Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
443 am EDT Thursday Aug 21 2014
occasional showers and thunderstorms can be expected through the
weekend as a series of disturbances move through the region.
Near term /through tonight/...
isolated rain showers finally exiting the eastern County Warning Area border along with vorticity
maximum. Fog being held in check a bit by mixing at the top of the
boundary layer. Still think patchy dense fog is possible...
especially in valleys...given the wet ground and mostly clear
Frontal boundary will continue to hang around the County Warning Area today and
tonight...although it will get a slight northward push later in this
period. Deeper moisture makes a run back to the east...with the 1.5
inch precipitable water line getting close to pit this afternoon. Expect some
pockets of up to 2500 j/kg of cape to develop...which will once
again allow for diurnal storm development. 0-6km bulk shear of
30-40 knots may be just enough to allow for some strong to
marginally severe storms...although organization will remain low.
Expecting higher coverage along/south of boundary with the juicier
airmass...and have the likely probability of precipitation targeted in this area. Chance
probability of precipitation are forecast in the fkl/duj far northern region.
Despite all this...am not sold enough on threat of heavy
rainfall/flooding to hoist any flood watches. Best organization and
rainfall potential will lie off to the south and west...where
isentropic upglide along with h250 upper divergence in rrq of modest
jet will allow for enhanced lift...not to mention the 2 inch plus
precipitable water values. Think the wet areas in eastern Ohio will be able to
escape any further impacts today.
Tonight...activity will diurnally decrease and clouds will break
somewhat. Cannot rule out a shower anywhere overnight though given
the airmass and front. There are hints in the models that some sort
of convective cluster/mesoscale convective system may attempt to form and ride through the
Ohio Valley. Again...expect this to be mainly off to the
southwest...but could see the zzv vicinity get clipped around 12z.
Will need to keep an eye on this.
Near or slightly above normal temperatures continue to be forecast.
Short term /Friday through Friday night/...
weak warm front lifts northeast early Friday...allowing copious deep
moisture to shift further east reaching the western edge of the forecast
area by daybreak. A surface low pressure system will slide along the
boundary Friday...supported aloft by a relatively weak 700 mb-500 mb SW trough.
Showers and thunderstorms should overspread a majority of the County Warning Area by
afternoon. Wind profiles are supportive of a few stronger storms
with roughly 30kts of 0-6km bulk shear...however...residual cloud
cover ahead of the system should act to keep surface temperatures and
ultimately instability limited.
Even with severe potential limited...showers and thunderstorms will
be able to tap into a warm and humid air mass with precipitable water values at or
just above 2"...values approaching 2sd above the mean.
Additionally...freezing levels should be nearing 13kft allowing for
impressive warm cloud layer depths. Moderate to heavy rain showers
could produce local water problems...especially in those areas
already saturated from Tuesday/Wednesday rains. Will continue to mention the
heavy rain threat in the severe weather potential statement Friday.
Temperatures Friday will be highly dependent on timing of wave and resultant
cloud cover as well as how far northeast the warm front advances.
Because of these factors...large spread exists across the MOS
guidance. Knocked down high temperatures Friday a degree or two...but tried to
stay closer to the warmer mav values as any breaks in the clouds
could shoot temperatures quickly up under the still potent August
Long term /Saturday through Wednesday/...
more clarity exists in the extended forecast with the latest round
of model guidance. By sun the surface low from the early weekend deepens
over the Piedmont of Virginia/NC pulling the deeper moisture south with
it. In the meantime...the upper ridge currently setting up over the
southern MS valley will build north further with its axis shifting east
through sun. This will work to deflect an advancing low pressure
system well north of the Great Lakes early next week...resulting in
a warming and drying trend. Temperatures should remain at or even
just above average into the middle of next week.
Aviation /09z Thursday through Monday/...
showers/storms have exited...with quiet weather overnight.
Clearing will allow fog to develop over the wet ground. River
valleys will get the thickest fog...although all terminals should
see it to some degree.
Fog lifts by 14z...with VFR cumulus developing. Best precipitation chances
will be across northern West Virginia and southeast Ohio this
afternoon...and thus prevailing rain showers were included at zzv and mgw
for a period. Thunderstorms in the vicinity to the north of there...while fkl/duj have the
least chance of getting wet. Any thunderstorms and rain will be capable of brief IFR
of course. Activity should die down again by the late evening
Outlook.../Thursday through Sunday/...
several crossing disturbances will cause periodic restrictions
into the weekend.