Area Forecast Discussion
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FXUS61 KPBZ 220045

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

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.


Some lower dewpoints have managed to sneak into the northern CWA
this evening, where values have fallen off into the 60s, which
has actually been enough to minimize 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.


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


- 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.


Generally VFR this evening, although a few storms may pop along
the old, lingering frontal boundary that should avoid most
terminals. Tonight, due to lingering moisture and recent
rainfall, especially south of PIT, expect areas of fog to
develop with MVFR/IFR restrictions. A few terminals may even get
down to LIFR, but with clouds expected to increase during the
predawn hours, did not feel confident enough to go that low. Fog
may lift a little earlier than normal if mixing can get going
ahead of the next shortwave. After 13Z or so, one or two round
of storms are possible. Handled with VCTS for much of the
daylight hours for now, pending improved timing in future
issuances. Gusty winds and hail will be possible with any storm.

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




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