Area Forecast Discussion
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FXUS61 KPBZ 201731

Area Forecast Discussion
National Weather Service Pittsburgh PA
131 PM EDT Sun Aug 20 2017

High pressure will maintain dry and seasonably warm weather into
tonight; however, a progressive flow pattern will support rapidly
changing weather into midweek.


High pressure will complete a slide across the region tonight
with warm, moist advection escalating with the advance of a
shortwave out of the Midwest. Despite a capped surface layer,
the latest NAM, RAP, and high res soundings indicate varying
degrees of elevated instability with the predawn moisture
return. Although, overall lift/forcing will be weak, think there
is enough convergence indicated on top of the boundary layer to
support a slight shower chance on the van of the aforementioned
Midwestern shortwave.

Otherwise, overnight lows will fall either side of the averages
with the western zones a few degrees above as per rising


That shortwave is the lead disturbance in a series of impulses
which will escalate rain chances into Tuesday. As for eclipse
viewing on Monday, local conditions are not looking as favorable
as preveious as that lead impulse is expected to at least spawn
mid level cloudiness, and possibly some isolated convection
with its morning to early afternoon passage. The good news is
that at moment, weak shortwave ridging is timed to suppress
precip during the peak time of the event.

By Tuesday, strong low pressure is forecast to dig between the
James Bay and Great Lakes and pull a cold front toward the Ohio
Valley. Given model-world agreement, likely precip probabilities
have been maintained with ramped-up severe expectations as per
a strengthened pressure gradient and resultant wind profile. That
potential remains slight at moment as warm mid level temperature
will likely retard overall instability. Nevertheless, prudence
dictates inclusion in the Hazardous Weather Outlook, especially
in light of the projected boundary layer warmth and moisture.

Prefrontal trough passage is timed to end the severe threat by
Tuesday eve, with early Wednesday frontal passage heralding
cooler temperature for the remainder of the week.


Broad surface high pressure under high-amplitude Eastern CONUS
troughing is progged to support dry weather and sub average
temperature into the weekend.


High pressure will ensure VFR conditions through this evening. A
low, but non-zero, chance of fog development again in the pre-
dawn hours Monday. Any fog development should be confined to
main stem river valleys, not nearly as expansive and intense as
Sunday morning`s.

Flow will remain light and variable through the period.

Widespread restriction potential returns with a late Tue/early
Wed cold front.




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