Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
533 AM EDT Wed Mar 17 2021

Valid 12Z Wed Mar 17 2021 - 12Z Sat Mar 20 2021


...Southern High Plains to the Lower Missouri Valley...
Day 1...
Rain changing to snow is expected across portions of southwestern
Kansas and the Oklahoma and Texas panhandles as a powerful upper
low moves from the Texas Panhandle into central Oklahoma later
this morning.  While widespread heavy snow accumulations are not
expected, probabilities for accumulations of 4-inches or more have
increased from the northeastern Texas Panhandle into southeastern
Kansas.

Probabilities for at least light accumulations have increased
farther east across portions of eastern Kansas into western
Missouri Wednesday night.  As the low continues to move east,
strong dynamics within the associated comma-head are expected to
support a changeover from rain to a wet snow Wednesday evening and
overnight across eastern Kansas and into western Missouri. 
Believe heavier precipitation rates should be sufficient to
overcome what will likely be marginal boundary layer temperatures
to produce an inch or two of slushy snow across some locations. 
While there is some concern for potentially heavier amounts,
confidence in the extent and magnitude of the heavier amounts
shown by the NAM and some of the hi-res guidance members was
limited.

...Northeast...
Day 2...
Models show the previously described low beginning to shear as it
moves east of the central Appalachians and begins to interact with
a  northern stream trough amplifying over the Northeast Thursday
night.  Moisture interacting with the northern stream boundary
will support precipitation spreading across the northern Mid
Atlantic and portions of the Northeast on Thursday into Friday. 
Rain changing to snow appears likely beginning Thursday afternoon
along the western New York-Pennsylvania border and then shifting
east through the evening and overnight into central New England
and then along the Maine coast.  While an inch or two of snow
appears likely, especially across the higher terrain, there is
some concern for locally heavier amounts here as well, as strong
low-to-mid level frontogenesis may help bolster precipitation
rates and the changeover -- resulting in brief period of moderate
to heavy snow for some locations.

...Western U.S....
Days 2-3...
A frontal band associated with a closed low/upper trough
approaching the Pacific Northwest will bring the next round of
mountain to snow into the western U.S. beginning late Thursday and
continuing into the weekend.  Areas with the greatest potential
for heavy snow accumulations include the Olympics, Cascades,
Klamath mountains and the Sierra.

For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Pereira