Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
422 PM EST Mon Feb 15 2021

Valid 00Z Tue Feb 16 2021 - 00Z Fri Feb 19 2021


...Great Lakes/OH Valley/northern Mid Atlantic to the Northeast...
Day 1...

Heavy snow developing across the Midwest is expected to continue
across the Great lakes/northern New York/northern New England
tonight with the synoptic system crossing into Canada tomorrow, so
the system tomorrow transitions to a period of lake effect snow
continuing near the south end of lake Michigan into northern IN,
and off Lake Erie and Ontario into lee shore areas, continuing up
into the western Adirondacks. 

South of the snow region, a transition to sleet and freezing rain
is forecast across the upper Ohio Valley and across PA, central to
eastern NY, and southern to central New England. The primary
uncertainty is the duration of each precipitation type, as
significant sleet accumulations are possible in northern
PA/NY/MA/southern VT before the change over to freezing rain.
Ice accumulations generally run a tenth to quarter inch across
western MD across PA to southeast NY and northern CT/interior MA,
with locally a third of an inch in favored cold spots.

...Pacific Northwest to the Lower-Mid Mississippi Valley and lower
Ohio Valley...

Days 1-3...

A winter storm will continue to develop over the Northwest as mid
level confluent flow channels a northwest to southeast jet across
the region. The confluent flow channels moisture streaming onshore
and inland, with ascent focused on low amplitude progressive
shortwaves combined with orographic enhancement.

Heavy snows are likely across the WA/OR Cascades, spreading
southeast across the Blue mountains, and then ranges of central to
southern ID, northwest WY and then the UT Wasatch, finally
reaching into the mountains of western CO.
As the leading shortwave continues to dig southeast, the threat
for heavy snow is expected to reach once again into the higher
elevations of the southern Rockies. 

By late Tuesday, snow is expected to develop once again across
portions of the southern Plains, with some potential for heavy
accumulations developing from the eastern Texas Panhandle across
Oklahoma, far northern TX, continuing east Tue night and Wednesday
across AR, southern MO, and then the lower OH Valley.

South of the snow area, the precip type transitions leads to sleet
across norther Texas to southern AR/northern LA and northern MS.
South of there,  freezing rain with heavy ice accumulations is
expected from eastern Texas through northern LA into adjacent
southern Arkansas and northern Mississippi.

...Mid Atlantic/Central Appalachians Day 3...
As the Upper trough in the central US drifts east, deep southwest
flow aloft develops with warm/moisture advection streaming across
the southern to central Appalachians and mid Atlantic results in
mixed precipitation redeveloping Wed night into Thu.
Cold air damming in place may lead to another freezing rain event,
with significant accumulations possible in southwest VA into
central WV and far western MD.  A low-moderate risk of quarter
inch of icing is indicated in these areas.  Further north, sleet
and snow are expected across the upper Ohio Valley and PA, so
there is potential for several inches of snow before a change over
to sleet.
The GFS and ECMWF are a bit more suppressed, with the axis of snow
further south in MD.

Given the recent pattern has had the southwest Atlantic high
persisting, this has resulted in the mid level warm advection
streaming further north and resulting in the snow axis shifting
north with the current event.  This is expected to repeat for the
Thu event as well (forecast snow axis shifts north out of MD into
PA and NY).

Petersen