PBZ issues Area Forecast Discussion (AFD) at Sep 25, 7:42 PM EDT412
FXUS61 KPBZ 252342
AFDPBZ
Area Forecast Discussion
National Weather Service Pittsburgh PA
742 PM EDT Wed Sep 25 2024
.SYNOPSIS...
Gradual lowering of precipitation chances is expected through
Thursday before a potentially unsettled pattern develops Friday
into early next week.
&&
.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
KEY MESSAGES:
- Scattered showers and thunderstorms continue today.
-------------------------------------------------------------------
Evening update...The current conditions feature a few showers
tracking across the northern forecast area along with a swath of
rain trying to enter the northern WV counties. A lot of the
moisture is dissipating as it tracks north into the area. Will
keep pops across the area and through the night as the moisture
streams north. Have added a fog potential for the breaks in the
precip overnight. The rest of the forecast remains on track.
Previous Discussion...Broad ascent with weak embedded vortices
will promote scattered showers and low probability thunderstorms
through tonight ahead of a stalled closed low sitting over the
lower Ohio River Valley. A lull between waves (with brief
subsidence) is occurring as of 2pm, with an uptick in activity
likely between 4pm-10pm as the next shortwave crosses and the
weak surface front approaches from the west. Despite 30-40kts
effective shear, excessive cloud cover will minimize sfc heating
and keep area instability generally below 1000 J/kg and keep
lightning (let alone severe) risks low. Dry antecedent
conditions and weak convergence also should limit any flooding
concerns.
Overnight, the closed Ohio Valley low will begin sagging SW
toward the western TN while a northern trough shifts toward New
England. This will lend to a downward trend in convective
activity given than ideal position relative to various waves.
Little air mass change and excessive cloud cover will maintain
above average temperature.
&&
.SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY NIGHT/...
KEY MESSAGES:
- Mostly dry with seasonable temperature Thursday
- Potential for increasing influence from Hurricane Helene
-------------------------------------------------------------------
Confidence is high in drier weather developing Thursday as the
closed low and remnants of Hurricane Helene remaining well to
the south and Nova Scotia troughing displaced northeast. The
moist airmass will be relatively unchanged, lending to mostly
cloudy skies and lower diurnal fluctuations.
Variation in outcomes increase starting Friday and are largely
defined by the positioning/movement of Hurricane Helene remnants
in conjunction with the west TN closed 500mb flow. Ensemble
means suggest rain chances starting Friday morning south of
Pittsburgh with the initial outer bands of the large area of
low pressure. This assumes the surface low reaching the western
NC/SC border Friday morning and lifting towards eastern KY by
mid-day. Other higher-resolution model forecasts are slower and
farther west with the low track, which favor the outer bands
staying south of the region. Current forecast represents a blend
of the two with higher/lower PoPs south of Pittsburgh possible
in scenario 1/2, respectively.
Low positioning will also influence surface gradient and the
onset of SE wind increases (remaining well below advisory
criteria). Confidence is high for slightly above average
temperature continuing while cloud cover remains plentiful.
&&
.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
KEY MESSAGES:
- Rain chances will continue through the weekend, but no day
looks like a washout.
-------------------------------------------------------------------
Ensembles are in fairly good agreement in the broad idea of
continued interaction between the TN closed low and Hurricane
Helene remnants Saturday into Monday. The combined low area will
be fairly stationary over the TN River Valley Saturday and
slowly weaken before drifting SE into early next week. Expect
periodic rain chances, abundant cloud cover, and above normal
temperature during this period. Despite the agreement, large
variations exist in the finer scale details of mesoscale
banding, jet placement, and low positioning that will greatly
determine timing/coverage/intensities of rain showers. Though
any given period could see showers, the limitations of
resolution for global models and general pattern recognition
suggest dry conditions are more likely than not with smaller
windows of rain that the forecast lends itself to.
Ensemble means favor a northern stream trough kicking the
sagging low out of the region by mid-week and ushering an
airmass change. That said, scenarios remain on the sagging low
staying farther south while ridging develops near the Great
Lakes; this would foster drier and warmer than forecasted
conditions.
&&
.AVIATION /00Z THURSDAY THROUGH MONDAY/...
Mainly VFR is expected this evening, with lingering MVFR at FKL
and DUJ, ahead of another shortwave embedded in SW flow on the
eastern side of an upper low across the Midwest. Expect
occasional showers to move across the area through overnight as
this next wave crosses the region. Deterioration to MVFR is
expected as the lower levels become saturated. Moisture pooling
around a weakening surface boundary should result in IFR at ZZV,
FKL and DUJ.
Rain should diminish Thursday as the shortwave exits. A gradual
improvement to VFR is expected by mid to late afternoon as
mixing results in an increase in ceiling heights.
.Outlook...
Light rain and restrictions are possible Friday, mainly south of
FKL-DUJ, as moisture streams north ahead of tropical system
Helene. VFR is expected Friday night and early Saturday before
an upper low spreads rain and restriction chances back across
the region late Saturday through early Monday.
&&
.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...None.
OH...None.
WV...None.
&&
$$
SYNOPSIS...Frazier
NEAR TERM...Frazier/Shallenberger
SHORT TERM...Frazier
LONG TERM...Frazier
AVIATION...WM
Source:
PBZ issues Area Forecast Discussion (AFD) at Sep 25, 7:42 PM EDT---------------
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