799
FXUS61 KPBZ 291732
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
132 PM EDT Sat Jun 29 2024

.SYNOPSIS...
Thunderstorms, some strong to severe, with primary threats of
damaging wind, a few tornadoes, and heavy rainfall leading to
flash flooding expected this afternoon into the evening.

&&

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

- Favorable environment for severe weather today. Damaging wind and
  tornadoes will be the main threat.
- Flash flooding will also be possible with heavy rainfall
  mainly from Pittsburgh metro and north.
-------------------------------------------------------------------

Low pressure currently moving across the upper Great Lakes has
resulted in broad and deep warm, very moist southerly flow
throughout the Ohio Valley and lower Great Lakes regions. A shield
of showers and thunderstorms to our west out ahead of a cold front
and with an attendant shortwave will arrive in our neck of the woods
after noontime today. Our severe weather threat will likely be tied
to this feature later this afternoon and evening.

The 12z PIT sounding supports the expected threats of damaging wind,
a few tornadoes, and heavy rainfall leading to flash flooding.
Notable elements on the sounding in regard to the flash flood threat
include a precipitable water value of 1.98" (above the daily
climatological max), warm cloud depths to around 14kft, and, above
the lowest 3 km or so, unidirectional west-southwest flow. Corfidi
propagation vectors of 10 knots or less across the area this morning
suggest slow moving and backbuilding potential with the convection
today. This, coupled with high PWATs even pushing 2.2" north of
I-80, signals a flash flood concern for roughly Pittsburgh and
north especially with any training showers and thunderstorms.
The latest hi res ensemble data supports a 60% chance of 1 hour
rainfall rates >0.5". Localized areas of 1-2" of rain aren`t
out of the question.

For the severe threat, despite morning cloud cover and showers,
temperatures are already in the mid 70s/low 80s, so
destabilization isn`t expected to be all that limited. Latest
ensemble joint probability is 60-90% for >1000 J/kg SBCAPE and
>30 kt of shear. Similar to the event this past week, the bulk
of the shear is packed into the lowest levels in the 25-35 knot
range which could support a tornado threat. Latest convective
models continue to suggest some organization into one or
multiple line segments that push across the area through the
afternoon and evening. The environment ahead of these storms
will support thunderstorm intensification to severe levels, with
damaging straight line wind and a few tornadoes being the
primary threat. The greatest tornado threat will reside
generally north of the I-70 corridor where 850mb and 700mb flow
will be stronger resulting in more bulk shear in the sfc-3km
layer and elongation of the low level hodographs. If any
discrete cells can fire ahead of the more organized segments,
deviation to a right moving motion vector of east/northeast will
be favorable for streamwise vorticity ingestation and increased
tornado potential. In addition, the bulk of the shear packed
into the lowest levels could also pose a QLCS tornado threat
with any bowing segments. Farther south, weaker winds and a more
unidirectional wind profile in the low levels appear to reduce
the tornado threat somewhat, though steep low level lapse rates
will still promote strong low level updrafts in thunderstorms
which can overcome the more marginal profiles as we`ve seen so
far this spring and summer, so the tornado threat certainly is
not zero in those areas. The Storm Prediction Center has
maintained a Slight Risk (Level 2 out of 5) for severe
thunderstorms across the entire area.

In terms of timing, the primary wave of thunderstorms carrying the
greatest severe threat pushes through between roughly 3-9pm, though
there is some indication of weak backbuilding occuring towards the
latter half of that window with storms lingering in areas south and
east of Pittsburgh until closer to midnight. Interestingly, some 12z
CAMs have backed off some on the threat today, though not entirely
buying this as those that do are struggling with resolution of the
upstream MCS and painting a much drier boundary layer with dew
points mixing out this afternoon which seems very low probability
given the moist southwest flow and morning rain.

Additionally, there may be redevelopment overnight into early Sunday
morning along the cold front itself as it pushes through the area.
Hi-res guidance suggests a low-end severe threat carries into this
overnight wave, though limiting factors will be a stabilizing
surface layer and available instability.

&&

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

- Cold front moves through Sunday morning.
- A few storms may be strong along it south and east of Pittsburgh.
- Drier air builds in with cooler temperatures.
-------------------------------------------------------------------

The cold front itself then moves through Sunday morning and carries
an additional lower confidence severe threat. Unfavorable mid to
late morning timing with at least some degree of cloud cover around
as even the lower end of the latest ensemble guidance indicates 40-
50% sky coverage through the morning hours lends less of a severe
threat, though that said, can`t rule out some stronger storms
capable of damaging wind southeast of Pittsburgh where a Marginal
Risk (1/5) remains.

We then finally get a push of drier air behind the front on Sunday
afternoon and evening as high pressure builds across the Great Lakes
and reinforces northerly flow in the wake of the boundary. Still a
bit of discrepancy in the 12z guidance on the timing of the front;
highs will be dependent on said timing as a quicker boundary could
keep PIT in the upper 70s while a slower push could result in mid
80s. Either way, clouds will scatter into Sunday evening, though a
passing upper wave looks to reinforce cloud coverage toward morning
on Monday. Still, overnight lows will be refreshing dipping into the
50s for most, though some 40s are possible (50-70% chance) in
the ridges and north of I-80.

Dry weather continues with high confidence through Monday night as
heights begin to rise and high pressure moves further east.
Temperatures remain below average on Monday with ensemble
probability <30% of highs topping 80 degrees.

&&

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

- Dry weather continues Tuesday with increasing temperatures.
- Showers and thunderstorms return Wednesday and Thursday with
  another low pressure system.
-------------------------------------------------------------------

Good agreement amongst the various ensemble clusters supports dry
weather on Tuesday as ECONUS ridging builds. Temperatures begin to
rebound as flow becomes more south of east and pushes highs back
toward or a couple degrees above normal.

By mid-week, the pattern becomes more unsettled as ensembles break
the ridge down and low pressure moves by to our north. Deep
southwest flow on Wednesday surges temperatures back to 5-10 degrees
above normal, though timing of a cold front passage may throw a
wrinkle in that. Ensembles show increasing deviation from the mean
come Wednesday night through the remainder of the week, so
confidence is lower, but a cold front looks to pass through sometime
late Wednesday into Thursday returning shower and thunderstorm
chances. Machine learning becomes more interested in severe chances
by mid-week as the front pushes into a warm, moist airmass, so this
will bare watching through the coming days.

&&

.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
Showers and thunderstorms will move across the area through the
afternoon and early evening as a shortwave trough crosses the
Upper Ohio Valley region. Included an MVFR to IFR TEMPO group
in the TAFs for the most likely time these storms affect area
airports. Thunderstorms are likely at KPIT between 20Z and 22Z.

The thunderstorms should move out of the area this evening as
the shortwave exits to the east. MVFR to IFR cigs are expected
late tonight with low level moisture in place. In addition,
scattered thunderstorms are expected toward Sunday morning as a
surface cold front crosses the region. MVFR cigs should continue
for a few hours after FROPA with low level moisture in place.
Improvement to VFR is expected by early Sunday afternoon as
mixing and dry advection occurs.


.Outlook...
Outside of early morning river valley fog, VFR is expected until
a slow moving front sets up across the region Wednesday and
Thursday.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...Flood Watch through late tonight for PAZ007>009-013>016-
     020>022-029-073-074-077-078.
OH...Flood Watch through late tonight for OHZ039>041-048>050.
WV...Flood Watch through late tonight for WVZ001-002.

&&

$$

SYNOPSIS...MLB
NEAR TERM...Cermak/Hefferan/MLB
SHORT TERM...MLB
LONG TERM...MLB
AVIATION...WM