Area Forecast Discussion
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FXUS61 KPBZ 260929

Area Forecast Discussion
National Weather Service Pittsburgh PA
529 AM EDT Wed Jul 26 2017

Today will be dry and a bit warmer. Showers and thunderstorms
return Thursday and Friday due to a cold front and upper level
low pressure system. Dry and seasonable weather returns this


No changes needed for the pre-sunrise update. Previous
discussion follows...

Quiet weather continues during this period as high pressure
slides off to the east today. River valley fog will lift by
mid-morning as the lack of cloud cover allows for decent heating
to take place. Some fairly flat cumulus should pop up this
afternoon, with generally dry mid levels. A return of southerly
flow will allow for some warming today, with temperatures
climbing back towards seasonal norms.

Clouds will be on the increase tonight ahead of the next cold
front/upper level wave to impact the region. Although upper
level moisture increases, the mid levels remain dry through
sunrise and the bulk of the lift remains off to the west,
leading to a dry forecast. The clouds and a bit more mixing will
allow low temperatures to rise back towards climatology as well.


Forecast confidence for this period remains low, although the
GFS and the ECMWF might be starting to settle on a preferred
track for the main surface low. A shortwave digs across the
western Great Lakes towards the upper Ohio Valley by Friday
afternoon. While this happens, the surface wave travels near the
Ohio River and just south of the Mason-Dixon line, reaching the
east coast by Saturday morning. This track will keep the frontal
boundary from crossing until late Thursday night/Friday morning,
while the deepening shortwave crosses Friday afternoon/evening.

What this means is an active period with plenty of opportunities
for showers and thunderstorms, particularly across the southern
CWA. Have the higher PoPs focused from Pittsburgh on south
initially, then more southeast with time. Despite frontal
passage by Friday, the crossing shortwave will provide support
for precipitation during the afternoon and evening hours,
despite the cooler air mass.

Areas south and east of Pittsburgh are targeted with a slight
risk of severe thunderstorms for Thursday afternoon and evening.
While 0-6km bulk shear of 30-40 knots would be sufficient for
some storm organization, instability appears to be in question.
Much will depend on how much heating can occur on Thursday. The
bigger risk may end up being flooding, as the heavier rain may
target the wetter areas of the CWA. PWATs approaching 2 inches
in spots plus warm cloud depths in excess of 12k feet could
support plenty of downpours. Will mention both the severe and
heavy rain threats in the HWO.

Temperatures will sink back a bit below climatology behind the
front by the end of this period.


A few showers may linger into Saturday morning as the shortwave
departs. Thereafter, Great Lakes high pressure will provide a
few dry days along with seasonable temperatures as midlevel flow
flattens. A front may approach from the northwest by the middle
of next week, but the timing remains highly uncertain.


Valley fog will dissipate through the morning, giving way to
VFR conditions. Clear skies will be marred only by some mid-
level afternoon cu-development.

Calm winds this morning will remain light and variable through
the day.

Restriction chances return Thursday as low pressure impacts the
Ohio Valley.




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