Thursday, June 29, 2017, 10:10 AM EDT
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Area Forecast Discussion
Valid At
Wed, Dec 31, 7:00pm EST
FXUS61 KPBZ 291236

Area Forecast Discussion
National Weather Service Pittsburgh PA
836 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.


Morning update to adjust sky and temperatures to match current
trends. The cool and dry morning will give way to a quick warm
up and increasing dewpoints. 

A shortwave trough will stream through the southern Great Lakes
this morning, reaching NW PA by this afternoon. This will
flatten out the current ridge and could generate some showers
and thunderstorms across the northern 1/3 of the forecast area.
Model solutions vary widely in terms of southern extent of
showers/thunderstorms this afternoon, or whether they'll exist
at all. Current RAP analysis seems very reasonable with skinny 
SB CAPE values nearing 1000J/kg along the north, given current
surface dewpoint forecast. This could develop in spite of
warm mid-level temperatures.

Model difference with temperatures aloft become important this 
afternoon. The NAM wants to cutoff the warming at 700mb 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 chance PoPs over 
areas north of PIT this afternoon. 

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 

Expect to see gusty south-southwesterly winds develop late this
morning and through the afternoon due to a strengthening wind 
profile aloft and rising mixing heights. 


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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