Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
750 am EDT Sat Jul 26 2014
the risk for showers and storms will return this weekend. The main
focus will be tonight and Sunday. Showers will continue on Monday
with cooler air arriving Tuesday.
Near term /until 6 PM this evening/...
minor changes to sky cover for the Erly morning updt based on satellite/near
term model trends.
Previous disc...shortwave trough will rush eastward this
morning...allowing for a reduction in cloud cover. Upper level
heights will respond with some ridging aloft. Warm air advection will ensue with
the rising heights and a southwesterly flow in the lower levels.
Current model runs are not showing much activity...if any today.
The aforementioned height rises will stall and leave the area
under northwest flow aloft. Upper level waves this afternoon will
remain west of the region...as will the real unstable air. A
frontal boundary... connected to a surface low over the
Midwest...will stretch across Lake Erie this afternoon. Its
possible that a shower or storm could develop late today...but
confidence is low. Model soundings are showing a pretty well
capped atmosphere with unimpressive shear values.
Clouds will increase again this afternoon as another upper level
wave rushes through.
Short term /6 PM this evening through Monday/...
activity should begin to increase tonight as the surface low
continues eastward into Illinois and atmosphere destabilizes. Models are
showing a nocturnal mesoscale convective system developing near the surface low tonight and
heading southeastward with the main upper level wave. Spokes of
energy will break away from the main shortwave and drift eastward
over western and southern portions of the area. Expect activity to
develop with these waves...but again...they will be moving into
more stable air. Shear values will increase tonight so if activity
gets rolling it should be sustainable. I would think that the
strongest activity would be west with the main mesoscale convective system.
Sunday sure looks like the day that will be most worrisome.
Atmosphere will continue its destabilization as the surface low
moves into western Ohio. A very buoyant atmosphere with strong
moisture convergence and shear will provide a threat for severe
weather. The southern half of the area seems to be particularly in
the bulls-eye. If models are correct...another mesoscale convective system should quickly
develop over western Ohio Sunday morning and move eastward with
the surface low. Widespread wind damage is possible if the mesoscale convective system
develops. Additionally...latest forecast hodographs are showing
helicity values reaching near 500 in the south. This will
increase the threat for tornado development. Heavy downpours are
also possible as precipitable waters reach close to 2 inches. Again...Sunday
afternoon will need to be closely watched...with the highest
threat across the south.
Complex will exit late Sunday after/early evening and the atmosphere
will begin to calm down. Another upper level disturbance will
cross late Sunday night/Monday morning...keeping in the risk for
showers and possibly a storm.
A secondary cold front...spinning around behind the upper level
disturbance crosses Monday afternoon. So showers will continue
with the risk of a storm as the boundary swings through.
Long term /Monday night through Friday/...
showers will end Monday night as entire system quickly moves
High pressure will provide a cool and comfortable midweek. Another
weak cold front may bring showers Thursday.
Aviation /12z Saturday through Wednesday/...
VFR conds expd today with a mix of diurnal cumulus and ac. A weak shortwave
approaches this evening...but with limited instability and moisture only included
vcsh in the tafs. An mesoscale convective system will likely affect ports S of pit lt tonight
into Erly Sun morning. With timing/location uncertainties maintained
a thunderstorms in the vicinity mention in the taf at this time with MVFR conds.
Outlook.../Sunday through Wednesday/...
advancing low pressure will likely bring thunderstorm rstrns sun...with lingering
rstrns into Monday as it/S associated cold front crosses.