Area Forecast Discussion
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FXUS61 KPBZ 291040

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

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


615 AM - Updated PoP fields in accordance with radar trends and
HRRR run...with higher values near and north of Pittsburgh, and a
lull in activity along the southeast slopes. Coverage will
increase through the day as moisture deepens.

Previous discussion...

Models in good agreement in dropping upper low into western
Kentucky today and stalling it there through tonight. Several
issues to deal with during the near term period, including QPF,
thunderstorm potential, and wind gusts tonight.

Heavier rainfall totals to this point have occurred along the
eastern slopes in Garrett/Tucker Counties, as well as in a small
band from western Fayette into Allegheny, with 0.5 to 1.0 inch of
rain. Expect best focus for precip will shift to the north of
Pittsburgh and along the eastern slopes as the morning progresses,
due to shifting of best upper diffluence and continued upslope
flow. This is similar to HRRR depiction. Accordingly, lesser
coverage is foreseen for southeast Ohio into northern West
Virginia. Coverage should improve this afternoon in these areas as
a few breaks in the clouds allow for instability showers and a few
thunderstorms to develop. Cannot rule out a strong storm or two
across the south given the high levels of deep shear, and even
some small hail is possible. However, overall low levels of
instability should keep this threat isolated at best.

Still concerned about potential QPF levels along and just west of the
eastern ridges as the Atlantic moisture tap continues through
tonight. The period for heaviest rain appears to be from late this
afternoon through 06Z tonight when moisture is deepest. Considered
issuance of a small flood watch for several counties in our
southeast. Given the dry conditions before tonight, the relative
underperformance of QPF to this point, and the apparent lull in
activity this morning, have elected to hold off on issuance at
this time, thinking that expected rainfall will fall over a long
enough period to prevent widespread issues. Will continue the HWO
mention and monitor the situation closely.

Final issue to consider is potential wind gust issues tonight
along western slopes of the ridges. With potential stable layer
above ridgetops and winds oriented favorably with ridgelines,
there is the potential for strong downsloping winds. Kept gusts
below advisory level for now, but this will also need to be


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 will continue to affect our region through the
period. Most precipitation will occur north of PIT early in the
TAF period, but activity is expected to fill in through the day as
moisture increases. Southwest PA/northern WV will start with VFR
ceilings, but those locations should join the rest of the
terminals with MVFR conditions as time goes on. Brief IFR cigs/vis
remain possible in any of the heavier showers. Best chance of
afternoon thunderstorms lies south of PIT where some breaks in the
clouds/better instability are possible, and have included VCTS in
a few forecasts. The heaviest rain is expected from early evening
through 06Z or so. Another concern, especially tonight, will be
gusty winds along and just west of the ridges. Some gusts of 30-40
knots cannot be ruled out near LBE.

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




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