Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
451 AM EST Tue Feb 19 2019

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


...Southern Plains and Ozark Region to the Northern Great Lakes...

A sharp upper trough is forecast to lift northeast out of the Four
Corners region on Tuesday.  Strong low level theta-e advection
into a region of enhanced upper divergence and strengthening low
to mid level frontogenesis is expected to support a blossoming
area of precipitation spreading north from the southern Plains and
lower Mississippi valley into the central Plains and mid
Mississippi valley on Tuesday.  Evaporative cooling into a dry
surface air mass is forecast to support a period of wintry
precipitation across areas as far south as north-central Texas. 
While widespread heavy ice accumulations are not expected, WPC Day
1 probabilities (ending 12Z Wed) show a Slight Risk for localized
amounts of 0.25 inch or more across portions of central Oklahoma
to northern Arkansas and southern Missouri, including the Ozark
region. 

Farther to the north and west, widespread snows are forecast to
develop and spread north across the region by late Tuesday. 
Models show the upper trough lifting steadily from the southern
High Plains into the central Plains Tuesday night.  While the
progressive nature of the system is expected to keep amounts in
check, strong mid to upper level forcing is forecast to support
widespread snows from Oklahoma north and east into the central
Plains and the lower Missouri and mid Mississippi valleys. 
Heaviest amounts ending Wednesday morning are expected to center
from eastern Nebraska to western Iowa, where increasing
deformation aloft is  expected to support some heavier totals. 
WPC Day 1 probabilities show a High Risk for accumulations of
4-inches or more and a Slight Risk for amounts of 8-inches or
greater across this area. 

As the system continues to advance steadily to the northeast,
snows will move across the upper Mississippi valley and northern
Great Lakes.  With the trough lifting quickly across the region,
WPC Day 2 probabilities (ending 12Z Thursday) indicate a High Risk
for accumulations of 4-inches or more from the Minnesota-Wisconsin
border to the U.P. of Michigan, however the probabilities for
amounts exceeding 8-inches drop off close to zero across much of
the region.

...Ohio valley Southern and Central Appalachians to the Mid
Atlantic...

Models continue to present a strong CAD signature developing
across the region as confluent flow aloft and strong high pressure
advance east from the Midwest into the Northeast and Mid Atlantic
on Tuesday.  There remains a significant threat for heavy icing
across portions of the southern and central Appalachians into the
adjacent Piedmont region as strong low theta-e advection and upper
divergence support precipitation spreading north over the top of
this low level cold air late Tuesday into Wednesday.  WPC Day 1
probabilities continue to show a Moderate Risk for ice
accumulations of 0.25-inch or more across western North Carolina. 
Within the deeper cold air, low to mid level frontogenesis and
strong upper divergence lifting north are expected to support a
period of moderate to heavy snow across portions of the Ohio
valley, central Appalachians and northern Mid Atlantic Wednesday
morning.  As suggested by the the WPC Day 2 probabilities, areas
from eastern West Virginia to central Maryland and south-central
Pennsylvania may see snow amounts of 8-inches or more before
changing over to a wintry mix on Wednesday.  Sleet and freezing
rain are expected to become the dominate precipitation types
across this area by late in the day, with significant icing
possible from eastern West Virginia and western Virginia northwest
into south-central Pennsylvania, where WPC Day 2 probabilities
indicate a Moderate Risk for ice accumulations of 0.25-inch or
more. 

As the high slides off to the east and warm air surges north, the
potential for significant snow/ice is expected to diminish
significantly for most areas farther to the north.  As the upper
trough advances east from the northern Great Lakes trough the St
Lawrence valley, portions of northern Maine may see some
significant snow accumulations on Thursday.

...Pacific Northwest to the Great Basin...

A well-defined shortwave diving south into the Pacific Northwest
Tuesday night is expected to support widespread mountain snows
spreading south and east from the Cascades into the Sierra and
Great Basin Tuesday night and Wednesday, with the probabilities
for heavy totals highest along the Cascades.

...Southwest to the Southern Rockies...

Models are showing a strong signal for a significant winter storm
with heavy snow accumulations for portions of the Southwest. 
Beginning early Thursday and continuing into early Friday, deep
southwesterly flow ahead of an upper low dropping south across
California is expected to support heavy precipitation into the
upslope regions of Arizona.  With upper level heights dropping to
2-3 standard deviations below normal, heavy snows will become
likely, particularly along the Mogollon Rim, where snow
accumulations of a foot or more can be expected by Friday morning.
 Lighter accumulations are forecast for the ranges of Southern
California into northern Baja, as well as for the mountains of
southern Utah.  Some heavier totals are expected for areas in
southwest Colorado and northwest Mexico, particularly the San Juan
mountains, where WPC Day 3 probabilities show a Moderate Risk for
accumulations of a foot or more.

Pereira