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fxus61 kpbz 271852 
afdpbz

Area forecast discussion
National Weather Service Pittsburgh PA
252 PM EDT Thu Jul 27 2017

Synopsis...
showers and thunderstorms return to the forecast today, with
rain chances continuing into Saturday. Heavy rainfall is
possible, especially southeast of Pittsburgh. Dry and
seasonable weather develops by Sunday.

&&

Near term /through tonight/...
a low-amplitude shortwave trough will move ewd today across the
Ohio Valley. Quality low-level moisture and decent lapse rates
through 500mb are present across the County Warning Area per modification of 12z
raobs.

With ample insolation ahead of ongoing convection, there is
little reason to expect ongoing storms to diminish through the
day ahead of the shortwave trough. Pops have been focused
highest especially across the srn third of the forecast area,
where warm frontogenesis is occuring in association with the
approaching wave, with a sharp precipitation gradient nwd from
the boundary.

It is entirely possible that very little convection occurs in
the far-nrn zones, if the hi-res models are to be believed. But,
given the ambient conditions and potential for heating, the
forecast will carry a slight chance of precipitation in this
area.

Conditions will favor storms capable of moderate to locally-
heavy rain given the considerable instability below the
freezing level of 13.1 kft and its attendant warm-rain
processes. The potential for wet updrafts and modest shear
suggest the possibility of downbursts from any vigorous storms,
but it is unclear if storms will achieve such vigor given the
weaker shear and warm air aloft above 500mb.

As the afternoon shortwave trough exits ewd and diurnal heating
wanes, a relative decrease in precipitation is expected this
evening. But a more-potent upper low will dig sewd overnight from
the Great Lakes region, leading to a sfc cyclone, sharpening
frontal boundary and increase in warm air advection into the
frontal zone. Showers and storms capable of very heavy rain are
expected to begin in earnest late tonight especially to the
south and east of Pittsburgh in the Mon basin.

&&

Short term /Friday through Saturday night/...
the upper low will continue to dig sewd on Fri and strengthen
through this timeframe while embarking on its slow trip across
wrn PA Friday night en Route to the Delmarva. A surface low
will also strengthen off the mid-Atlantic coast in response to
its approach. The departure of the low continues to trend
slower on the models as its strength increases and becomes
better resolved, with impacts continuing to linger into Sat at
least as vorticity maxima pivot around the parent low.

Strong 850-700 mb frontogenesis across southern PA may couple
with moist easterly flow to support heavy rain. Nearly-
saturated low levels and a deep warm-cloud layer also support
the heavy rain potential. Localized rain amounts in excess of
two inches are possible, which could promote flooding. A Flash
Flood Watch has been issued beginning early Fri morning and
continuing into early Sat morning.

High pops continue across the region through Fri evening as the
upper low crosses, with a slow taper toward the southeast
during the night. Little to no severe risk exists during this
period. The main concern likely will be heavy to excessive
rainfall, especially south and east of Pittsburgh.

In response to the slower model trends with the system's
departure, the decrease in pops was slowed through Sat evening,
with all precipitation exiting the area late Sat night.

&&

Long term /Sunday through Thursday/...
a strong upper ridge over the wrn Continental U.S. Will dominate the
weather pattern for the remainder of the forecast, spelling some
semblance of mean troughing along the East Coast.

Great Lakes high pressure should provide mostly-dry but cooler
weather early next week. Heights begin to rise thereafter,
which will spell a warm-up toward climatological averages.

A frontal boundary will sag into the region mid-week, although
there remains uncertainty in the timing of its arrival and exact
placement. Given the presence of this feature and a moistening
boundary layer, low pops were included in the forecast to keep
the forecast during these periods closer to climatological
average.

&&

Aviation /19z Thursday through Tuesday/...
mainly VFR conditions are expected through the evening. Shower
and thunderstorm chances are expected to increase this afternoon,
mainly S of I-70, with the approach and passage of a shortwave
and associated sfc boundary. Maintained thunderstorms in the vicinity mention for ports S
of pit, with some tempo restrictions at zzv and mgw where tstm
occurrence is more probable.

Precip should diminish this evening with exiting shortwave
support, though increasing low-level moisture near the stalled
sfc boundary should result in MVFR cigs for Ohio ports overnight,
spreading to most other ports by sunrise.

Conditions should remain MVFR with local IFR S of I-80 in
showers through Fri as an upper low approaches and a surface
wave along the boundary strengthens.

Outlook...
restrictions are likely through Sat with slow-moving low
pressure.

&&

Pbz watches/warnings/advisories...
Maryland...Flash Flood Watch from Friday morning through late Friday
night for mdz001.
Ohio...none.
PA...Flash Flood Watch from Friday morning through late Friday
night for paz014>016-020>023-029-031-073>076.
WV...Flash Flood Watch from Friday morning through late Friday
night for wvz001>004-012-021-509>514.

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