Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
415 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/...
shortwave trough will rush eastward this morning...ending the
showers and 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 /08z Saturday through Wednesday/...
VFR in the tafs...with caveats. Shortwave moving across through
10z or so is providing mainly middle clouds and a few
sprinkles/-shra. The rain showers will move out but the middle clouds will
remain through much of the daylight hours. Some scattered cumulus may form
underneath. Cannot rule out a few rain showers/thunderstorms and rain in daytime heating but
thought chances a bit low to include at this point.
Things get interesting towards the tail end of the taf
period...particularly across eastern Ohio and southern WV...where the
outskirts of an mesoscale convective system could roll through after 02z. Included thunderstorms in the vicinity to
hint at this...but occurrence/timing are still uncertain. Brief
IFR or worse would of course be possible in any thunderstorms and rain...along with
Outlook.../Saturday night through Wednesday/...
low pressure will likely bring shower/thunderstorm restrictions
late Sat night and sun. Additional restrictions are possible Monday
as the associated coldfront crosses.