Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
919 am EDT Thursday Jul 31 2014
temperatures will remain near normal with daily chances for showers
and thunderstorms Friday through Sunday. Dry and mild conditions to
return by the beginning of next week.
Near term /through tonight/...
morning update made to add scattered showers to the zones over
Westmoreland and Indiana as an upper level shortwave departs. No
other changes needed.
Previous discussion follows...
as the trough axis shifts east of the area today the overall chance
for precipitation wanes. 00z NAM/GFS continue to show some
residual deep moisture lasting through the early afternoon
hours...mainly across our northern tier of counties. With the cold pool
aloft still in play...low to middle level lapse rates will once again
generate mostly cloud cover...but some isolated showers could
develop. Hi-res WRF model runs also supportive of showers
developing near or north of I-80 again today.
00z NAM has the next system beginning to pull moisture northward
from the Atlantic off the southeast coast tonight. This will advect high
clouds into the area but with the low level flow primarily out of the
southeast...downsloping will keep the majority of our area dry through
early Friday. At this time timing differences exist among models for the
onset of precipitation spreading south to north Friday.
Short term /Friday through Saturday/...
flow aloft will become more southwesterly as the upper trough
begins to fill and lift to the northeast. This will allow for
increased moisture advection into the forecast area with precipitable waters
just shy of 1.5" by Saturday afternoon. There will be a slight
chance of thunderstorm development by late Friday morning mainly
over the ridges...but coverage will increase to include all zones
during the afternoon as a shortwave trough transitions over the
area late Friday afternoon through Saturday morning. Thunderstorm
chances will continue through Saturday as models indicate enough
instability and model forecasts show the area under a favorable region
of the upper jet...with eastern zones having the best shot of
seeing any development. Without any apparent trigger at this time...its
possible cloud coverage may keep things too stable for storms to
trigger so may need to adjust probability of precipitation in the coming days as forecast
confidence regarding this increases. More of the same on Sunday as
there is minimal change in the overall setup...however there is
some disagreement among operational models whether or not a second
shortwave will cross the region...which will would be the
difference between scattered coverage areawide or minimal coverage
with a focus over the ridges. Temperatures will be at to slightly
below seasonal average through the weekend.
Long term /Saturday night through Wednesday/...
mainly dry and mild conditions can be expected through the first
half of the week as the longwave pattern becomes more zonal and dry
west/northwest flow sets up across the region. Operational models
vary greatly on the idea of showers/thunderstorms returning by
midweek so have included a slight chance on Wednesday...increasing
on Thursday based on ensemble guidance.
Aviation /13z Thursday through Monday/...
some IFR stratus and fog will lift by 14z. From kbvi to kduj. Broad
upper level trough swings through the today with enough moisture
available to generate some isolated showers mainly north of kpit
into early afternoon.
Next restrictions could come overnight tonight into Friday as the
high clouds from the next system could be slow to arrive.
Additional light fog development is expected.
Outlook.../Thursday night through Monday/...
a low pressure system over the weekend could bring additional