998
FXUS61 KPBZ 162225
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
625 PM EDT Fri Aug 16 2024

.SYNOPSIS...
A slow moving upper level system will promote rounds of showers
and thunderstorms through Monday, with convection peaking Sunday
afternoon. There is a low probability for severe weather
Saturday afternoon and evening.

&&

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

- Trends suggest a lowering threat for severe weather this
  afternoon.

- Showers and thunderstorms will remain fairly scattered while
  tapering off into Saturday morning.

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The evening update feature adjusting pops and lower thunder
probs as lack of instability this afternoon and quickly waning
daytime heating is keeping convection for getting deeper. In
fact, showers are having a difficult time breaking the cap. Have
lowered pops and thunder chances through 00Z.

Previous discussion...Shortwave movement southeast of a well
developed upper level low over NW Wisconsin continues to promote
areas of showers and thunderstorms primarily along and south of
I-70 early this afternoon. Splitting flow aloft is aiding to
the development of a more northern stream area of lift (well N
and W of region) and a southern stream area of lift (favoring
locations south of I-70). The expectation is for the southern
stream wave to move ENE through this evening and offering some
NE expansion of convective activity ahead of the mid-level 700mb
warm front. Little to no instability within this environment
means damaging wind and high rainfall rates are not expected.

Hi-res CAMS suggest potential for new convective development
along a weak surface trough near central OH late this afternoon,
and moving said convection east. Confidence is lowering in this
occurrence, however, due to excessive cloud cover in the area
continuing to limit destabilization (with waning upper support
and lack of notable cooling aloft). As a result, the probability
of isolated damaging wind threats developing appears to be
lowering at this time. If greater destabilization does occur
(resulting in SBCAPE values around 1000 J/kg), then the
highlighted marginal severe threat by SPC is closer to reality.

Subsidence behind the shortwave movement and loss of diurnal
heating will lead to a downward trend of precipitation chances
after 03z (11pm) tonight. Temperature will be mild with varying
cloud decks, averaging 5 to 10 degrees above the daily average.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/...
KEY MESSAGES:

- Low probability for strong to severe thunderstorms exist
  Saturday, favoring a wind and isolated flash flood threat.

- Widespread convection is favored Sunday, but poses a more
  limited hazard risk.

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The organized upper level low will creep ESE towards the eastern
Great Lakes Saturday into Sunday, with varying shortwave
movement through its base crossing the Upper Ohio River Valley
region. This will promote periods of showers and thunderstorms
favoring the afternoon and evening hours and tapering off with
the loss of diurnal heating.

For Saturday, there may be lingering light rain showers east of
Pittsburgh from prior shortwave movement during the morning.
Otherwise, generally dry conditions will exist until the
afternoon and evening with the approach of a pre-frontal trough
and the likely thunderstorm development along and ahead of this
boundary (in the form a broken convective line). There is a
marginal risk for damaging wind with these thunderstorms due to
potential SBCAPE reaching 2000-2500 J/kg and notable mid-level
dry air overcoming weaker lapse rates and more limited deep
layer shear. Confidence in the overall threat coverage is only
on the medium side as probabilistic guidance suggests the
potential for lingering cloud cover to reduce the degree of
instability while area forcing proves too weak for greater storm
organization amid the likely weak shear/lapse rates. Progression
of the trough eastward is expected through 06z, with dry air and
loss of heating contributing to a downward trend in
precipitation chances.

For Sunday, influence from the nearly overhead cold core trough
center will lower convective temps and allow for more widespread
thunderstorm development after 15z. There is some uncertainty in
favored locations due to slight variations in trough positioning
that may create subsidence holes, but the overall
pattern/environment should support on and off storms through the
evening. The lack of strong instability despite 90th percentile
PWATS and some forward motion of storms should preclude a flood
threat.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
KEY MESSAGES:


- Residual shortwave movement favor lower probability showers
  Monday along with abnormally cool temperature.

- Long range trends favor dry weather and near normal
  temperature.
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Ensembles indicate fairly minor variations in trough progression
Monday into Tuesday, lending to the likelihood of the trough
axis shifting east of the region and NW flow remaining in place.
Lingering, diurnally driven showers will taper off Monday
evening while area temperature ends well below the daily
average. Tuesday will then be dry with the residual cold air
advection leading to temperature around 10 degrees below the
daily average.

Long range analysis favors a continuation of the fairly
amplified pattern of ridging over the Rockies and troughing
along the east coast. This should position the region within the
drier NW flow pattern where precipitation chances will be
heavily reliant on strong shortwave movement over the ridge.
Latest trends don`t favor this and thus suggest a prolonged
period of dry weather and seasonable temperature Wednesday
through the end of the week.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
Meager instability will build across the region this afternoon,
limited by cloud coverage across most of the area save eastern
OH where some breaks are more likely. A shortwave will cross
the Upper Ohio Valley region this afternoon and early evening.
This should result in scattered showers and thunderstorms
developing, but confidence in coverage low given limited
instability and lack of surface forcing mechanism. Continued to
carry only VCTS in the TAFs for the most likely time of
occurrence.

Instability will diminish this evening with a few showers
possible until the shortwave completes its passage. Ceilings
will then lower overnight with lingering low level moisture
resulting in restrictions to MVFR to IFR, primarily after 06z
, with probability for MVFR at 60-90% and IFR around 50-80%.
Even at any sites that do receive rain today, fog chances are
lower with wind expected to remain elevated overnight.

.Outlook...
Periodic restrictions and showers and thunderstorms are possible
through early Monday as low pressure slowly crosses the region.
The greatest coverage of showers/thunderstorms will be in the
afternoon and early evening. VFR returns by Tuesday under high
pressure.

&&

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

&&

$$

SYNOPSIS...Frazier
NEAR TERM...Frazier/Shallenberger
SHORT TERM...Frazier
LONG TERM...Frazier
AVIATION...MLB/88