Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Pittsburgh PA
1231 PM EDT Sat Aug 23 2014
showers and thunderstorms will become less common early next week.
A slight reprieve from humidity is expected Monday and Tuesday...
but temperatures are expected to climb into the middle to upper 80s.
Rain chances will increase middle-week as a frontal boundary impacts
Near term /through Sunday night/...
after widespread morning fog and stratus...cumulus beginning to form as moist air mass
heats to cnvctv temperature ahead of aprchg wave in northerly flow aloft.
Slow-moving showers and eventually thunderstorms will develop along a stalled
boundary with north-S orientation from near DuBois to near Morgantown...
and in the rdgs as a result of orographic lift.
Increasing cnvctv covering expected late this afternoon through middle-evening as 500 mb shortwave
trough aprchs. Northerly steering flow will direct this cnvctn southward along
the boundary...which may lead to training. In addition...substantial
portion of cape profile is located blw freezing level...which suggests
that warm rain processes will dominate and precipitation will be very
Although precipitable waters are lower than one typically would expect for flash
flooding during this time of year...cnvctn has overperformed for the
last several days...leading to daily episodes of flash flooding. The
episode last night focused on Westmoreland and Fayette counties...
where cnvctv threat will present today.
Consequently...a Flash Flood Watch seems like it would be prudent
for the higher terrain counties...where flash flood guidance is quite
low. Conds will be monitored early this afternoon...with a decision
expected before 2pm.
Overnight cnvctn in the eastern rdgs will subside late tonight. Given
lingering moisture and clearing sky...fog is expected to form again tonight amid
light wind. Some may be lclly dense...and a dense fog adzy may be
warranted in at least a portion of the forecast area once it is clear
where the boundary becomes situated and where additional rain reinforces
the saturated air mass.
Upper ridge over the Midwest builds eastward on sun...with surface hipres and
subsidence aiding in westward evacuation of low-level moisture. Any isolated
morning showers will dissipate quickly as air mass dries. Maxima in the lower
80s can be expected...with a slight lowering of humidity.
Ridge will amplify further Sun night...leading to clearing sky and decrg
humidity as dry air advects westward. Minima Sun night will be cooler than
the past few days...reaching the upper 50s-middle 60s. Kramar
Short term /Monday through Tuesday/...
Monday...with the axis of the ridge directly over the area and
dry air in place...cloud develop should even be limited. Abundant
sunshine will help keep temperatures close to or above normals. By late
Monday the upper ridge axis tilts west to east allowing a little
more moisture to stream across the northern periphery of the high. Some
clouds may develop into Monday night/tues...but the dry weather will
Long term /Tuesday night through Friday/...
ridging will persist through midweek...keeping the trend of warm
and dry weather in place. Model guidance continues to struggle
with the details surrounding a potential tropical system in the
Atlantic. While this system should have no direct impact on our
immediate area...the broad scale pattern...including timing the
breakdown of the ridge...will be impacted. A cold front will be
in the neighborhood toward the end of the week with associated
shower chances. Tax
Aviation /15z Saturday through Wednesday/...
occasional LIFR fog/ceilings noted at some terminals with MVFR/IFR at
others...both resulting from stagnant saturated air mass in the region.
Fog expected to lift by 14z...with broken ceilings between 1-3kft expected until
this afternoon. After 17z or so...VFR conds should persist.
IFR/LIFR fog/ceilings could return late tonight as air mass not expected to be
Outlook.../Sunday through Wednesday/...
Hipres will build into the region sun-Tue. Modest drying at surface may
preclude fog development through at least Tuesday. Showers and thunderstorms will become
increasingly likely Wednesday afternoon and evening...with attendant visibility and ceiling