Area Forecast Discussion
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FXUS61 KPBZ 130936

Area Forecast Discussion
National Weather Service Pittsburgh PA
436 AM EST Wed Dec 13 2017

Bitter cold wind chills this morning. A widespread snow event
is expected to bring accumulating snow to the region through
tonight. Below normal temperatures will continue until we return
to near normal for the weekend.


Remnant lake effect snow showers and flurries continue to
be relegated farther north this morning as flow turns more
westerly in the lower levels. Even though moisture fetches are
getting progressively worse for the majority of the area, and
dry air is incoming, stability profiles are still favorable
through the dendritic growth zone. This combined with the
dendritic growth zone being so close to the surface is resulting
in a few flurries continuing. As flow turns continues to trend
southwesterly into the morning hours, these showers should
diminish and be relegated farther northeast until they are out
of the area.

All eyes then turn toward the northwest as a fast-moving clipper
system approaches the area. Warm advection ahead of it should
start to develop snow in isentropic ascent. However, dry air
will be encroaching rapidly from the southwest in a large scale
dry slot associated with the system. What is interesting is that
it remains saturated below 700 mb in this dry slot, however the
warm advection into the dry slot brings the temperatures at 700
mb above -12C, thus precluding efficient crystal development.
Thus, it seems as the system dives southeast through the area,
there will be a fairly stark diving line between accumulating
snow and essentially dry conditions.

With deeper moisture farther north and better deformation lift
to the north of the surface low track, and a more optimal
overlay of the dendritic growth layer with the moisture and lift
profile, a very efficient snow seems to be in the cards. Models
have really trended quite closely together with regard to the
surface track of the system from around Fort Wayne, IN, at 7 PM
to just west of Pittsburgh at 1 AM tonight with reorganization
offshore quickly thereafter. That said, optimal forcing for
ascent being north of this track and consensus favoring roughly
0.2-0.25 inches of QPF with roughly 20:1 ratios means a swath of
4-5 inches of snow centered on the U.S. 422 corridor. Amounts to
the south of there will taper quickly, while to the north, they
will taper far more slowly. That said, winter weather advisories
were issued from Columbiana County eastward to Indiana County to
focus the attention on the heaviest snowfall. This does not mean
it will not snow in metropolitan Pittsburgh, however. But a less
efficient profile, substantial dry air aloft, and less
consistent forcing will make widespread 3 inches amounts tough
to muster.

As the surface low tracks eastward, moisture and backside
deformation wrap in a period of snow showers for all locations
by tonight. This may be the only measurable snowfall that the
southernmost locations in the CWA see during the event. That
said, large scale deformation combined with a period of upslope
into the ridges could accumulate a quick 3 inches there, so
winter weather advisories have been hoisted for the terrain.


With the formation of the coastal system, isallobaric flow
across the terrain again increases through Thursday morning.
This will cause wind gusts to extend from Wednesday night into
Thursday for the ridges. However, because of the location of the
formation of the coastal low, geostrophic flow will not favor
much more than a short window of lake effect snow showers in the
wake of this system. As a result, things will trend dry pretty
quickly on Thursday in continental cold advection.

Dry weather continues until at least Friday when a weak system
moves through the central Great Lakes. Warm advection ahead of
it is rather weak, and forcing is rather weak as well. Thus,
just a few snow showers are expected. Behind that system, dry
weather returns on Saturday in nearly neutral thermal advection.


A general retrogression of the upper trough toward the high
plains seems in store for next weekend. This will result in more
normal temperatures returning to the region, but with the
threat of a southern jet branch storm that should bring
primarily rain across the area on Sunday.


With restrictions overnight and this morning being confined to
lingering snow bands, have decided on a general VFR forecast.
Included the mention of restrictions at FKL and DUJ, as these
ports will have the best chance to see those overnight bands.
Also mentioned restrictions at LBE as the flow overnight will
remain advantageous for additional snow showers/lower clouds
over and along the ridges.

Winds will pickup again by late morning, and swing toward the
south. Gusts to 20kts are possible.

MVFR cigs will begin to overspread the area late in the day and
into the evening hours, as the next system rushes toward the
area. Snow showers will move in as well, and provide
restrictions to visibility, but these could be confined to
ports from PIT northward.

Next chance for restrictions is Friday into Saturday.


MD...Winter Weather Advisory from 7 PM this evening to 10 AM EST
     Thursday for MDZ001.
     Wind Chill Advisory until 1 PM EST this afternoon for MDZ001.
OH...Winter Weather Advisory from 4 PM this afternoon to 7 AM EST
     Thursday for OHZ041.
PA...Winter Weather Advisory from 4 PM this afternoon to 7 AM EST
     Thursday for PAZ007>009-013>016-020-022-023-074.
     Winter Weather Advisory from 7 PM this evening to 10 AM EST
     Thursday for PAZ076.
WV...Winter Weather Advisory from 7 PM this evening to 10 AM EST
     Thursday for WVZ512-514.
     Wind Chill Advisory until 1 PM EST this afternoon for


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