568
FXUS61 KPBZ 212358
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
758 PM EDT Tue May 21 2024

.SYNOPSIS...
Seasonable warm weather is expected through Wednesday before the
first in a series of upper level disturbances promote periods of
showers and thunderstorms. Severe potential appears highest
Wednesday afternoon and evening, though impacts can`t be ruled
out through the weekend.

&&

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

- Well-above normal temperatures into the overnight.
  _____________________________________________________________

Broad eastern CONUS ridging and surface high pressure will
continue to promote well above normal temperature this evening.
Diurnal heating through the day and orographic assist was
lacking this evening in the ability to produce convection across
the area. Thus, pops were removed for the evening.

Increasing cirrus overnight ahead of an upper shortwave and its
associated nocturnal convection should further buoy overnight
temperature. Low readings are likely to be 15 degrees above the
daily average and could approach record values for high minimum
temperature (Zanesville/New Philadelphia/Wheeling favored).

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
KEY MESSAGES:

- Strong to severe thunderstorms possible Wednesday afternoon
  into Thursday morning.
- Damaging wind is the primary threat, while tornadic threat
  favors northwest PA.
- Showers and thunderstorms are likely to continue ahead of a
  cold front Thursday.
  _____________________________________________________________

Shortwave movement ahead of the main trough axis will push a
decaying line of showers and thunderstorms towards the region
Wednesday morning. The stable airmass should prevent this
convection from continuing/maintaining east, but its residual
outflow boundary appears to be the key for afternoon convective
initiation within an increasingly unstable environment (upper
level shortwave associated with this convection will weaken and
result in less convective support). Mean SBCAPE values between
1250-2000 J/kg combined with 25-30kts shear offer heightened
probabilities for damaging wind and large hail; a smaller
corridor of enhanced low-level helicity around NW PA could also
favor isolated tornado development. This appears to be a "if
storm initiation can occur" severe threat environment where
convection won`t be widespread through the early evening hours
as the main upper trough remains too far west.

Overall convection coverage is likely to increase overnight into
Thursday morning as a more well-defined upper trough lifts
through the Great Lakes and pushes a surface cold front
eastward. Though surface based instability will wane with the
loss of heating, increased shear with residual elevated
instability could maintain the damaging wind and hail threat
during this period. As the upper trough lifts NE Thursday
morning, the surface boundary will stall in a WSW-ENE
orientation through the middle of the forecast area; this
residual boundary combined with afternoon heating should favor
additional thunderstorms south of the boundary until the front
drops south of the area Thursday night with additional shortwave
movement.

Though forward prorogation of storms and lack of stronger
forcing should limit flooding hazards, elevated PWAT values
and potential for some training during the day Thursday (as the
boundary becomes more parallel to the upper flow) could lead to
localized flooding.

&&

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

- Variability remains high on long term forecast and may be
  predicated on prior day`s convective outcomes.
- Overall, periodic showers and thunderstorms with slightly
  above normal temperature is favored through the holiday
  weekend.
  ____________________________________________________________

Ensembles favor high pressure north of the region establishing
more seasonable and dry weather to at least start the day Friday
in the wake of Thursday`s shortwave passage. However, global
models suggest the potential for a weak shortwave to lift
northeast out of the TN River Valley through Friday evening to
promote showers and thunderstorms. Either way, the overall
pattern will usher additional rounds of shortwave crossings
through the Upper Ohio River Valley as it remains in between
broad ridging over the SE CONUS and troughing over the northern
Great Plains. Expect periods of showers and thunderstorms with
near to slightly above normal temperature, with potential for
severe hazards depending on evolution of prior day`s convection.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
VFR conditions will prevail overnight with high confidence as
just a few cirrus stream overhead. Wind will go light and likely
favor a southerly direction as a gradient establishes ahead of
approaching low pressure.

Wednesday morning will see increasing and lowering cirrus
coverage while wind picks up out of the southwest owing to the
aforementioned gradient and mixing tapping into 20-30 knot wind
aloft. Shower and thunderstorm chances increase Wednesday
afternoon and continue into the overnight hours with the
passage of a low pressure system; timing on the arrival of the
first wave of storms remains uncertain. Most likely timing for
development will be after 17z, however hi res ensemble guidance
indicates a low confidence chance for development as early as
15z primarily north of PIT followed by additional showers and
storms further south. Outside of thunderstorms, there is high
confidence that VFR prevails. Thunderstorms will be capable of
producing hail as well as gusty and erratic wind, but confidence
in timing of impacts is low.

Showers and storms will continue into the overnight hours
Wednesday night as the cold front completes its passage. Wind
will shift to the northwest in its wake. MVFR CIG restrictions
then become more likely overnight (40-60%) with increasing low
level moisture.

.Outlook...
Periodic thunderstorm chances and associated restrictions are
likely through the weekend as an active weather pattern
continues.

&&

.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