Area Forecast Discussion
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FXUS61 KPBZ 290522 AAC

Area Forecast Discussion...UPDATED
National Weather Service Pittsburgh PA
122 AM EDT Thu Jun 29 2017

Temperatures will continue to warm into the weekend, and
humidity levels will also increase. This will return the risk
for showers and storms, mainly Friday and Saturday.


No changes to the overnight period.

Upper level heights will slowly increase this morning, in
response to a weak shortwave trough which will swing through the
Lower Great Lakes. This shortwave will reach NW PA by this
afternoon. Models are indicating that most of the energy from
the wave will be used to flatten the ridge. In addition, warm
air advection will be well underway in the lower and mid-levels,
which will work to provide a nice cap in the atmosphere.
Looking at forecast soundings across the area, the temperature
profile will become dry adiabatic this morning, causing an
increase in low-level lapse rates, and mixing heights will also
quickly rise. Model difference with temperatures aloft become
important this afternoon. The NAM wants to cutoff the warming at
700mb late this afternoon, while the GFS does not. Thus, the
GFS shows the cap holding through the day, while the NAM
dissolves it. So will find a compromise between the models and
focus on the area of shortwave induced ascent. With strong low-
level lapse rates and increasing surface instability, will
include PoPs across the north this afternoon. I believe the
activity will initially be scattered and would develop on the
eastern edge of the afternoon shortwave. Speed shear does
increase this afternoon, especially over northern counties, so
its not out of the question that updrafts could intensify and
allow for convection to become more organized. Another point to
keep in mind, if any convection does intensify, there will also
be outflow boundaries. These boundaries would lead to further
development, which could expand the risk for storms. This will
need to be watched as the day progresses.

Also expect to see some gusty south-southwesterly winds develop
late this morning and through the afternoon due to a
strengthening wind profile aloft and rising mixing heights.
A cu field should develop with daytime heating, as parcels will
have no trouble rising to the cap.

The increasing cloud cover could hold temps down just a bit, but
think that we will still see above normal readings with the
strong warm air advection and well mixed atmosphere.


Any convection will dissipate tonight with sunset and the
absence of large scale ascent. Weak riding aloft is expected as
well. Models are trying to squeeze out some QPF tonight, likely
due to the passing shortwave trough and brief lowering of 500mb
heights, but at this point, not sure if that is realistic.
Temperatures aloft begin to warm again with the passage of the
wave and deep moisture is not available. For now will keep most
of the tonight period dry, but this will need to be reevaluated
in future model runs.

Friday looks similar to today, but with even greater warming
aloft and rising heights in the morning. This would mean the
atmosphere would again be capped. Surface instability will be
increasing due to a rise in surface dewpoints, but again, this
is dependent on which model hits those numbers correctly. If
anything, Friday looks like more popup summer time convection,
so will just go with low chance PoPs for the entire area. Temps
Friday will be above normal and humidity levels will be much
more noticeable.


The Midwestern upper trough is progged to advance E Saturday
driving a cold front across the area. Dry weather is expected to
return Sun/Mon as surface ridging builds underneath the upper
trough. Another trough is progged to advance E from the Midwest
and Plains regions Tue, though with uncertainty in timing and
location limited POPs to the chance category for now on the 4th
of July.


Mainly VFR conditions are expected through the TAF period.
Mid/high clouds should continue streaming E from weakening
thunderstorms to the W.

CU rule/model soundings show SCT to BKN CU by late morning,
though bases should still be VFR. A shortwave trough is progged
to move across Srn Ontario/NW PA by afternoon with sct
showers/tstms expected. Mentioned a VCTS for FKL/DUJ, though
capping warmth aloft should limit tstm potential elsewhere. A
tightening pressure gradient and mixing should also result in
gusty SW winds today.

Restriction chances will increase Fri and Sat with the approach
and passage of a cold front.




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