Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Tue Mar 19 2019

Valid 12Z Tue Mar 19 2019 - 12Z Fri Mar 22 2019


...Western U.S....

A well-defined shortwave trough will continue to slide southeast
from the northern into the central Plains on Tuesday. 
Northeasterly flow behind the associated front pushing south
through the Rockies and High Plains will support accumulating
snows along the Colorado Front Range into the Sangre de Cristo
Mountains, with WPC probabilities indicating a Moderate to High
Risk for amounts of 4-inches or more across portions of the Sangre
de Cristos Mountains during the Day 1 period (ending 12Z
Wednesday).

Farther west, models continue to show a negatively-tilted trough
swinging east into California, with accumulating snows developing
along the Sierra, with WPC Day 1 probabilities indicating a Slight
Risk for 4-inches or more for the western slopes above 6000 ft,
with Moderate Risk for areas above 8000 ft.

As energy continues to move into the base of the trough, models
show a closed low developing over California and the lower
Colorado basin late Wednesday-early Thursday.  Snows are expected
to continue across the Sierra while extending farther east into
the central Nevada and southern Utah ranges, with WPC Day 2
probabilities (ending 12Z Thursday) indicating a Moderate Risk for
accumulations of 4-inches or more for portions of these areas. 
The overnight models have trended slower with this system and as a
result probabilities are lower for areas farther east, with the
latest WPC probabilities now indicating only a Slight Risk for
accumulations of 4-inches or more across the San Juan Mountains in
southwest Colorado and northwest New Mexico on Day 2.

Probabilities do increase across this area by Day 3 as the upper
low does move farther east across the Great Basin on Thursday. 
Left-exit region upper jet forcing is expected to support
accumulating snows across the southern Utah ranges into the
Wasatch and farther east into the Colorado and northern New Mexico
ranges.  WPC probabilities for the Day 3 period (ending 12Z
Friday) show a Moderate Risk for 4-inches or more across much of
this area, with a Moderate Risk for 8-inches or more across the
San Juans in southwest Colorado.

...Northeast...

The previously noted shortwave dropping through the Plains on
Tuesday is forecast to swing southeast into the southeastern U.S.
early Thursday before lifting to the north and phasing with a
trough digging across the Great Lakes into the Ohio valley. 
Overnight consensus has shown a significant shift west with the
associated surface low developing and lifting north off of the Mid
Atlantic to Northeast coasts on Thursday into early Friday.  This
has raised the potential for heavier precipitation, including
accumulating snows for portions of the interior northern Mid
Atlantic and the Northeast.  However, the intensity and track of
the low remains a source of uncertainty, with the overnight models
showing a good deal of spread with respect to both.  Therefore
confidence is limited at best with respect to snow accumulations. 
The latest WPC probabilities show the best chance for significant
snows centering over northern New England, with a Moderate Risk
for accumulations of the 4-inches or more over northern Maine. 
However, given the uncertainty and poor run-to-run model
continuity over the past 24-hrs, these amounts may change
significantly in subsequent runs.

Pereira