300
FXUS61 KPBZ 261725
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
125 PM EDT Wed Jun 26 2024

.SYNOPSIS...
Thunderstorms capable of producing damaging wind and localized
flooding are expected this afternoon and evening ahead of a cold
front. Dry and seasonable weather will return Thursday, with
rising temperature expected Friday.

&&

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

- Strong to severe storms are expected between 1pm to 8pm.
  Damaging winds will be the main threat. An isolated tornadoes
  and hail as large as a quarter can`t be ruled out.
- Storm Prediction Center (SPC) has expanded the Slight Risk for
  severe weather across the region, and Weather Prediction
  Center (WPC) has continued the Marginal risk for flooding.

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130pm Update:
The area stretching from northeast Ohio to northwest PA has
growing confidence in seeing localized flash flooding and storm-
driven damaging wind (among other potential hazards). Analysis
shows an overlapping maxima of 700-400mb convergence, surface
convergence, rising PWAT values and slow storm motion in order
to maximize 1-2"/hr or higher rainfall rate potential, which
will shift into northwest PA over the next 2-3 hours and last
roughly through 00z/8pm EDT. The severe threat, with a Severe
Thunderstorm Watch now issued, over northwest PA highlights the
area where 2000-3000 J/kg SBCAPE has developed and increasing
40-50kt effective shear; timing of the damaging wind and lower
probability hail/tornadic threat is between 19z (3pm) and 00z
(8pm).

Farther south, cloud cover from weak vorticity advection (see
discussion below) has continued to limit destabilization. The
expectation now has the upper trough and frontal approach as the
necessary ingredients to create stronger lift for convective
activity. PoPs have been adjusted as such, with potential for
flash flood or damaging wind in these regions (generally south
of the watch) as being lower than previous outlooks.

11am Update:
Weak vorticity advection from the lower Ohio River Valley will
continue to provide enough ascent within a weakly unstable
(elevated) environment to push areas of light rain and low
probability thunderstorms into southeast Ohio over the next two
hours. PoP forecast has been updated to account for this
feature; the expectation/focus remains on this afternoon with
convection firing along/near Lake Erie and moving SE.

Rest of the Discussion...
An well-organized upper shortwave trough is expected to drop
southeast from the western Great Lakes today and cross the Upper
Ohio River Valley region through tonight, pushing through a
surface cold front. Ahead of this feature, moist advection is
helping to create a storm environment featuring above 90th
percentile PWAT values, 30-40kts of effective shear, and between
1000-3000 J/kb SBCAPE (the latter two increasing towards those
values by the afternoon). The resulting chemistry of the large-
scale synoptic pattern and meso setup should feature fairly
widespread thunderstorm development initializing near the cold
front along Lake Erie and traversing ESE between 1pm to 10pm
EDT. Environment and model-supported outlooks favor a damaging
wind and localized flooding threat for the stronger, more
organized thunderstorms (which should favor northwest PA);
however, lower probabilities still exist for large hail (due to
for deep updrafts) and a tornado (lower LCL heights and modest
sfc-1km shear).

Shower and thunderstorm activity is likely to weaken late
tonight as surface instability wanes along with the passage of
the cold front. Much of the severe threat should subside by 10pm
EDT while lingering post-frontal showers and convective
stratiform rain regions exit the higher terrain of WV by early
Thursday morning. Dry air and residual mixing of a NW post-
frontal wind should mitigate morning fog risk, but residual
boundary layer moisture in river valleys (favoring locations SE
of Pittsburgh) could have patchy fog development.

&&

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

- Dry weather returns.
- Temperature rises above normal again Friday.

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

Cool, dry northwest flow will develop Thursday as high pressure
builds in from the northwest. Residual moisture and delay in
subsidence is likely to allow for lingering stratocu Thursday
morning that turns into scattered to occasionally broken
afternoon cumulus. Cold advection and cloud cover will result in
more seasonable afternoon temperature while post-frontal mixing
may support NW wind gusts around 20 mph.

The surface high will shift over New England by Friday as
heights aloft rise ahead of the next upper level shortwave
trough. Positioning of the high will support weak warm, moist
advection amid mostly clear skies to raise area temperature
about 5 degrees above the daily average. Broad ascent may
increase mid to high level clouds and even low probability
showers Friday night as weak perturbations ahead of the main
upper level trough axis.

&&

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

- Shower and thunderstorm chances return Saturday into Sunday
  with the next low pressure system.
- Low probabilities exist for a damaging wind and flooding
  threat with these thunderstorms.

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

Broad ascent within warm, moist southwesterly flow ahead of the
main upper level trough axis will support increasing shower and
thunderstorm probabilities starting early Saturday morning.
Arrival of a more notable embedded shortwave and the surface
cold front Saturday afternoon and evening is likely to help
concentrate convective activity (currently favoring NW PA) and
spreading it southeast through Sunday morning.

Variation in storm timing and environment remains large due to
differences in upper trough depth/timing as well as convective
evolution prior to its (and cold fronts) arrival. Latest
analysis suggests a low probability risk for damaging wind
(mean values currently show 30-40kts of 0-6km shear) and
localized flooding (tall-skinny CAPE values around 1500 J/kg
with PWATs approaching daily maxima). Will mention threat in HWO
and monitor evolving forecasts for this period.

Dry weather and seasonable temperature is likely by late Sunday
into Monday as the upper trough axis moves east and high
pressure builds in from the western Great Lakes.

&&

.AVIATION /17Z WEDNESDAY THROUGH SUNDAY/...
A broken band of rain showers is currently tracking across the
state of Ohio along a low-level jet. Impacts have only been
noted to visibility at terminal for a brief period.

Restrictions appear promising to start between 20Z and 03Z, as
convection develops along a frontal boundary near Lake Erie and
a shortwave ejects southeast; TEMPO groups continue for all
sites with scattered coverage. Strong to severe wind (40mph to
65mph) gusts could be observed with passing downdrafts and hail
up to the size of quarters could also develop in isolated storms
between 20Z to 00Z. Probability of thunderstorms rapidly
decreases after 04Z (MGW/LBE/DUJ will likely be the last to be
impacted) as the trough ventures east.

MVFR is expected as the thunderstorms end, with a period of
showers.

IFR is likely (LIFR possible) overnight with low level moisture
in place. Restrictions in stratus/fog are possible with all
sites; TEMPOS will be added in the afternoon update.


.Outlook...

VFR is then expected through Friday under high pressure.

Restriction are possible Friday night with a crossing warm
front.

Restrictions are likely Saturday and Saturday night with
showers and thunderstorms ahead of and along a crossing cold
front. VFR returns Sunday under high pressure.

&&

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

&&

$$

SYNOPSIS...Frazier
NEAR TERM...Frazier
SHORT TERM...Frazier
LONG TERM...Frazier
AVIATION...WM/Hefferan