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623
FXUS61 KPBZ 282200
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
600 PM EDT Wed Sep 28 2016

.SYNOPSIS...
Upper level low pressure will bring rain to our region for the
rest of the week.

&&

.NEAR TERM /THROUGH THURSDAY/...
Southeasterly flow will continue to increase over the area as
moisture starts to push northwestward toward the ridges from the
Mid-Atlantic coast. An upper low poised to our west will move
exceedingly little save for a bit of a southerly drift with the
main change likely to be the tilt in the upper level system from
positive to neutral. The change in the tilt of the system will
allow for added upper level diffluence over western Pennsylvania
later this evening, which should allow for shower development to
spread northeastward from central Ohio toward northwestern
Pennsylvania.

Increasing southeasterly flow is likely to duke it out with
dynamical developments aloft for areas generally from Pittsburgh
southward to the west of the ridges overnight. With increasing
downslope flow off the ridges and dry air in place, the chances of
rain should be exceedingly low. However, an influx of middle and
upper level moisture coupled with the development of the ridge
entrance region of the jet will make precluding rainfall very
difficult. Thus, through the night, while the chances of rainfall
in Ohio, along I-80, and in the ridges become/remain very high,
from Pittsburgh southward to the west of the ridges, activity will
likely be much more scattered.

Additionally, the increase in momentum off the surface as a result
of the southward sag of the low should preclude inversion
formation overnight to a degree. This will be particularly true
in the ridges and their immediate lee. While soundings suggest the
unstable layer should exist solely below the ridgetop high, gusts
will be possible generally from Fayette County northward in the
lee of the ridges. Momentum profiles do increase overnight,
however the best momentum and profile for wind gusts looks to hold
off until Thursday night/Friday morning, when wind advisories may
be necessary. Fries

&&

.SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
The aforementioned low is forecast to briefly stall over the
Middle Ohio River Valley on Thursday, before lifting northward and
filling once again through the weekend. The concern for the period
is for Thursday when an Atlantic moisture tap is forecast to
stream across I 68 through Garrett County and over areas generally
along and north of the PA turnpike, and deepen the overall
moisture supply in the column.

Currently-progged rain-supporting dynamics for that period are
limited given the lack of convergence/mid level frontogenetical
forcing in this mature stage of the low. The better probability
for heavy rain off the immediate eastern upslope areas should thus
be convection based and instability is not impressive.

Have used general River Forecast Center QPF for todays prog of
the event in conjunction with WPC QPF probability maps. The bottom
line is no flash flood watch until precursor rainfall tonight at
least defines a better threat area in conjunction with moisture
convergence into the occluded frontal zone.

As the Atlantic moisture tap in interrupted by Friday, the sustained
rainfall threat will be alleviated with more scattered convection
providing the rain chances.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
The low is forecast to lift northward and fill through the weekend
with dry flow returning and diminishing rain chances for the
region, and supporting seasonably warm temperature.

&&

.AVIATION /22Z WEDNESDAY THROUGH MONDAY/...
VFR will predominate into the evening, but deterioration can be
expected tonight and into Thursday as rain becomes likely with
moisture streaming into the eastern flank of the upper low.

.OUTLOOK /THURSDAY NIGHT THROUGH SUNDAY/...
Periodic restrictions can be expected into the weekend as the
upper level low lingers, then slow lifts north of the region.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
MD...None.
OH...None.
PA...None.
WV...None.

&&

$$