Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
519 am EDT Monday Aug 31 2015
warm...humid conditions and sparse rain chances are expected
through the upcoming week.
Near term /through tonight/...
a weakening and opening upper low will slowly transition east of
the Appalachians primarily to our south through the day. A
weakening deformation zone will pass the southeastern portion of
the County Warning Area early this morning. This will allow for some light rain
shower development and more significant cloud cover...however this
will be a rather fleeting passage as the system is quickly
weakening and shearing off to the east of the ridges. It will
largely be following the modestly coupled upper jet structure
that transitions toward New Jersey as the day progresses. This
will leave our area in an area of weak middle-level troughing as an
upper ridge probability of precipitation from the High Plains toward the northern Great
Lakes later in the day. This will stretch out what little sheared
vorticity there is largely along and south of I-70. Deeper
moisture will largely remain along and south of this
corridor...thus this is where the best probability of precipitation were concentrated on
for the afternoon and evening hours. Deeper moisture and
exceedingly weak forcing may be just enough to overcome warm air
aloft and a generally unfavorable environment for convection due
to exceedingly low shear values through the day.
Elsewhere...nothing more than low-topped weak pulse convection is
expected and only mid-afternoon/early evening slight chance probability of precipitation
were carried for this eventuality.
Even though sunshine will be unlikely to be pervasive until later
in the morning and into the afternoon due to high clouds around
the opening upper low...850 mb temperatures jump toward
+18c...which means that generally middle to a few upper 80s will
be possible by afternoon. This is even with poor mixing.
However...the temperature will only be part of the equation. Due
to dewpoints currently largely in the upper 60s and little chance
of dry air being entrained downward due to poor mixing
today...heat index values will likely run up toward 90f by
afternoon in many places.
Cooling tonight will likely be a slow process. Ample boundary
layer moisture seems poised to interact a virtually clear sky with
little wind. However...residual heating in urban areas seems
likely to keep lows in the city center near 70f again...while
outlying areas saturate to the dewpoints in the middle and upper
60s yet again with patchy fog developing...especially around the
river valleys. Fries
Short term /Tuesday through Thursday/...
synoptic pattern favors the building of a large ridge over the
eastern Continental U.S. Tuesday. Despite this...low-level moisture lingers
across southern zones enough to generate isolated
showers/thunderstorms with strengthening low-level lapse rates.
With warm middle-level temperatures in place...keeping probability of precipitation limited to
slight realm...even as an embedded weak trough passes through the
ridge Tuesday evening. The upper ridge then amplifies by middle-week
reinforcing the trend to drier and warmer conditions. Tax
Long term /Thursday night through Sunday/...
no major changes to the extended period as the impressive eastern
Continental U.S. Ridge further amplifies reaching into eastern Canada. Cannot
rule out diurnal rain chances...especially in the higher terrain
as the center of the upper high moves north of the area and return
flow establishes a moisture feed from the middle- Atlantic.
Still...largely dry conditions will persist into and through the
weekend. Temperatures will play out like a broken record...nearly
the same everyday...several degrees above average through the
period. The next real chance of a rain producing system for the
entire forecast area could come with a front slated sometime early
to middle of next week. Tax
Aviation /09z Monday through Friday/...
areas of MVFR and some IFR fog will continue through sunrise under
very weak flow and ample boundary layer moisture. This should
burn off toward 14z. Winds will be light from the southwest. A few
showers and thunderstorms will be possible by afternoon...however
their coverage will be isolated enough so as not to mention at
this time. Fries
Outlook...Monday night through Friday... general VFR except for
patchy morning fog the remainder of the week.