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FXUS61 KPBZ 201232

Area Forecast Discussion
National Weather Service Pittsburgh PA
832 AM EDT Thu Jul 20 2017

Thunderstorm chances will escalate today with severe storms
possible. Active weather remains in place through the weekend,
with the risk of severe weather returning Saturday.


Only minor tweaks made to sky and hourly temperatures for this
update. Forecast remains on track. With the midwest MCS diving
towards the lower Ohio Valley and another shortwave tracking
across southern Ontario, it appears that support for any
significant convective threat will hold off until mainly after
21Z, when lift improves. Current PoP fields handle this well,
with only scattered possibilities across the northwest counties
prior to that time. Latest HRRR trends indicate that timing
could be an hour or two later still...this will be monitored.

Previous discussion...

Thunderstorms will become likely over the I 80 corridor this 
afternoon, and those are expected to spread southward toward the
I 70 corridor this evening.

Those storms are anticipated with a shortwave that supported 
the MCS that pummeled the midwest overnight. Upper Ohio 
temperatures are again forecast to rise through the 80s/near 90 
today, and those readings coupled with near 70 F dewpoints will 
not only be uncomfortable, but will fuel severe-healthy 
instability profiles.

As mid level flow increases to near 50 kt with the approaching 
trough, deep layer shear will escalate and support an organized 
severe threat into the evening hours as the surface front and/or
outflow ploughs into the warm, humid unstable airmass over the 
immediate area. Expect a thunderstorm line or short bowing 
segments to provide a fairly widespread damaging wind threat. In
addition, with backed surface wind under west, to northwest flow
aloft, the threat of the isolated tornado cannot be ruled out,
especially in light of chaotic outflow boundary expectations.
These threats have been mentioned in the morning Hazardous
Weather Outlook.


The latest deterministic model trends are that this evenings 
convection and frontal passage will be decisive enough to drive 
the boundary south of the region for Friday. POPs for Friday 
were thus reduced, then revamped for the approach of another 
strong shortwave on Friday night, which may spawn more severe 
storms given the projected wind profile. Current timing does not
favor that scenario, hence the marginal SPC outlook for the time

Models indicate a repetitive pattern for Saturday and cold
frontal passage for Sunday. Likely POPs were thus maintained for
the weekend along with the beginning of a downward temperature


The pattern of shortwaves crossing in zonal/northwest flow is 
expected to give way to Great Lakes-to-Northeastern-CONUS 
troughing during the first half of next week. Temperature will 
thus moderate back toward, and a few degrees under the average
and rain chances will decline into mid week.


High confidence forecast save for the onset and duration of
thunderstorms this afternoon and early evening. These times
could vary by 2-3 hours and flight category between 6-12Z

An upper level disturbance crossing this afternoon will bring a
line of strong to severe storms moving from north to south 
affecting likely all terminals if it develops like many of the 
high resolution models indicate. Inserted VCTS for all terminals
starting around 20Z north and 0Z south of the mason dixon line.
Brief restrictions to IFR vis can be expected if an airport is 
impacted by a storm. In wake of the storms, last 6-9 hours of 
forecast its hard to tell if MVFR fog will develop or will there
be a canopy of stratocu or just VFR? For now, rolled with an 
optimistic forecast no cigs below 030.  

Additional rounds of storms are expected through the weekend, 
and morning fog is possible as well.



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