Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Pittsburgh PA
949 PM EDT Thursday Aug 21 2014
unsettled weather will continue through the weekend with
occasional showers and thunderstorms.
Near term /through Friday/...
945pm update...spotty showers across the north. Keeping an eye on
large mesoscale convective system that has rapidly developed over Indiana. Using
forecasted 300-700mb thickness...this complex should just skirt to
the west of my area tonight. Its possible that an outflow from
storms within the complex could spread eastward and promote some
development over the far west...because of this have included
higher probability of precipitation there. Temperatures adjusted to latest lamp numbers.
Showers and thunderstorms supported by upper level
wave is now moving across the upper Ohio Valley. Added likely probability of precipitation
to the north as upper wave passes through in the afternoon with
coverage of convection expected to increase. Visible satellite
imagery shows ample cloud coverage across Pittsburgh and south.
Hence...optimal daytime heating will not occur in that
region...however...increasing moisture will aid in instability
yielding showers and thunderstorms. To the north...sufficient
heating due to less clouds have ramped up instability. Rap
analysis shows relatively stronger shear in this zone as well.
Therefore...isolated strong to marginally severe storms are
possible this afternoon north of Pittsburgh.
Greater coverage of convection is expected to wane later in the
evening due to loss of heating. Some showers may still linger
across the area overnight due to the nearly stationary surface
Short term /Friday night through Sunday/...
as the next shortwave crosses the region from a weakening and
exiting upper low...axis of heavy rainfall will shift east on
Friday as the boundary begins to slowly move. Uncertainty still
exists for any flooding as models are in minor disagreement for
the placement of heavy rain. Used model blend of GFS/NAM for probability of precipitation
and HPC for quantitative precipitation forecast.
Upper level ridge develops through the weekend bringing slightly
warmer than normal temperatures. A few shortwaves topping the ridge
brings the chance of showers and thunderstorms...at least for
Saturday but overall coverage should be small.
Long term /Sunday night through Thursday/...
more clarity exists in the extended forecast with the latest round
of model guidance. Upper ridge building north 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. Looking a little farther out in time than usual...no effects
are expected locally from anything that may develop in the tropics.
Aviation /02z Friday through Tuesday/...
low confidence forecast overnight with the risk for MVFR/IFR fog
and ceilings developing overnight. Still seeing some areas of MVFR
ceilings that never left from earlier in the day. Will go with general
VFR early then deteriorate conditions later. Showers/storms will
move back in Friday morning.
Outlook.../Friday through Tuesday/...
several crossing disturbances will cause periodic restrictions into
the weekend...with VFR conditions expected Monday and Tuesday.