528
FXUS61 KPBZ 121111
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
711 AM EDT Sat Sep 12 2026

.WHAT HAS CHANGED...
The Flood Watch has been expanded to the rest of the forecast
area. Isolated severe storm threats also now encompass more of
the area today.

&&

.KEY MESSAGES...
1) Widespread rain and isolated thunderstorms expected on Saturday
with flooding and isolated damaging gusts possible

2) Still benchley for severe thunderstorms Sunday afternoon and
evening with a cold front, but the level of the threat is
uncertain

3) A brief bout of dry weather returns early next week

&&

.DISCUSSION...
KEY MESSAGE 1...
A renewed push of moisture is forecast today in strengthening
south/southwest flow. This is occurring ahead of a shortwave
trough that is riding up the Ohio Valley and will cross our
region during the late afternoon and evening. Expected
precipitable water values of 1.8 to 2.0 inches during this
period represent values above the 98th percentile areawide. With
initial high clouds providing only modest hindrance to heating,
no capping of note on model soundings, and increasing support
for lift from warm advection and the approaching wave, CAMs
suggest that scattered showers and thunderstorms may begin to
fire during the late morning and early afternoon hours. The
environment will already be supportive of warm rain processes at
that point given the increasing PWAT, increasingly saturated
column, and rising warm cloud depths.

As the afternoon wears on, these factors continue to improve,
and a more robust shield of rain and embedded thunderstorms is
likely as the shortwave approaches. Areas east of Pittsburgh
may see enhanced low-level convergence as strengthening
south/southwest flow butts up against southeast flow just to our
east as surface high pressure moves further away from the New
England coast. This could locally enhance rainfall rates to
concerning levels. HREF probabilities of greater than 1" are
generally 60 to 90 percent in the previous Flood Watch areas,
with maximum 24-hour rainfall totals ending at 06Z Sunday
showing several 3-5" bullseyes. 24-hour LPPM values of 2-4"
focus especially on areas northeast of Pittsburgh. Such totals
represent the high-end scenario, but the environment is
certainly capable of supporting them. While these areas have the
highest risk of flooding, elected to expand the Flood Watch to
the rest of the CWA. Much of the CWA has soil moisture levels in
excess of 50 percent according to MRMS, including portions of
eastern OH/northwest PA not previously covered by the Watch.
Although low-level forcing may not be as robust as areas to the
east, the same favorable environmental parameters for heavy rain
(high PWAT/saturated soundings/etc) are present, and at least
isolated flooding issues are possible. WPC has expanded the
Slight Risk for excessive rainfall to virtually all of the
forecast area.

In addition to the flooding threat, a window for strong to
isolated severe thunderstorms exists. Joint 10km neighborhood
probabilities of 1000 J/kg of SBCAPE and 30 knots of 0-6km shear
reach the 60-80 percent level across much of the area during
the noon to 6 PM period. While mid- level lapse rates will be
poor and soundings eventually lead to tall and skinny CAPE
profiles, there may be enough storm organization and steep low-
level lapse rates to produce damaging wind potential, with
small-scale bowing structures possible. Even a tornado cannot be
totally ruled out, thanks to the low-level southeast flow noted
above allowing for backed surface winds.

The heavy rain threat should begin to wane during the late
evening/midnight period as the shortwave crosses, leaving some
scattered showers in its wake. Lingering low-level moisture
should lead to abundant stratus coverage by Sunday morning.

KEY MESSAGE 2...
Another severe storm threat could emerge on Sunday, as
developing Great Lakes troughing drops a surface cold front into
our region during the late afternoon and evening hours. The
trough will provide sufficient deep shear on the order of 35-45
knots, with higher values closer to the lakes. Moisture
certainly remains present with surface dewpoints remaining in
the upper 60s to around 70. However, there remains some question
as to the level of clearing and thus destabilization that can
take place, as ensembles continue to suggest a range of sky
cover outcomes. This results in 25th/75th percentile SBCAPE
spreads of below 500 J/kg to above 1100-1200 J/kg. Also, the
subsident warm layer at 800mb noted by the previous shift
continues to show up in some HREF members, but not the RRFS.
Thus, uncertainty in the level of severe threat continues.
However, the front and the level of shear do suggest that even
lower-end CAPE levels may not be sufficient to totally suppress
convection, and some CAMs do show at least widely scattered
storm coverage during frontal passage. Given the shear, any
storm will be capable of marginally severe wind and hail
threats. A more widespread threat could emerge if later runs
show better potential for destabilization and a lack of capping.

KEY MESSAGE 3...
Minor timing issues remain, but the trough axis is still slated
to cross the region at some point Monday. At least scattered
showers will likely linger through Sunday night and perhaps into
Monday morning, before mid-level ridging provides a dry
interlude by Monday afternoon. This break may last until
Tuesday night or Wednesday, depending on the progress of the
next front.

&&

.AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
Patchy MVFR cigs are possible at DUJ by mid morning with
increasing moisture in SE flow. Otherwise, VFR is expected
through early afternoon, though increasing cumulus and mid level
clouds are expected ahead of an approaching shortwave trough.
Expect a broken line of thunderstorms to develop as the
shortwave trough approaches this afternoon and evening. Included
a tempo for the most likely time to see thunderstorms. IFR and
heavy rain are expected in some of these storms.

Expect MVFR in showers for 3 or 4 hours after the storms exit
late this evening and overnight. IFR in fog and stratus is then
expected to develop tonight with low level moisture in place.

Outlook...
IFR should continue into Sunday morning before a gradual
improvement to VFR by afternoon. A crossing cold front will
bring scattered thunderstorm chances to the area late Sunday
afternoon and evening. Outside of morning valley fog, VFR
returns Monday and Tuesday under high pressure. Shower and
thunderstorm chances increase from N-S on Wednesday with an
approaching cold front.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...Flood Watch from noon EDT today through late tonight for
     PAZ007>009-013>016-020>022-029-031-073>078.
OH...Flood Watch from noon EDT today through late tonight for
     OHZ039>041-048>050-057>059-068-069.
WV...Flood Watch from noon EDT today through late tonight for
     WVZ001>004-012-021-509>514.

&&

$$

DISCUSSION...CL
AVIATION...WM