Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
256 PM EDT Tuesday Sep 30 2014
after widespread rain departs the region this evening...temperatures
will be on the rebound again for the rest of the work week. A cold
front will brings widespread rain Friday and Friday night before
much colder air arrives this weekend.
Near term /through Wednesday/...
big temperature contrast across the forecast region with upper 50s
in the northwest behind the frontal band of showers and 80 degrees
ahead of the band. Northern portion of the band is beginning to fall
apart with some convective cells developing in advance of the
line...while the southern portion has done a better job holding
together...with some convective elements developing along the line
itself. Should not be any threat of thunderstorms behind the band.
However...with upper level low dropping southeast from Lake Erie
into western Pennsylvania...have kept some probability of precipitation into the forecast
through much of the night. Some isolated echoes are beginning to
appear across north central Ohio. With the proximity of the upper
low...have kept mostly cloudy skies in the forecast through
Wednesday morning...with diminishing trends in the afternoon. As a
result...think that stratus should prevent fog development in the
morning. Have added slight chance probability of precipitation for much of the forecast area
Wednesday as well due to proximity of upper low. Although
temperatures have dropped significantly behind the front...expect
that temperatures will rebound to near seasonal levels tomorrow.
Short term /Wednesday night through Friday/...
large scale ridging quickly builds back into the area Wednesday
night into Thursday. In fact...the model consensus does favor some
sort of a weak warm frontal passage Wednesday night into Thursday
morning which does manage to briefly increase moisture pooling
along it. This may yield an increase in clouds Wednesday night
into Thursday...however given the weakness of the boundary and
weak saturation...partly cloudy skies will be featured in the
Deep southwesterly flow becomes established on Thursday and
continues right into early Friday as the next system bears down
on the area. The 12z guidance has largely slowed relative to the
00z guidance and favors a late afternoon/evening cold frontal
passage on Friday. Probability of precipitation were slowed to account for this. What is
also notable is the increase in temperatures for Friday across the
eastern County Warning Area on the GFS/ECMWF. Both depict lower 80s in the eastern
reaches of the County Warning Area under a very well mixed lower atmosphere in the
dry pre-frontal environment. This will be in Stark contrast to the
western County Warning Area...where drastically increased layer moisture...500-700
j/kg of cape and strong shear will be resulting in very strong
chances of rain showers with a few thunderstorms charging eastward
into the County Warning Area along with much colder conditions to start the
extended forecast. Fries
Long term /Friday night through Tuesday/...
the cold front will likely be slicing through the area Friday
evening...with showers and thunderstorms winding down from west to
east as it does so. Strong winds will be likely along the front as
it does so owing to roughly 30-35 kts of momentum off the surface
to mix down with the frontal passage. This combined with a drastic
air mass density discontinuity should yield a fairly windy period
from Friday afternoon into Friday night.
Much colder and still unstable conditions will dominate the region
through the weekend as an upper low settles in overhead by
Saturday. Model soundings suggest strong low level lapse rates
with continued cold advection should allow for a diurnal increase
in showers under the upper low on Saturday. With wet bulb zero
heights falling like a rock...there may be some p-type
considerations...possibly some graupel or something of that
nature...however the general story will be the transition to much
colder conditions into the weekend.
Beyond the weekend...a general continuation of an eastern trough
and western flat/broad ridging scenario seems to play out. This
should yield weaker and faster moving systems over the top of the
ridge and into the eastern U.S. With generally cool conditions
Aviation /19z Tuesday through Sunday/...
model forecasts indicated that the frontal band would pass through
with VFR conditions...while observations at most sites have dropped
to IFR. With models not handling the passage well...have generally
used upstream observations and trends for creating 18z tafs. Think
that any sort of thunderstorm threat should be contained to
duj/lbe/mgw at this point. Behind the band...expect that sites
should drop to IFR for a few hours. However...one concern is that
IFR conditions observed upstream may be due to Lake Erie
influence...which would not be as strong at local terminals. In that
case...widespread IFR conditions may not occur. By evening...expect
that clouds will remain widespread due to upper level low. With MVFR
stratus expected overnight...have not added fog at any terminals. As
stratus scatters out Wednesday morning...remaining clouds should
lift to VFR conditions.
Outlook.../Wednesday through Sunday/...
the next chance for widespread restrictions will come with a Friday/Friday
night cold front.