Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
322 PM EDT Tuesday Aug 4 2015
generally dry and sunny conditions are expected through
Wednesday night. Rain chances increase on Thursday as an upper-
level disturbance approaches the region.
Near term /through Wednesday night/...
a trough at 500 mb will linger over eastern Canada today...maintaining
general westerly flow aloft in the Great Lakes region. High pressure at
the surface will build eastward from Indiana/Ohio and will aid in
drying the low levels for most of the forecast area and mostly
sunny-partly cloudy conditions for the day.
The primary exception will be in the northern zones...where a secondary
frontal surge overnight will be preceded by a plume of enhanced
low-level moisture...evident this afternoon by the more robust cumulus
field present. In this area...models indicate existence of modest
instability and generate light precipitation. With the three
ingredients for convection present...probability of precipitation were increased to chance
category overnight as the frontal surge translates southeastward.
Amid continued high pressure...Wednesday will feature considerable
sun in the morning...giving way to partly cloudy conditions as
cold air advection and strong heating yield a broad stratocu
field. Dry air and high pressure justify eliminating probability of precipitation almost
everywhere Wednesday and Wednesday night.
Maxima will be slightly below normal with the presence of the
unseasonably cool upper trough...and minima also below normal
owing to dry air and generally efficient radiational cooling.
Short term /Thursday through Friday night/...
deep shortwave trough will dig into the Midwest Thursday morning.
A surface low will accompany the upper-level system and move
across Missouri into Illinois. Still some significant differences
between different model depictions. 12z NAM is the fastest and has
the highest precipitation amounts...but this is likely due to
unrealistic vertical lift. 12z GFS seemed to be the dry
outlier with the bulk of the system remaining to the south...but
the 12z European model (ecmwf) came in farther south than its previous run. Have
still bumped up probability of precipitation to categorical across the southeastern
ridges...but have made a wider gradient overall for probability of precipitation from
north to south. North of Interstate 80...only some slight chance
probability of precipitation were included. Model soundings still show no instability to
work with...so have kept thunder out of the forecast. Bulk of
rain should be east of the region by Friday morning...with
conditions clearing out Friday evening. With extensive cloud
cover...temperatures will be a few degrees cooler.
Long term /Saturday through Tuesday/...
models still trying to determine a solution for a surface wave
which will move out of the Midwest and cross the middle-Atlantic late
this week and into the first half of the weekend. After this
point...a low confidence forecast due to model discrepancies in
timing of shortwaves in the northwest flow aloft.
Aviation /19z Tuesday through Sunday/...
high pressure at the surface will continue the drying trend
currently observed at all terminals...leading to a VFR forecast for
all tafs. Amid deep mixing...west-northwesterly wind will increase and become
gusty to 15-20 knots this afternoon.
A plume of moisture ahead of a secondary frontal surge may
generate showers at kfkl/kduj overnight tonight...but VFR
conditions otherwise are expected to persist through Wednesday
night at all terminals.
Outlook...Wednesday night through Sunday...
the next chance for restrictions is expected with a Thursday
kpbz radar remains down. Technicians advise that a return to
service is expected late this afternoon.
Gregs knob National oceanic and atmospheric administration weather radio is off the air with a failed
transmitter. Restoration is expected within 24 hours.