993
FXUS61 KPBZ 250504
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
104 AM EDT Wed Sep 25 2024

.SYNOPSIS...
Scattered showers and a few thunderstorms will move across the
area today, ahead of a slow moving cold front. Unsettled weather
continues into the weekend, although no day looks like a
washout.

&&

.NEAR TERM /THROUGH TODAY/...
KEY MESSAGES:

- Lull in the activity overnight.
- Scattered showers and thunderstorms possible today.

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Lowered PoPs overnight as the main moisture plume, and
associated shortwave trough, has shifted eastward. Temperatures
remain mild overnight.

Area will be stuck between an upper low digging southward over
the midwest and an amplifying ridge just off the east coast. As
the 500mb low drifts southward, minor height rises are expected
here today. This will actually direct another moisture plume on
the eastern side of the upper low back over the region.
Additionally, another wave of energy rotating around the 500mb
low, will get caught up in the southwest flow aloft and move
northeastward with the plume. This will mean another day with
the threat for scattered showers and possibly a thunderstorm.
Locating the areas with the highest risk for rain today will be
predicated on where the moisture plume ends up and the track of
the shortwave. Seeing some disagreement in the model solutions
because of varying outcomes on the path of the large upper low
to the west and the amount of amplification of the Atlantic
ridge. Latest ensemble data if focusing higher QPF over the
eastern half of the region, while the latest CAM models are
pushing the main axis of QPF a bit further west. Will lean on
the NBM which is providing a solution closer to that of the GEFS
and EPS. Cloud cover, warm air advection aloft, and limited
instability will keep the risk for convection lowered today.

Temperatures today will again be above normal.

&&

.SHORT TERM /TONIGHT THROUGH FRIDAY/...
KEY MESSAGES:

- Decreasing rain chances Thursday and Friday.
- Still plenty of clouds and warm temperatures.

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As we keep an eye on the southward drifting 500mb low that will
sink from the midwest toward the lower Mississippi valley on
Thursday, another large upper low will slowly move across
southern Canada, just north of New York. The focus for the
southern stream energy will be how much of the moisture from
Helene will be absorbed into it, and the focus of the northern
stream of energy will be how much will it deflect this moisture
from the upper Ohio Valley. Models are actually in good
agreement that these two systems will not phase which will work
to limit rainfall potential through Friday.

A surface cold front, associated with a surface low that will
eventually be absorbed into the northern upper low, will cross
the region on Thursday. The front will work like a barrier to
the deep, tropical moisture streaming northward on the eastern
side of the inland upper low. As the plume of moisture
encounters the front and the northwest flow aloft on the
southwest flank of the Canadian low, it will be suppressed and
spread westward back toward the inland 500mb low. As this battle
is ongoing, the remnants of Helene will move inland over the
Gulf coast states and eventually the southeastern US and head
northward. However, the remnant low will then begin to turn
westward back toward the upper low keeping all of the tropical
rainfall well south of the region. Additionally, as the remnants
of Helene get absorbed into the inland upper low, a massive dry
slot will develop over a large portion of the eastern CONUS and
drive northward over our neck of the woods sometime Friday.

As always with such a complex setup, which is made even more so
by the intrusion of a tropical system, there remains
uncertainty in the details.

The pattern on Thursday, area resting between the two large
upper level systems and the surface cold front drifting south of
the region, would suggest that much of the day should be dry
with lower chances probabilities for rainfall over the far south
which will be closer to the stalling cold front.

The surface front will likely be shoved northward on Friday as
the northern 500mb low moves eastward over New England and the
remnants of Helene move over the Tennessee Valley. This could
lead to showers developing along the front and drifting
northward. However, the large dry slot will also be moving
northward over the east on Friday and drier air to our north,
advecting southward on the back side of the exiting northern
low, will work to decrease overall moisture associated with the
cold front. High levels of uncertainty for Friday with how far
north any potential showers will push and how long they will the
be sustained as drier air becomes more prevalent.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
KEY MESSAGES:

- Rain chances will continue through the weekend, but no day
  looks like a washout.
-------------------------------------------------------------------

Large upper low to the southwest, which by the weekend will have
completely absorbed the remnants of Helene, will remain
basically stationary and weaken as it gets squeezed between a
developing ridge over the Gulf of Mexico and another ridge over
the Great Lakes region. The area will likely remain under the
influence of the filling upper low, but this no sure bet. It
will keep our area unsettled through the weekend with the risk
for showers both Saturday and Sunday.

By Monday and Tuesday, the remnants of the upper low will open
up into more of a weak, but broad trough, that will eventually
deepen as energy dropping southeastward through western Great
Lakes phases with it. This would likely mean a continuation of
the unsettled pattern and cooler temperatures.

&&

.AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
Deteriorating conditions are expected early this morning with
areawide MVFR/IFR cigs expected by 12z. Light showers are
expected to become more widespread near dawn, with iso/sct thunderstorm
chances increasing in the afternoon. Cigs will begin to improve
after 18z, with an 80% chance of VFR at all terminals except
FKL/DUJ. Afternoon rain will be sporadic, favoring locations
northwest of PIT.

Cigs will once again lower Wednesday night, with fog development
also possible in areas that receive heavy rainfall.

.Outlook...
Widespread restrictions will linger into Thursday before VFR is
expected to end the week. Periodic rain chance is then expected
through the weekend.

&&

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

&&

$$

SYNOPSIS...WM/22
NEAR TERM...22
SHORT TERM...22
LONG TERM...22
AVIATION...Rackley