Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
503 PM EST Sat Feb 13 2021

Valid 00Z Sun Feb 14 2021 - 00Z Wed Feb 17 2021

...Mid Atlantic to Long Island NY and Coastal Southern New
England...
Day 1...
A wave of low pressure at the sfc is forecast to move east
northeast south of Long Island NY and southern New England.
Surface cold air that is below freezing is in place for the
northern mid Atlantic to southern New England. The layer near 850
mb rises above freezing, so a mixture of sleet and freezing rain
moves north across the northern Mid Atlantic to Long Island NY and
coastal southern New England.  The precip tapers Sunday as the
wave departs and drier air aloft advects across the region.  The
models are in good agreement on the storm track/intensity, so the
models were equally weighted in the probability forecasts.

...Pacific Northwest...
Day 1...
The models indicate confluent flow aloft results in a strong west
northwest to east southeast jet stream across the Pacific
Northwest.
A few surges of 700 mb ascent cross the OR Cascades, weakening as
you go north across the WA Cascades.
Heavy accumulations are likely for portions of the OR Cascades,
with additional accumulations of 1-2 feet expected across the
higher elevations of the Oregon Cascades.

...UT/CO/AZ/NM...
Day 1...
The upper trough this evening across UT and AZ drifts east with
time across the ranges of CO/NM.  Upper divergence maxima along
the path of the upper trough helps focus snow across the San Juan
Mountains of northern NM/southern CO, and Sacramento Mountains of
NM.
Secondary upper divergence maxima in northwest flow behind the
trough focus continuing ascent in southeast CO to northwest NM on
Sun.
Locally heavy snow are expected with a foot likely in the San Juan
Mountains and portions of the central UT Wasatch.

...Central and Southern Plains to the Lower Mississippi Valley/TN
Valley...
Days 1-2...

...A major freezing rain event is forecast from southeast Texas
across central Louisiana, and western to northern Mississippi...

Supported by low level easterly flow and an upper jet couplet,
snow will begin to spread east of the central Rockies into the
High Plains tonight. As the upper trough begins to move east
across the southern Rockies, favorable upper jet forcing, along
with a low level baroclinic zone will support heavy snows
developing across the TX/OK panhandles to central Oklahoma.
Northern Texas will be in a precip type transition zone, with a
period of sleet before changing to snow.

As the upper trough continues farther east, drawing colder air
across the southern Plains, a wintry mix, including accumulating
snow is forecast to extend as far south as South Texas Sunday
night.  Sleet and freezing rain is likely to develop along the
central to eastern Texas Gulf coast and across eastern Texas into
the lower Mississippi valley Monday morning, with significant ice
accumulations possible across eastern Texas into northern
Louisiana and northern Mississippi, northern Alabama, continuing
to central Tennessee, eastern Kentucky, and southwest West
Virginia. 

A stripe of 0.25 inches of freezing rain is possible across
central LA to western/northern MS. A tenth of an inch is possible
down to the Gulf coast of southwest LA and eastern Texas. The
probability of icing greater than 0.10 inches if 50-75 percent
across central TN to eastern KY.

The event winds down on Day 3 as the cyclone moves steadily
northeast up into the northeastern part of the country.

...Ohio Valley/Great Lakes to the Northeast...
Day 2/3...

A widespread mixed winter precipitation event is expected on Day 2
in the upper Ohio Valley and central Appalachians, and continuing
across the lower Great lakes, New York, and New England Day 3.
Phasing northern and southern streams shortwaves leas to an
expansion of the precipitation shield across the Ohio Valley
Monday, with snow in the northern Ohio Valley to the lower Great
Lakes.  Further south, the precip changes to sleet and then
freezing rain across central to eastern KY and southern WV.

On Tue, the low may reform off the northern Mid Atlantic to
southern New England region. 
Those models that advect warmer air 850-700 mb north across Y/New
England create a change over from snow to sleet and freezing rain.
The further north/west you go in PA/NY/New England, the greater
the chances of snow are. There is potential for up to a foot of
snow in portions of northwest PA to western and northern NY and
VT. Enhancement near the southern end of Lake Michigan leads to
several inches of snow in northern IN, and enhancement off Lake
Superior likewise leads to several inches of snow in the eastern
UP of Michigan.

Further south from New York City to coastal southern New England,
the greater the chances are of a change over to rain.
In between is a likely areas of multiple precip types with
uncertainty on the duration of each.
The highest probability of significant icing is from southern WV
across south central PA/northern NJ, southeast NY, and southern
New England.


Petersen