Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
439 AM EST Wed Feb 17 2021

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


...Southern Plains to the Northeast...
Days 1-3...
The next round of winter weather will continue to develop across
eastern Texas and the lower Mississippi Valley as shortwave energy
moves through the base of the large-scale trough centered over the
southern High Plains on Wednesday.  Strong ascent along the
right-entrance region of the upper jet and a low-to-mid level
baroclinic zone is expected to support moderate to heavy
precipitation developing across eastern Texas into the lower
Mississippi valley later this morning.  This will likely include
heavy snow across portions of southern Arkansas, with freezing
rain and heavy ice accumulations unfolding farther to the south
from eastern Texas through central and northern Louisiana into
southeastern Arkansas and northern Mississippi.  Latest PWPF shows
50 percent or greater probabilities for snow accumulations of
4-inches or more centered across southern Arkansas.  PWPF also
shows 50 percent or greater probabilities for ice accumulations of
0.25-inch or more extending from the central Piney Woods area of
eastern Texas to northern Mississippi. 

As this energy begins to lift out to the northeast, heavier
precipitation will begin to spread out through the Tennessee and
Ohio valleys later in the day before reaching the central
Appalachians during the overnight hours.  Latest PWPF suggests a
sold stripe of 2-4 inch snowfall accumulations, with locally
heavier totals, extending from northern Mississippi and western
Tennessee through eastern Kentucky.  Probabilities for
accumulations of 4-inches or more are higher across portions of
the Allegheny Mountains from southern West Virginia into western
Virginia, where local orographic effects in addition to
large-scale forcing are more likely to support heavier snow
amounts.  Meanwhile, moisture and warm aloft moving across the top
of arctic high pressure will set the stage for a prolonged
freezing rain event, with heavy ice accumulations expected. 
Freezing rain developing Wednesday night may continue through
Thursday into Friday.  Latest PWPF shows 50 percent probabilities
or greater for ice accumulations of 0.50-inch or more extending
from the northern North Carolina Piedmont and southwestern
Virginia into central Virginia.  Farther to the north, a period of
snow likely changing to a wintry mix is expected along the I-95
corridor from DC through Philadelphia, while snow is expected to
remain the predominant precipitation type for areas farther north
and west on Thursday.

A coastal low is forecast to develop and track northeast off of
the Mid Atlantic coast on Thursday, staying well southeast of Long
Island and southern New England on Friday.  Overall trend of the
guidance has shifted farther southeast with the track of the low. 
This has also brought the axis of heavier snow accumulations
farther to the southeast across the northern Mid Atlantic and
Northeast.  While the threat for heavy snow appears to be
decreasing across western to central Pennsylvania, Upstate New
York and northern to central New England, the shift in track is
supporting an increase in probabilities for accumulations of
4-inches of snow or more along the I-95 corridor from the New York
Metro through southern New England.  Even as the low continues to
track northeast well off the coast, light snows are expected to
continue across portions the Northeast as a northern stream wave
moves from the Great Lakes into the Northeast on Saturday.

...Pacific Northwest to the Northern Rockies...
Days 2-3...
A strong ridge is expected to bring a period of dry weather as it
moves east across the region Wednesday into early Thursday. 
However, unsettled weather is forecast to return during the day on
Thursday as mid-energy/upper jet flattens the ridge.  A weakening
frontal boundary, followed by deep westerly flow will support
another round of locally heavy snows over the Olympics and
Cascades, with light accumulations spreading east across the
Intermountain West into the Rockies.  Periods of mountain snow
will likely continue through Friday.  Then on Saturday, a
well-defined shortwave moving inland is expected to extend the
threat for snow farther to the south into California and the Great
Basin.

Pereira