Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
432 AM EST Wed Nov 27 2019

Valid 12Z Wed Nov 27 2019 - 12Z Sat Nov 30 2019


Day 1...

...Western U.S....
A powerful low moving into Oregon this morning is expected to
gradually weaken as it moves further inland on Wednesday. 
However, shortwave energy digging south of the low will continue
to carve out an anomalously deep upper trough along the West
coast, with 500mb heights lowering to 3-4 standard deviations
below normal across much of California on Wednesday.  Strong upper
forcing and onshore flow ahead of a shortwave moving though the
base of the trough will likely support heavy snowfall
accumulations along the central into the southern Sierra, where
widespread accumulations of 1-2 feet can be expected.  Heavy
snowfall amounts are also expected  along the coastal ranges of
Southern California.  Additionally, significant accumulations are
also expected farther east across the southern Nevada and
southwestern Utah mountains into the Wasatch and Uintas. Meanwhile
deep southwesterly flow ahead of the low will direct moisture and
energy across southern Idaho and western Montana - supporting
heavy mountain snows.

...Southern Rockies/High Plains...
Models show a subtropical shortwave and stripe of deep moisture
lifting from west of Baja northeastward ahead of the amplifying
trough to the west.  Thermal profiles suggest precipitation will
begin as snow over the mountains of central New Mexico, with a
wintry mix farther to the east across the plains into the Texas
Panhandle.  Some areas in the mountains may see locally heavy snow
accumulations, while lighter snow and ice accumulations are
expected farther east.

...Upper Midwest...
A negatively-tilted upper low is forecast to deepen further as it
tracks from the Mississippi valley into the upper Great Lakes
Wednesday morning.  Low-to-mid level frontogenesis will continue
to support bands of moderate to locally heavy snow, with heavy
accumulations over northern Wisconsin and the U.P. of Michigan
Wednesday morning.  As the low moves into southeastern Ontario,
strong northerly winds of the backside of the system will
encourage lake effect snow showers - further bolstering snow
totals over the U.P.  WPC probabilities indicate that
accumulations of 8-inches, with locally heavier totals are likely
along the shore of Lake Superior.

...Northeast...
As the previously noted upper low moves into Canada, models show
the surface low over southeastern Ontario quickly weakening as a
new low forms over the Gulf of Maine.  Mixed precipitation
developing north of the low will result in minor ice
accumulations, with some light to moderate snowfall accumulations
over northern New England.

Day 2...

...Southwestern U.S. to the Central Rockies...
Shortwave energy moving through the base of the upper trough will
bring an additional round of heavy precipitation to Southern
California and the Southwest. More heavy snows are likely for the
coastal ranges on Thursday, before spreading east into the
southern Nevada and Utah mountains overnight.  As the upper trough
shifts east Thursday night, rain changing to snow is expected
along the Mogollon Rim in Arizona, with some locally heavy
accumulations possible by Friday morning.  Meanwhile a prolonged
period of snow is expected to result in heavy accumulations across
the San Juan mountains in southwestern Colorado.

Day 3...

...Southwestern U.S. to the Central Rockies..
Snow is expected to continue through early Friday, with additional
heavy accumulations possible along the Mogollon Rim in Arizona as
well as along the San Juans in Colorado.  Then as the upper trough
begins to lift out of the northeast, snows are expected to
diminish from southwest to northeast across the region late
Friday.

...Northern Rockies and High Plains...
After moving across the Southwest, models show the previously
noted shortwave lifting across the central Rockies Friday evening
before amplifying and a deepening closed low develops over the
northern Plains Saturday morning.  A rapidly intensifying surface
low is forecast to track north from eastern Colorado into western
South Dakota Friday night.  This will spread heavy snow across the
northern High Plains into the Rockies, with strong low-to-mid
level frontogenesis expected to help enhance totals across
portions of western South Dakota, eastern Wyoming and eastern to
central Montana. 

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

Pereira