too cold with surface temperatures and often
overdoes ice. This threat will need to continue to be monitored
including as it moves into MN Thursday.


...Arizona/New Mexico...
Days 1-2...

The remnants of the northern stream trough persist/are reinforced
AZ/NM through Thursday. Limited moisture drawn north toward the
surface front is mainly wrung out on the Mogollon Rim where snow
elevation is around 5000ft. Day 1 WPC probabilities for six inches
are high on the White Mountains of AZ with moderate probabilities
for four inches along the Mogollon Rim and other ranges of AZ/NM.
High pressure building over the central Rockies tonight through
Thursday directs air from the Great Plains into northeastern NM
which remains under the mid/upper level trough enhancing
orographic lift with moderate Day 2 probabilities for six inches
over the Sangre de Christo mountains.


Jackson