Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
430 AM EST Sun Feb 17 2019

Valid 12Z Sun Feb 17 2019 - 12Z Wed Feb 20 2019


..Northern Plains to the Northeast...

A mid to upper level low closing off over the northern Plains and
upper Midwest Sunday morning is expected to deamplify and quickly
shear east ahead of an amplifying trough over eastern Canada. 
This is expected to support a stripe of light to moderate snows
extending all the way from eastern South Dakota to southern New
England on Day 1.  A wintry mix, resulting in some light
accumulations is forecast to the south over portions of the Ohio
valley, central Appalachians, the interior Mid Atlantic and into
southern New York and New England.  Bolstered by low to mid level
theta-e advection and frontogenesis, the general trend of the
overnight model runs was for slightly heavier totals across the
impacted areas in the Northeast.  WPC Day 1 probabilities now show
a Slight Risk or great for accumulations of 4-inches or more from
lakes Erie and Ontario eastward to eastern Massachusetts.

...Southern California to the Southern Rockies...

On Sunday into early Monday, shortwave energy moving into the base
of a broad upper trough centered over the southwestern U.S. is
expected to support significant snowfall totals from higher
elevations of the mountains of Southern California into the
Southwest and southern Great Basin.  Snows are expected to wane
across California while extending into the southern Rockies as the
upper trough shifts farther to the east on Monday.  Bolstered by
low to mid level frontogenesis, in addition to favorable upper
forcing, some of the heaviest two day totals are forecast to focus
along the Mogollon Rim into the White Mountains of central
Arizona, as well as the San Juan and Sangre de Cristo mountains in
Colorado and New Mexico.  Across these areas, WPC probabilities
for the 48-hr period ending 12Z Tuesday are 50 percent are greater
for accumulations of 8-inches or more.  Additional light
accumulations are expected to continue across portions of the
central and southern Rockies into late Tuesday, diminishing by
early Wednesday as the upper trough lifts northeast into the
Plains.

...Central Plains into the Lower Missouri and Mid Mississippi
Valleys...

Probabilities for significant snows increase across the region
late Tuesday into early Wednesday as the previously noted trough
lifts in the central Plains.  Consensus of the overnight models
showed the better mid to upper level forcing supporting heavier
totals across Iowa where the WPC Day 3 probabilities (ending 12Z
Wednesday) indicate a Moderate Risk for accumulations of 4-inches
or more.

...Southern and Central Appalachians...

As as strong area of high pressure shifts east into the Northeast,
models show a well-defined CAD signature developing over the East
by Tuesday.  Strong low to mid level theta-e advection ahead of
the trough moving into the central U.S. will support precipitation
spreading north over this shallow dome of cold air, producing
mixed precipitation with the potential for significant ice
accumulations across the region by early Wednesday.   WPC Day 3
probabilities (ending 12z Wednesday) indicate a Moderate Risk for
ice accumulations of 0.25 inch or more extending from western
North Carolina into southwestern Virginia.

Pereira