Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
641 PM EDT Friday Sep 4 2015
warm and humid conditions will continue through the weekend.
Near term /through Saturday/...
with 630pm update... have increased probability of precipitation across eastern half of
the forecast area through the next few hours as thunderstorm
coverage has expanded more than expected. With no flow vertically
through the atmosphere...downdrafts are managing to create some
gusty winds at the surface as thunderstorms collapse on
themselves. Coverage of thunderstorms should diminish with loss of
daytime heating. Previous discussion follows.
Shortwave/weakness in the upper ridge will maintain the chance
for showers and thunderstorms through the evening. In
addition...residual outflow boundaries from this mornings complex
will also be a contributing factor...and a few storms have already
developed in the higher terrain. With no shear/upper level
flow...most of these storms will be rather short lived.
Thus...slight to low chance probability of precipitation were maintained through the
period with most of the convection expected to dissipate with the
loss of heating at nightfall. While temperatures will be
contingent on cloud cover and coverage of storms this
afternoon...generally stayed with a persistence forecast with
temperatures remaining 5 to 10 degrees above seasonal averages.
Short term /Saturday night through Monday night/...
h500 ridge will reestablish itself over the weekend and continue
to keep most weather systems at Bay. Weak flow out of the east and
southeast...around a surface high to our east...will bring a
slightly drier and more stable airmass on Saturday. With precipitable waters
falling and h500 temperatures rising...expecting less diurnal
convective coverage. Limiting probability of precipitation to slight chance...with areas
north and east of pit staying Dry.
Ridge reaches its best strength on Sunday with h500 heights around
590dm. Think capping will be strong enough to suppress convection
and will keep the dry forecast going. If probability of precipitation need to be added
eventually...it would be along the ridges.
The ridge starts to weaken on Monday as a large upper low across
Canada moves eastward...causing heights downstream to begin to
fall. Still...the lack of moisture...continued capping...and
persistent sprawling surface high will keep probability of precipitation below 15. Any
significant precipitation chance will wait until midweek.
Temperatures were based on a blend of 12z bias-corrected mosguide
and previous numbers...with continued values of around 10 degrees
Long term /Tuesday through Friday/...
the strong middle/upper ridge is prognosticated to remain through early
next week...with continued warm and muggy conditions. Relief from
the quite warm conditions may come with cold front later Wednesday
into Thursday...as well as an increasing chance for thunderstorms.
Aviation /00z Saturday through Wednesday/...
expect prevailing VFR conditions for much of the afternoon and
evening with a small chance of restrictions in any shower or
thunderstorm that passes directly over a taf site. With limited
confidence in any one specific port being impacted...opted to
leave ts mention out for now.
Expect another round of fog overnight with plenty of low level
moisture and little mixing. IFR fog will be possible in locations
that received rainfall during the day.
Outlook...Saturday night through Wednesday...
general VFR with brief restrictions in early morning fog is
anticipated each day.