Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
633 am EDT Tuesday may 26 2015
warm and humid conditions will persist through the shortened work
week...with shower and thunderstorm chances each day.
Near term /through tonight/...
625 am...updated to reduce probability of precipitation in wake of first shortwave exiting
the northern County Warning Area. Still expect a lull during the late
morning...before next ripple arrives in the southeast counties.
Expect daytime heating to get scattered rain showers/thunderstorms and rain going this
afternoon in advance of next more significant shortwave. Also
updated sky/hourly temperature grids.
First in a series of shortwaves is currently riding northeastward
across the area with some rain showers. No thunder so far as warm air
aloft is keeping instability in check. Expect this activity to
continue to lift northward this morning...with a lull in activity
during the late morning hours.
Daytime heating will help to increase cape during the afternoon
hours...but a trigger will be lacking through midafternoon and
thus will keep probability of precipitation at the lower end of chance. This will change
toward the end of the day as a more potent shortwave trough
approaches from the southwest. Have likely probability of precipitation spreading across
the region through tonight as a result.
Storm Prediction Center has a slight risk of severe thunderstorms up over most of
Ohio. Think that better midlevel flow and instability will be
realized to the south and west of our County Warning Area during the
afternoon/evening...towards the middle and lower Ohio Valley.
However storms that develop could propagate towards the WV/Ohio
border late today with some strong and potentially damaging wind
gusts. Will mention potential in severe weather potential statement but do not foresee a
significant severe outbreak. Storms have the potential to be
efficient rain producers as well. Precipitable waters in the 1.5 to 1.75 range
are near the high end of climatology...and warm cloud depths will
likely exceed 10k feet tonight. It does appear that storms will
move with sufficient speed to deter excessive rainfall for the
most part...but poor drainage locations may need to be watched.
Temperatures were mainly tweaked downward slightly today to get
more in line with model consensus. Little change needed for
Short term /Wednesday through Friday night/...
the first half of the period will consist of unsettled weather as
an upper level shortwave trough is prognosticated to be over the Great
Lakes and shift eastward. Deep moisture will be maintained over
the region allowed by southwest flow. Therefore...chance to likely
probability of precipitation were maintained and adjusted slightly based on latest
A lull in precipitation can be expected late Thursday into Friday
as brief high pressure builds under a weak ridge in the wake of
the shortwave trough.
Precipitation chances rise again on Saturday supported by an
approaching surface boundary and a weak split flow aloft. The front
is expected to slowly sag across the forecast area through Saturday
as predicted by European model (ecmwf) and GFS solutions.
Forecasted temperatures will be above normal through the period.
Long term /Saturday through Monday/...
various long range solutions agree on stubborn boundary laid
across the forecast area through the weekend. Therefore chance
probability of precipitation were preserved with the support of a series of upper level
disturbances across the upper Ohio Valley. GFS and European model (ecmwf) differ
over the outcome of an approaching trough from the Midwest in
terms of magnitude...position...and timing. Therefore persistence
was heavily used in the latter half of the period and low chance
probability of precipitation were introduced.
Aviation /12z Tuesday through Saturday/...
generally VFR conditions are expected in the mean today. Some MVFR
stratocu may briefly impact Ohio/northwest PA terminals this morning
before mixing out. Otherwise...VFR cumulus/ac forecast for much of the
day. Still expect thunderstorms in the vicinity to handle any scattered convection this
afternoon as timing and coverage should not warrant anything more
firm in the taf. Best chance of strong storms will be in Ohio
during the late afternoon/evening. This will occur with the
arrival of a more potent midlevel shortwave...which will create
better precipitation coverage from west to east tonight. It will
still take much of the night to sink ceilings down to MVFR level
though. Cannot rule out spotty IFR late in areas that receive
decent rainfall. Winds will generally be around 10 knots or less
out of the south and southwest.
Outlook.../Tuesday night through Saturday/...
abundant moisture and disturbances in southwest flow are likely
to spawn periodic restrictions through Wednesday.