261
FXUS61 KPBZ 281804
AFDPBZ

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

.SYNOPSIS...
Quiet weather expected today under high pressure. A warm front
brings showers and thunderstorms to region early Saturday. The
severe weather increases Saturday and evening ahead of a cold
front. Tornadoes and damaging winds will be the main threat.

&&

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

- Dry weather and light wind expected today.
- Rain chances increase tonight with approaching low pressure.

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8:30am Update: No major chances have been made to this forecast
period.

Surface high pressure shifts eastward off the northeast Atlantic
coast today as low pressure upstream advances from the northern
Plains into the Great Lakes region. We remain warm and dry
through much of today under the departing high pressure, with
high temperatures running around 5 degrees above normal.

Southerly flow 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.

&&

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

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

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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 possibly 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 4 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 north and west of Pittsburgh) at least initially.
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. These factors, along with abundant buoyancy
and warm cloud depths >13kft supporting warm rain processes,
are an indicator for slow-moving or backbuilding thunderstorms
capable of producing torrential downpours. The Weather
Prediction Center has highlighted much of the area in a Marginal
Risk (Level 1 of 5) for excessive rainfall leading to flooding,
with the low-end risk reflecting the isolated nature of flash
flooding and its dependence on thunderstorms. If the pattern
evolves such that a more widespread coverage is expected, an
upgrade to a Slight Risk (Level 2 of 4) is certainly not out of
the question.

&&

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

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

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A cold front sweeps through the area late Saturday night or
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 on Thursday.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/..

VFR conditions will continue to prevail late evening into the
overnight time period. South-southeasterly will prompt warm,
moist advection and increase low-lvl cumulus this evening.

Upper-lvl clouds will increase overnight with an approaching
disturbance from the west. The first round of rain, associated
with a warm front is expected between 08Z to 12Z Saturday,
probability of thunder associated with convection is between 10%
to 40%, TEMPOS were added for heavy downpours that could
restrict visibility.

A second wave of thunderstorms appears promising between 18Z to
23Z Saturday. Storms could be considered strong to severe around
terminals and wind gusts between 45mph to 60mph. However,
confidence is not high on the spatial displacement of storms.
Therefore, TEMPOS of wind gusts between 25kt to 30kts have been
added. MVFR to IFR may develop for FKL/DUJ with sufficient low-
level moisture.

.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...None.
OH...None.
WV...None.

&&

$$

SYNOPSIS...Hefferan
NEAR TERM...Cermak/Hefferan
SHORT TERM...Cermak/Craven
LONG TERM...Cermak/Craven
AVIATION...Hefferan/Milcarek