Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
414 am EDT Tuesday Sep 2 2014
a cold front will bring widespread showers and storms today.
Near term /until 6 PM this evening/...
a weak boundary will cross the region this afternoon. Looking at
current data...more concerned with the potential for heavy rain
then severe. Atmospheric instability does not stand out with sb
convective available potential energy generally less then 1000j/kg and lifted indexes around -1.
Additionally...middle clouds will rapidly increase ahead of frontal
passage. Models are showing a pretty descent shortwave trough
riding along the boundary...but it weakens and gets sheared apart
as it reaches the area. Cannot totally rule out isolated
strong/severe storms as a line of strong low level forcing will
develop with frontal passage and 0-6km bulk shear values will rise to near
50kts. My greater concern will be the threat for heavy downpours.
Moisture convergence in the lower levels strengthens early this
afternoon and precipitable waters rise to almost 2 inches. Surface dewpoints
will still be around 70...which will be close to a tropical
regime in which the rain drops will be created. Front does get
trucking along once the trough swings through...which will help to
limit the amount of time heavier rain can fall over the same
areas. Will continue with high probability of precipitation with frontal passage and the mention of
Temperatures will be a bit tricky with pre-frontal clouds and showers...
so have lowered temperatures just a bit from previous forecast. Expect
another very humid day.
Short term /6 PM this evening through Thursday/...
front quickly exits this evening as shortwave trough swings
through. It will take most of the night...but lower surface
dewpoints will eventually sink southward. At this point...not
quite sure show long the less humid airmass can remain in place.
With the surface high drifting eastward...the low level flow will
become southerly once again on Thursday. Because of this...see
Wednesday as being comfortable...but the humidity creeps back in
on Thursday. Surface high and weak ridging aloft should provide
enough subsidence to keep the area activity free Wednesday and
Long term /Thursday night through Monday/...
latest model data is speeding up The Crossing of the next real
cold front. Will speed up the entrance of probability of precipitation Friday afternoon.
Front will take its time crossing the region...meaning shower and
storm chances will be pushed into Saturday. Much cooler and drier
air for the end of the long term with broad surface high building
in from the northwest.
Aviation /08z Tuesday through Saturday/...
sufficient surface wind and middle level clouds are expected to preclude
serious restrictions early this morning...although scattered showers may
degrade conditions especially at ports north of pit. Showers and thunderstorms
will become more numerous aftr daybreak as a weak cold front encroaches on
the upper Ohio region. Expect IFR visibility in the stronger cells.
Outlook.../tonight through Saturday/...
restrictions in fog and St are possible Wednesday morning. More
restrictions are possible with a late week cold front.