Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 PM EDT Mon Mar 11 2019

Valid 00Z Tue Mar 12 2019 - 00Z Fri Mar 15 2019

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

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

A closed low near Baja California will open and eject eastward to
the TX Panhandle Wednesday morning. This feature will then phase
with northern stream energy digging from the Pacific Northwest to
re-amplify the upper trough, close it off, and tilt it negatively.
As this occurs, a 120+ kt jet will rotate below the trough,
enhancing mid and upper level diffluence to increase synoptic
ascent across the Plains. Beneath this robust ascent, surface low
pressure will develop in the Lee of the Rockies, and then move
slowly while rapidly intensifying on Wednesday. On Thursday, the
mid-level trough will begin to fill and eventually open into a
wave, allowing the surface low to eject northeast towards the
Great Lakes by Friday morning.

The robust synoptic ascent 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+
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
NAM/GFS/ECMWF and the ensemble means are in relatively good
agreement with the placement of the low and the heaviest snow band
NW of the track, and SREF probabilities for 6"/6 hours are high
across SE WY, western NE, and SW SD where 1-2"/hr snowfall rates
are possible. The heaviest snow is currently expected from the
front range of CO, northeast through WY/NE and SD, where WPC
probabilities are greater than 70 percent for 12 inches Wednesday
into Thursday, which matches well with the highest probabilities
for heavy snow from the most recent CIPS analogs. A much larger
area of 6" is probable north, west, and east of the heaviest snow
region.

On Thursday, the low will move begin to move towards the Great
Lakes while weakening. This will spread continued snow
northeastward into MN, with wraparound snow possible into eastern
NE, but amounts are expected to be much lighter and WPC
probabilities are high for 6 inches across SD and eastern ND as
the swath of heavy snow continues northeast, with low
accumulations possible into eastern NE and MN.

To the southwest of the heavy snow, there is likely to be a stripe
of moderate freezing rain as cold air wraps behind the low in
response to a cold front dropping southeast ahead of a high
pressure. As surface temperatures cool first, robust forcing into
the warm nose will produce a period of freezing rain, and WPC
probabilities show a high risk for 0.1 inches from northern NE
northeast into southern MN. Heavy rain rates, and a quick
transition to snow will likely preclude accretions from reaching
0.25 inches.



...Four Corners...
Days 1-3...

Deep closed low near Baja California will move slowly eastward
across southern Arizona before lifting northeast Wednesday. As
this occurs, a secondary impulse will drop southward from the NW
to phase with the parent trough, causing rapid re-amplification as
the system pulls off into the Plains. A surface low is expected to
develop beneath this feature in the Lee of the Rockies in eastern
Colorado, before strengthening and lifting northeast through the
remainder of the forecast period.

Two rounds of heavy snow are likely across the terrain from the
Mogollon Rim into New Mexico, and northward through the Wasatch,
San Juans, Sangre De Cristos, and Colorado Rockies. The first is
expected Day 1 as Pacific Jet energy and abundant moisture ride
into the region ahead of the closed low. This will initially be a
very warm air mass with snow levels as high as 8000 ft ahead of
the upper feature. Anomalously high PWATs and ascent due to jet
level diffluence, height falls, and robust orographic enhancement
on the strong 700mb southerly winds. Day 1 probabilities are
moderate for 12 inches in the White Mountains of Arizona, the San
Juans and Sangre De Cristos, with heavy snow also likely in the
Sierra Madre of northern Mexico. 4 inch WPC probabilities are high
across the remainder of the terrain, but only above 6000 feet.

As the closed low begins to shift eastward it phases with the
secondary low, enhancing snowfall potential across these same
areas. Intense synoptic ascent, even stronger moist advection, and
rapidly lowering snow levels has the potential to produce heavy
snow as low as 2000 ft in UT/CO, and down to 6000 ft in northern
Mexico. WPC probabilities on Day 2 are high for 12 inches in the
San Juans and much of the high terrain of Colorado, with moderate
probabilities in the White Mountains, Wasatch, and high terrain of
New Mexico. Probabilities for 6 inches are high nearly everywhere
above 4000 ft.

By day 3 the system will pull away and dry continental air will
flood in from the northwest. This should bring a rapid end to the
snow from west to east, with just light accumulations expected the
first half of Thursday.


...Pacific Northwest to the Northern Rockies...
Days 1-2...

A shortwave and accompanying 120kt Pacific Jet will dive into the
Pacific Northwest tonight and early on Tuesday. As this feature
shifts southeast towards the Great Basin, a weak atmospheric river
will shift onshore on prolonged westerly 850-700mb flow across the
Pacific. Snow levels will range from as low as 1000 ft in the
Washington Cascades, to 3000 feet in the Northern Rockies to the
mountains of Northern California, so the big cities of Seattle and
Portland are not expected to receive snowfall with this system.
However, significant moist advection and upslope enhancement will
produce heavy snow in the Cascades of Washington, where WPC
probabilities are high for 12 inches of snow or more. Heavy snow
will spill east and south from there as well, with high
probabilities for 6 inches existing across much of the terrain
above the aforementioned snow levels. As the trough and its
associated jet shift further southeast into Day 2, snowfall will
wane from NW to SE, with modest probabilities for 4 inches in the
Northern Rockies Tuesday night into Wednesday.


Weiss