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004
FXUS61 KPBZ 212336
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
736 PM EDT Fri Jul 21 2017

.SYNOPSIS...
Yet another muggy night tonight keeping those air conditioners
running.  Saturday could be active with more than one round of
storms moving through the region.  Sunday, pop up storms forecast,
along with very warm and humid weather.  Cooler and less humid early
next week.

&&

.NEAR TERM /THROUGH SATURDAY/...
Some lower dewpoints have managed to sneak into the northern CWA
this evening, dropping dewpoints values there into the 60s,
which is actually enough to minimized instability. Farther
south, while dewpoints are higher, earlier convection and
convective debris have managed to suppress temperatures enough
such that instability is fairly measure. In between however,
there exists a chasm between two easily-identifiable boundaries
on radar imagery where low level convergence is maximized, and
thus convective initiation may be fostered. While we are getting
well beyond the convective maximum of the diurnal cycle, ample
low level instability combined with low level convergence may be
enough to spark off a shower or storm down the heart of the CWA
for the next few hours.

With abundant low level moisture in place, dying cumulus, and
only modest incoming upper level convective blow off, fog
development should be fairly efficient overnight. Areas of fog
were added to the grids ahead of the next incoming wave, which
should arrive by morning, at which time fog will likely mix our
rapidly. Fries

All signs are pointing to an active Saturday with more than one
round of severe storms.  MCS development over the northern plains
this afternoon will be moving into western Ohio around daybreak.
There should be enough destabilization through the morning once any
morning fog burns off.  H7-H5 lapse rates approach 6.5ckm-1 while
MUcapes push 2.0kjkg-1.  Wind shear is impressive as well with
nearly 40 knots of shear.  All of this yields to a damaging wind
threat with a line of storms from late morning in eastern Ohio to
early afternoon in western Pennsylvania and northern West Virginia.
Atmosphere keeps its tropical characteristics with PWATS around two
inches and warm cloud depths nearing 13KFT.  This will lead to heavy
downpours with any storm.  the concern for flash flooding is not
high with the initial line of convection, however given there may be
around round of weather /albeit maybe not severe/ in the evening, if
repeated areas get hit then high water is a concern.

Daytime highs are tricky with precipitation forecast to occur during
peak heating.  It would not be surprising to see some places stay in
the lower 80s if convection moves through midday and residual cloud
cover holds until mid and late afternoon.  Overall temperatures were
a blend of the bias corrected all and raw blend.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/...
Line of showers and storms could be ongoing through locations mainly
south of I-70 in the evening.  With dewpoints still around 70F
atmosphere remains uncapped through the overnight hours.  This
combined with another embedded wave traversing in the mean flow and
residual outflow boundaries will warrant the need to keep pops in
the forecast for the overnight.  A severe weather threat remains a
concern with the round of weather coming through in the evening
given decent upper level support and atmosphere favorable for
upstream support.

Sunday confidence is lower in organized convection moving through
the region however we still remain in northwest flow with
disturbances riding through.  BUFR soundings show main threat being
damaging winds given surface delta theta e values nearing -30 around
10-15K feet supporting wet microbursts.  Still hard to pinpoint an
exact time but at this juncture will go with afternoon and evening,
Thermodynamic instability is impressive over far sw PA, northern WV
and eastern OH, however shear is not with 20 kts of mean flow.
Locations north US 422 may be spared Sunday per less instability.
Flash flooding remains a threat, but as stated earlier 20 kts of
mean flow will be keep storms moving, we just have to watch out for
areas that get hit Saturday so training does not become an issue.

Activity wanes Sunday night with the loss of daytime heating, but
again expect a muggy night with temps well into the 60s and lower
70s.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
Highlights:
- Lower humidity
- Active weather regime end of next week returns

Mid level trough sweeps through Monday afternoon so this will be the
last round of showers / storms before a welcome break occurs.  Given
time of occurrence and associated steeper lapse rates with the
cooler air aloft, there is a low risk of severe weather during peak
heating hours.  H5 heights build Tuesday into Wednesday offering a
reprieve from daily chance of storms.  After Tuesday being the
coolest day of the week, temperatures ramp back up towards normal
values in the lower 80s.

A two standard deviation below normal trough establishes across
eastern Canada Wednesday night through the balance of the week. This
will return passing shortwave troughs permitting the return of
precipitation under northwest flow.  Much the case for this week,
thunderstorm complexes could traverse the region given we are on the
southern periphery of the trough, however confidence not high enough
to warrant anything greater than climo pops.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
Cu field will bounce around low vfr/mvfr through the afternoon.
A MCS will continue to pass to the south with vicinity thunder
maintained at MGW. A second MCV over northern Indiana may spawn
additional showers and storms across the upper Ohio Valley,but
at this time, confidence is not high enough for inclusion.

Diurnal cumulus will diminish, with another night of fog
possible. The models are hitting it hard everywhere with so
much moisture in the low levels, but this may be overdone. Will
generally maintain MVFR visibility except in those places that
had IFR this morning and add some where rain fell near terminals
today.

.OUTLOOK...
Periodic restrictions are expected through the weekend in
morning fog and daily rounds of thunderstorms.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
MD...None.
OH...None.
PA...None.
WV...None.

&&

$$