314
FXUS61 KPBZ 290137
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
937 PM EDT Fri Jun 28 2024

.SYNOPSIS...
Quiet weather can be expected this evening under high pressure.
A warm front brings showers and thunderstorms to the region
early Saturday. The probability of severe weather increases
Saturday afternoon and evening ahead of a cold front.
Torrential rain, tornadoes and damaging wind all will be
possible.

&&

.NEAR TERM /UNTIL MIDNIGHT TONIGHT/...
KEY MESSAGES:

- Dry weather and light wind expected through this evening

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Cumulus field has broadened in response to upstream ascent
spreading ewd across the upper Midwest, with cirrus from showers
in sern MI and far-nrn OH spreading across the area north of
Pittsburgh. These showers may graze the region north of I-80
this evening on their current trajectory, but confidence remains
low in accumulating precipitation reaching the ground with
abundant dry air to overcome between H8 and H5. Some modest
updates to the sky and PoP/Wx grids have been made to reflect
the near-term evolution. Previous discussion follows...

Southerly flow has gradually allowed rich moisture to move back
across the Ohio River with a decent cumulus field and dew point
of 69F at KHTS this moisture is now poised to return rapidly
later tonight.

&&

.SHORT TERM /MIDNIGHT TONIGHT THROUGH SATURDAY NIGHT/...
KEY MESSAGES:

- Showers and thunderstorms expected Saturday and Saturday night
  as a cold front approaches the area. Some storms could become
  severe.
- The primary threats will be damaging winds and possibly a few
  tornadoes.
- Flash flooding will also be possible due to torrential
  downpours in the heavier thunderstorms, mainly from the
  Pittsburgh metro on north.

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Southerly flow late tonight will prompt a warm, moist airmass
to return to the region ahead of an approaching low pressure
system. Overnight low temperatures will likely range in the
upper-60s to low-70s; 10 degrees above the climatological
average.

The warm front lifting north and increasing flow just above the
nocturnal inversion should quickly push 850 mb theta-e values
into the 335-340K range after midnight and this will result in
development of elevated showers and thunderstorms especially
toward daybreak in the northern Ohio counties of our northwest
forecast area.

Saturday is shaping up to be another potential severe weather
day across the region. Low pressure advancing eastward from the
upper Great Lakes into southern Ontario will result in
southerly flow developing across the Ohio Valley and lower Great
Lakes tonight, bringing moisture and showers back into the
area. Ongoing convection is anticipated Saturday morning, most
likely over eastern Ohio and northwest PA where a weak warm
frontal zone will reside along with deeper moisture. Despite
the negative impacts this morning round may have on diurnal
heating, convective-allowing models continue to suggest
moderate instability developing by mid to late afternoon, with
HREF probabilities for 1000+ J/kg SBCAPE exceeding 60% areawide.
Meanwhile, deep-layer bulk shear profiles will be supportive of
organized storm modes (sfc-6km around 40kts), including bowing
segments and supercells.

What catches my eye when interrogating model-derived soundings
for Saturday afternoon is how they compare to Wednesday`s setup.
There are striking similarities between the two, most notably
the fact that instability and wind shear are packed in the
lowest ~4km of the atmosphere, the former being driven by steep
850-700mb lapse rates and the latter by jets at those two
respective levels overspreading the region ahead of the
advancing cold front. These two factors combined could support
strong low-level updrafts where your best streamwise vorticity
will also reside, leading to low-level mesocyclones that can
rapidly spin up and locally enhance tornado potential. We saw
this manifest in the QLCS that moved through southwest PA on
Wednesday, which produced 5 confirmed tornadoes. This will be
something to watch closely as this system evolves. The Storm
Prediction Center has highlighted this local enhanced tornado
threat with a 5% tornado probability contour extending across
portions of western PA and eastern OH (the rest of the region
remains in a 2% probability area).

The final threat to discuss for Saturday`s convection is flash
flooding. The setup may be such that there is training, or at
least multiple rounds, of thunderstorms in some areas
(particularly Along the I-70 corridor on north). Eventually,
the cold front will sweep through and storm motion will become
more progressive before ending altogether behind the front.
However, ahead of the front, corfidi vectors decrease to 5-10
knots in a moisture- rich environment characterized by PWATS
exceeding 2 inches, nearly an inch above normal levels for this
time of year. 850 mb theta-e values rise into 345-350K range and
there is decent prolonged deep layer moisture transport into the
area.  These factors, along with abundant buoyancy and warm
cloud depths >13kft supporting warm rain processes, are an
indicator for backbuilding and training thunderstorms capable
of producing torrential downpours. The Weather Prediction Center
has highlighted much of the area in a Slight Risk (Level 2 of
5) for excessive rainfall leading to flooding.

Issued the Flash Flood Watch for roughly northern half of
forecast area, where widespread 1 to 2 inch rainfall is expected
and locally 3-4 inches could fall.

&&

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

- Dry weather expected Sunday through mid week under building
  high pressure.
- Showers and thunderstorms return Wednesday and Thursday with
  another low pressure system.

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

A cold front sweeps through the area early Sunday morning,
bringing an end to showers and thunderstorms and ushering in a
drier airmass. This will bring dry weather and a slight cool
down Sunday and Monday with temperatures returning to normal or
just below normal levels for this time of year.

Dry weather persists through Wednesday under high pressure. The
upper ridge then pushes off to the east Wednesday night and
low pressure moves back into the Great Lakes region, bringing
showers and thunderstorms back to the area Wednesday and Thursday.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
VFR will prevail through much of the overnight. Warm/moist
advection in advance of an approaching disturbance will increase
mid-level clouds overnight and eventually increase rain chances
before 12z Saturday. Thunder is possible with morning showers,
but probabilities remains low for now, so have opted to exclude
TS mention. MVFR to IFR ceilings are expected north of PIT after
around 14z until mid-afternoon.

Some clearing is anticipated late morning/early afternoon before
more robust convection is expected ahead of and along a crossing
cold front. Strong to severe thunderstorms are possible
generally after 18z, with a most likely timeframe in the 20-22z
range at PIT.

Brief clearing is expected Saturday evening by around 00z
Sunday, though additional rain is expected along the cold
front by early Sunday morning.

.Outlook...

MVFR conditions may continue north of PIT with lingering low-lvl
moisture. Widespread VFR will likely return Sun afternoon under
high pressure and is likely to persist Mon and Tue.

&&

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

&&

$$

SYNOPSIS...Craven
NEAR TERM...Craven/Kramar
SHORT TERM...Craven
LONG TERM...Craven
AVIATION...Hefferan/Rackley