Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
603 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/...
relatively flat cumulus has managed to develop across the region
today...while the more vertically enhanced variety has remained to
the north of Lake Erie for the most part. As the middle and upper
trough that has resided over the central Great Lakes through the
day translates eastward overnight...the colder middle-level
temperatures should likewise move eastward as well. It is in this
middle-level cold pool that shower activity has generally manifest
itself this afternoon...and given the trajectory of this air...it
would seem the bulk of this activity should remain north of our
Model guidance is in fairly good agreement in terms of tracking a
jet streak from the western Great Lakes and across the upper Ohio
Valley tonight. However...the simple presence of the right
entrance region of an upper jet streak is not sufficient for
shower development. Because of the fact that radar is devoid of
precipitation in the right entrance region of the jet streak
currently in Wisconsin...and model middle-level relative humidity values average
around 20 percent overnight...and that instability will be waning
greatly by its arrival in our area...shower chances were removed
from the forecast overnight. It would seem the best shot at any
clouds overnight will come from transient cirrus currently over
Illinois...which trajectories indicate should sag southeastward
generally south of I-70 overnight. Fries
Previous discussion for Wednesday and Wednesday night...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 /22z 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
gregs knob National oceanic and atmospheric administration weather radio is off the air with a failed