Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
1108 am EDT Tuesday Jul 28 2015
warm weather will continue until a cold front passes on Thursday
Near term /through tonight/...
rather tricky near-term forecast as models are all over the place
with temperatures and dewpoints. Looking at the NAM which keeps
surface dewpoints near 70...we could see storms develop as the
cloud-free skies would result sufficient surface-based instability.
However... water vapor imagery shows a good deal of dry air and
subsidence over the region today...which our 12z sounding showed
extended as far down as 850mb. Assuming the boundary layer mixes
up to 750 or 760mb and the absence of any notable moisture
advection ... should result in surface dewpoints dropping into
the low 60s across the south and middle to upper 50s across central
and northern zones. Have also nudged up forecast maximum temperatures
by a couple degrees with most areas cloud free today and 850mb
temperatures nearing 23c. All in all even with the slight increase
in temperatures...it should feel at least a little better if lower
dewpoints are realized.
Short term /Wednesday through Friday/...
the short term period will feature the approach and passage of a
cold front...currently slated for a Wednesday night/Thursday sweep
of the region. Another warm Wednesday should be expected before
the relief arrives with highs around the 90 mark and heat indices
in the middle 90s.
As for precipitation with the front...night timing and very warm middle
level temperatures...-4 to -5 c at 500 mb as per GFS and
NAM...will limit instability and explosiveness...while little
forcing for ascent is indicated either. Precipitation probabilities in
the chance/scattered category will thus suffice for now.
Surface high pressure that is projected to build in the wake of the
front with weak troughing aloft will result in a dry close to the
short term period with near average temperatures.
Long term /Friday night through Monday/...
surface ridging under a broad eastern Continental U.S. Upper trough should
keep mainly dry and seasonally warm weather through the
weekend...though included unmentionable slight chance probability of precipitation
Saturday as a weakening shortwave crosses. A stronger shortwave is
prognosticated for early next week with better chances for rain. Followed
close to the ensemble based wpc guidance to smooth out individual
model differences through the period.
Aviation /15z Tuesday through Saturday/...
with the exception of early morning patchy MVFR fog...VFR
conditions with diurnal cumulus and light winds are expected through
tonight under weak surface ridging.
Outlook...Wednesday through Saturday...
restrictions are possible with a late Wednesday/early Thursday