Area Forecast Discussion
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FXUS61 KPBZ 281006

Area Forecast Discussion
National Weather Service Pittsburgh PA
606 AM EDT Wed Sep 28 2016

An upper level low pressure system will bring rain to our region
for the second half of the week, with heavy rainfall totals
possible in some areas.


605 AM - Made some minor modifications to Sky/PoP fields for
radar and satellite trends, mainly to slow down increase later
this morning in southwest PA where downslope flow may slow
precipitation advance. Forecast mostly on track otherwise.

Previous discussion...

Main event this period is the advance of the deep upper low from
Lake Michigan this morning to near Louisville by 12Z Thursday.
Models are in good agreement with this general scenario. PoPs
today will increase from both the west and east with time. Steep
low-level lapse rates plus a ribbon of isentropic lift will help
to promote shower development in Ohio this afternoon, while
the development of moist onshore flow will help to increase
precipitation coverage along the ridges. The lowest PoPs end up
being in the south-central CWA, furthest from these influences.

Tonight, the loss of instability will allow shower coverage to
decrease across the southwestern CWA. However, continued moist
southeast flow and column saturation will necessitate likely PoPs
north of I-76. The NAM is more bullish with precipitation in the
eastern CWA as it develops a fairly significant deformation zone
and divergence aloft, while other models are more subdued
overnight for us, keeping the better forcing to the east. While
the exact NAM scenario is not expected, and the heavier rain
should remain to our east tonight, the general idea of an enhanced
band of precipitation cannot be ruled out.

Temperatures were adjusted to depict a bit less of a diurnal
range between today and tonight, given the moistening pattern.


Cut-off low pressure system will stall out near the middle Ohio
River Valley Thursday, pin-wheeling through the latter part of the
week. Initially kept pops a little lower across the southern half of
the forecast area, as a dry slot and strong downsloping flow will
keep showers scattered. Later Thursday afternoon, the low
retrogrades to our southwest, bringing a frontal zone over the area
that will result in more organized rain production.

Model guidance continues to depict a strong 850mb jet developing
through the forecast area Thursday night. While the bulk of this
50kt+ jet will remain just above the the inversion, some stronger
wind gusts should scrape the ridge tops, and potentially impact the
western downslope areas of the Laurel and Chestnut Ridges.
Additionally, this strong jet will transport a plume of anomalously
high PWATS over the area. This could bring efficient rainfall
production Thursday night into Friday morning, primarily to our
southeastern zones. Exact placement of the frontal zone and
subsequent rain production is still somewhat nebulous. This along
with dry antecedent conditions is tempering flash flood concerns at
the moment, but several inches of rain is possible over a 12 to 24
hour period.

Rain will linger through much of Friday, before drier air wrapping
around the system pushes into the southern zones. Diurnal
temperatures spreads will be relatively compressed under the
influence of the low. Highs will be near average values.


Stubborn low pressure system will begin to accelerate northward over
the early part of the weekend. As it does, light rain chances will
gradually cease with drier air continuing to feed in from the south.
Some differences exist in timing the departing low, but by early
next week, the low should be swept away by the upper-level jet.
Building high pressure will restore dry conditions will slightly
above average temperatures.


Still foresee mostly VFR conditions today, although clouds will
be on the increase as a strong upper level low pressure center
drops into the Ohio Valley. Showers are first expected by this
afternoon in southeast Ohio near the upper low, and along the
eastern ridges as easterly flow allows moisture to pool. Cannot
rule out an afternoon thunderstorm in southeast Ohio. MVFR
ceilings will develop tonight as moisture increases and showers
become a bit more widespread, perhaps along the eastern ridges
first due to upslope flow.

Periodic restrictions will remain through Saturday as the upper
level low lingers over the region.




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