Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Pittsburgh PA
754 PM EDT Tuesday Jul 29 2014
temperatures will moderate toward normal by late week with the best
chance for showers on Wednesday.
Near term /through Wednesday/...
8pm update...only minor changes to overnight. Adjusted cloud cover
and nudged temperatures closer to latest guidance.
Northwesterly cold advection will begin to wane this evening as
low level flow should switch around to the west or southwest ahead
of the next incoming system. The boundary associated with this
system currently runs from roughly Detroit to Fort Wayne...with at
least some nominal scattering out of the stratocumulus ahead of
it. This should yield some breaks in the clouds for an hour or two
before the wind shifts back to the northwest and cold advection
stratocumulus again fills back in. Some shower activity is noted
along the boundary...and given instability parameters with said
lifting mechanism...a shower or two cannot be ruled out overnight
as the boundary passes. Regardless...yet another cold night is in
store for the region.
A middle-level wave looks to translate southward through the area on
Wednesday...with steepening middle-level lapse rates and very low wet
bulb zero heights down toward 7 kft. With precipitation cooling
and steep lapse rates and fairly low dewpoints below this
level...some graupel is not out of the question. However...cape
values below 200 j/kg and very slim positive area in the charge
separation layer make thunder a tough thing to call for. As
such...probability of precipitation were raised quite a bit from the previous
forecast...while thunder was removed from the forecast. Highs were
summarily reduced to account for significant precipitation
Short term /Wednesday night through Friday/...
shortwave exits Wednesday night with showers ending. Upper level
trough then begin to lift out as closed low begins to shift west
with heights rising off the East Coast. Temperatures will
gradually warm through the period and approach seasonal averages
Friday. Showers chances will linger mainly toward I-80 Thursday
afternoon closer to lake moisture and colder temperatures aloft.
On Friday flow aloft will begin to shift to the southwest allowing
deeper moisture to advect north. Models differ on how fast and
where this will occur...but would expect better moisture to begin
to move up along the ridges before increasing farther west toward
Ohio. For now chance probability of precipitation most locations Friday.
Long term /Friday night through Tuesday/...
although the mean trough maintains through the weekend...h500
heights will have recovered to the point that allows seasonable
temperatures. Dew point will remain comfortable without any
significant moisture influx from the south. A shortwave may provide
decent rain showers/thunderstorms and rain coverage Saturday afternoon/evening...while
Sunday will see a return of more sparse precipitation behind this
disturbance. There are hints that the trough may try to lift out
early next week...at least temporarily. This may lead to little or
no afternoon precipitation by Tuesday...as well as continued seasonable
Aviation /00z Wednesday through Sunday/...
stratocu layer will quickly decrease during the early part of the
forecast period. However...another area of clouds will swing
through from the northwest with the next upper level wave. Will
keep in a VFR forecast for the night and for most ports on
Wednesday. The exception will be at fkl and duj where MVFR ceilings
with showers are possible with the passage of a weak surface
boundary Wednesday afternoon.
Outlook.../Wednesday night through Sunday/...
no widespread rstrns expd.