Area Forecast Discussion
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FXUS61 KPBZ 250533

Area Forecast Discussion
National Weather Service Pittsburgh PA
133 AM EDT Thu May 25 2017

Slow moving low pressure will keep rain in the forecast through
early Friday. Rain chances return for part of the weekend with
low pressure.


Ascent with increasing shortwave support ahead of approaching
low pressure, and increasing moisture should continue to result
in an increase in showers again overnight. Gusty SE winds
around 25kt should continue along the ridges with elevated low
level flow and a temperature inversion. Temperatures are
expected to only drop a couple of degrees from current levels
with the cloud cover and rain.


Widespread morning rain will subside early in the day as a
closed upper low deepens in the Ohio Valley and a dry slot
asserts itself. As the moist boundary layer in the wake of the
morning rain has a brief window to heat, cold air will be
advected aloft, leading to steepening mid-level lapse rates and
increasing instability despite only a minimal change in surface

Convection-allowing models suggest that showers and storms will
be generated quickly in this increasingly-unstable airmass, with
a cluster of storms expected to fill in the dry slot by mid-
afternoon Thu. Instability is not expected to be sufficient for
organized severe weather, but moderate rain rates will be
possible in any storms.

By Friday, the upper low will have shifted toward New England
with northwesterly flow in control and drier air arriving. This
respite will be rather short-lived, however, as the H5 ridge
axis will shift through the area by Saturday with deep swly
flow becoming established. This shift will allow increasingly-
humid and warmer air to arrive for the weekend. However, it
will also bring an increasing chance for showers and


The pattern going into next week looks to continue to amplify as
an upper low again drops into the Mississippi Valley with a
downstream ridge popping up rapidly along the eastern seaboard.
The Ohio Valley will be stuck between these two areas, with
broad southwesterly flow remaining in place and abundant moisture
streaming toward the area. As a result, chances for convection will
remain in the forecast with temperature near normal.

Of interest in the long term is that synoptic models are
offering hints at a series of MCSs developing in the Plains and
moving ewd toward the Ohio Valley along the nrn edge of the
instability axis. These MCSs could impact the region Saturday
and Sunday during a busy holiday weekend, so anyone with outdoor
plans should monitor forecasts toward the end of the week.


An elongated slug of deeper moisture is approaching from the
south and moving into MGW as of 06z. This should overspread most
other sites in the next couple of hours east of ZZV. All sites
should deterioriate to at least MVFR as saturation overcomes the
effects of SE downslope flow off the ridges. Farther north,
FKL/DUJ should be quickly mired in IFR conditions for the
remainder of the night.

Atlantic moisture fetch is cut off on Thursday as low level flow
turns SW. This should shunt low level saturation progressively
NEward, allowing for MVFR then VFR conditions to return to most
sites with scattered showers still about as the upper low
settles overhead. However, as it moves NEward Thursday evening,
low level moisture will quickly wrap back in and trend cigs
decidedly downward into Thursday night. Fries

Showers continue through much of the period with episodic




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