Area Forecast Discussion
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FXUS61 KPBZ 291425

Area Forecast Discussion
National Weather Service Pittsburgh PA
1025 AM EDT Thu Sep 29 2016

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


The late morning update feature the addition of a flash flood
watch for Eastern Preston, Tucker, and Garrett Counties, as well
as HWO severe mention for high shear/better instability potential
areas to the south and west of Pittsburgh.

Deep, mature, stacked low pressure continues to spin over the Mid
Ohio Valley with surface occluded front objectively analyzed as
curving across Central WV. The systems mid level dry slot as
shunted widespread rain toward, and north of the I 80 corridor
into the mid level convergence zone. The Atlantic moisture stream
is still expected to develop generally north of the PA turnpike,
so no changes were made to categorical POPs across those areas.

Convective development into Tucker, Garrett, and Preston counties
will provide the main heavy rain/high water threat into Friday
given flash flood guidance and upslope enhancement. Reduced cloud
cover as per the latest visible images will aid in destabilization
today. A flash flood watch was thus issued for those areas, especially
in light of the band of 2 to 3 inch rainfall which fell on the
eastern slopes last night.

In addition, expect convection along the aforementioned front to
become a player today with SPC marginal risk maintained, and
dependant on cloud cover/instability magnitude for areas south of
the PA turnpike. These areas will be closely monitored for severe
given the magnitude of deep layer shear on the eastern flank of
the low. A hazardous weather outlook mention was thus included for
this potential also.

The potential for high downslope wind gusts from the Morgantown-
to-Latrobe vicinities will also be monitored given inversion
levels and low level jet expectations, but current limitations
based on surface wind orientation as per development of the
surface low along the aforementioned front to the south have
provided some decision time.


Rain and gusty winds will linger through Saturday as the
nearly stationary mature low pressure system gradually fills. A
strong 60kt+ jet at 850mb is progged to stream across our southeast
ridges through Friday morning. Still some questions as to the
orthogonal flow and just how much of this wind will be able to
break to the surface, so at the moment, the near Advisory gusts
and HWO mention were maintained from previous forecast.

Strong downsloping flow will continue through the day Friday as the
parent low moves very little. This, along with the deep Atlantic
moisture feed and convergence zone lifting north will result in a
relative minima in shower activity just west of the ridges through
center of the forecast area. This rain shadow effect is reflected in
most operational model qpf fields and PoPs and WPC collaborated qpf
have been trimmed back accordingly.

As the low dislodges and slowly creeps northward Saturday, the dry
slot wrapping around the eastern edge of the circulation will push
into our area. This will bring an end to any organized rain bands,
leaving only scattered, light rain showers mainly across the north.


Lingering low pressure system will slowly drift over the southern
Great Lakes Sunday into Monday. While this will keep rain chances in
the forecast across the north, the aforementioned dry air will erode
residual moisture. By Tuesday, the low will finally move east of the
area, and high pressure will build into our west. This will bring a
return to drier and warmer conditions with temperatures returning to
above-average through the end of the week.


An upper low aloft will pull deeper moisture across the ports
today with low MVFR across northern terminals becoming IFR as
widespread showers move in late this morning. Elsewhere conditions
will deteriorate to MVFR or remain so through the day with showers
and isolated thunderstorms after 16Z. East winds will strengthen
with gusts over 20kts most locations through the afternoon.

Periodic restrictions can be expected into the weekend as the
upper level low lingers, then slowly lifts north of the region.


MD...Flash Flood Watch through Friday afternoon FOR MDZ001.
WV...Flash Flood Watch through Friday afternoon FOR WVZ512>514.


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