513
FXUS61 KPBZ 271215
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
815 AM EDT Fri Sep 27 2024

.SYNOPSIS...
Broad low pressure over the Tennessee River Valley and influence
from Hurricane Helene remnants will create periodic rain
chances through Monday. Gusty wind is likely this afternoon and
evening.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
KEY MESSAGES:

- Wind Advisory in effect for a portion of the ridge counties.
- Light rain showers expected to spread northward this
  afternoon.

-------------------------------------------------------------------

The remnants of Helene will rapidly phase with a large and deep
upper low over the TN valley this morning. Moisture north of
Helene will surge northward before the phase. Initially this
should just lower cloud cover as most of the rainfall will
rotate westward around the stacked system. A plume of Atlantic
moisture will develop later this morning and spread across the
region during the afternoon. A shortwave trough, rotating around
the eastern side of the upper low, will cross with the Atlantic
moisture. This will spread an area of light rain showers over
the region from south to north during the afternoon. This rain
looks to be light and short lived as a large dry slot, surging
northward east of the upper low, will eat away at the Atlantic
moisture plume causing much of the light rain to dissipate as
evening approaches.

The biggest concern today will be the increasing winds over the
ridges this afternoon and evening and the resulting downsloping
effects. Latest HREF probs of gusts >45mph are above 80% over a
large portion of the ridges. Bufkit soundings are showing 50 to
60kts just above the ridge tops later this afternoon. Will be
expanding the wind advisory to include the rest of the WV ridge
counties and will extend it later into the evening.

Temperatures will remain above normal today.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SUNDAY/...
KEY MESSAGES:

- Wind gusts will decrease tonight.
- Light rain from Helene may pass through portions of the
  region.

-------------------------------------------------------------------

Light rain cannot be completely ruled out Friday night and
Saturday. However, we will remain under the influence of the
upper level dry slot, at least through Saturday morning, and
most of the best forcing will be situated close to the upper
low.

Clouds will continue to linger with outflow associated with
Helene into Saturday afternoon.

The upper low will fill and meander northeastward on Sunday.
Rain chances will increase Sunday as waves of energy, rotating
around the low, cross the region. However, moderate uncertainty
remains in the exact track of the upper low. Most of the rain
Sunday looks light with probs of >0.25 sitting at around 40%.

Temperatures will trend above average (by 10 degrees) through
this time period with deep southerly flow.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
KEY MESSAGES:

- Rain chances will be dependent on how quickly the closer upper
  low transitions eastward Monday
- Ensembles favor a trough kicker to arrive Tuesday into
  Wednesday, with dry and seasonable weather developing
  thereafter
-------------------------------------------------------------------

Latest model trends favor a gradual eastward track of the closed
upper low that will continue to weaken on Monday. Subtle low
positioning will be key in determining areas of greater forcing,
but means values suggests rainfall chances trend eastward
Monday.

Ensembles favor the weakening area of low pressure to shift
toward the Atlantic coast Tuesday into Wednesday morning thanks
to a kicker upper trough diving out of central Canada. This
feature would likely offer a last chance for precipitation ahead
of a surface cold front before a drier and more seasonable
airmass develops. There are scenarios where the broad low
pressure sags south rather than moving east while Great Lakes
ridging develop mid week; if this plays out, look for rising
temperature trends and potentially a reintroduction of low
probability afternoon thunderstorms.

&&

.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
An approaching light rain band associated with an outer band of
Helene will lift across the region this morning, bringing non-
restrictive light rain and at least low VFR ceilings. Some
guidance, notably LAMP, wants to generate MVFR ceilings in this
rain, but HREF is more skeptical of this, and upstream obs are
VFR for the most part. Kept MVFR ceilings for a period this
afternoon, with low confidence. Low VFR ceilings should return
this afternoon as the rain tapers.


A tightening pressure gradient will result in gusty east wind
today. Gusts from 20-25 kt are likely for most locations, with
gusts to 30kt with some downsloping at LBE. Wind will decouple
this evening somewhat and diminish at the surface, but with
mixing still likely to continue in the column, lingering low-
level moisture is more likely to manifest as low MVFR/IFR
ceilings tonight as opposed to fog. Depending on when decoupling
occurs and the departure of the low-level jet, a period of low-
level wind shear will be possible at least during the late
evening/early overnight hours, but confidence in this is too low
for inclusion at this time.

.Outlook...
Occasional restrictions and light rain are likely Sunday
through early Tuesday as low pressure drifts across the Upper
Ohio Valley region.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...None.
OH...None.
WV...Wind Advisory from noon today to 10 PM EDT this evening for
     WVZ510>514.

&&

$$

SYNOPSIS...Frazier
NEAR TERM...22
SHORT TERM...22
LONG TERM...Frazier
AVIATION...CL