-- Discussion containing changed information from previous version are highlighted. --
566
FXUS61 KPBZ 240059
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
859 PM EDT Thu May 23 2024

.SYNOPSIS...
-- Changed Discussion --
Scattered showers and storms are likely south of Pittsburgh
into the overnight period, south of a stationary front. Periodic
thunderstorm chances are expected through the holiday weekend
as a series of upper level disturbances cross the region.
-- End Changed Discussion --

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
-- Changed Discussion --
KEY MESSAGES:

-A disturbance to our south will provide support for scattered
 showers and storms into the overnight hours; probabilities are
 high south of I-70 for convection redevelopment.
-Fog is expected to develop south of Pittsburgh during the
 overnight hours under warm, moist airmass.

_____________________________________________________________

A wavy stationary boundary remains in place across eastern
Ohio, then dipping ESE into southwest PA in the general area of
I-70. A shortwave advancing towards the area has provided the
spark for what thus far have been scattered showers and
thunderstorms. A couple in eastern Ohio were able likely get
tall enough to produce subsevere hail/wind gusts; these storms
formed on the edge of a higher area of downdraft CAPE (700-900
J/kg) that could have fueled 45-55 MPH gusts. Other concerning
trend is east/west movement of storms in Washington County PA in
the vicinity of the boundary. Expect the threat of strong
storms to wane over the next few hours as diurnal
cooling/outflow boundary layer stabilization occur. Localized
heavy rainfall will then become the concern for a few hours
after that. Precipitable water values of 1.2 to 1.3 inches south
of the boundary may fuel downpours, and any additional training
may lead to isolated high water issues. To the north of the
boundary, reduced cloud cover and a quiet night are foreseen.

The potential for fog late tonight increases in the vicinity of
the stalled boundary, and where rain has been able to soak the
ground due to storms. A gradient in overnight lows from the
lower 50s north of the front to the lower 60s near the Mason-
Dixon Line is expected.
-- End Changed Discussion --

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH SATURDAY/...
KEY MESSAGES:


-Lingering showers from a passing shortwave may last through
 late morning Friday south of Pittsburgh, PA.
-Probability of showers and storms decrease Friday afternoon.
-A crossing shortwave will return the risk for showers and
 storms early Saturday
 _____________________________________________________________

Remnant moisture south of Pittsburgh,PA may keep convection
ongoing south of I-70 into the late morning hours of Friday with
an exiting shortwave. Elsewhere, high pressure will likely keep
quiet, dry weather is expected. Afternoon high temperatures for
Friday and overnight lows early Saturday will range 5 to 10
degrees above average.

Ridging will break down across the Ohio River Valley early
Saturday morning as an ejecting low out of the northern Dakota
tracks into the Great Lakes. With a flux of low-lvl moisture
early Saturday morning, our environment has the potential for
strong to severe storms and excessive rainfall late Saturday
afternoon/evening. MUCAPE ranges between 1500J/kg to 3000J/kg
over western PA/eastern OH. Given a storm with a nice updraft,
hail and downbursts may be a threat. However, some models
suggest very little triggering for lift despite a decent
environment. So confidence is still somewhat low on this
scenario.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
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 and into next week.
  ____________________________________________________________

A strong, well-organized low pressure system is becoming the
favored pattern towards the end of holiday weekend in the Ohio
River Valley. Timing and the position of the center of the low,
along with environmental moisture profiles, are currently not
in phase between the long range models and some of the newer
runs of the CAMs. There is a very large difference between
800J/kg (GFS) to 3000J/kg (NAM). At the moment, the Storm
Prediction Center (SPC) has focused the convection outlook
southwest of our county warning area (CWA). However,
probabilities of organized convection (a combination of shear
and instability) may shift northeast if model runs come to an
agreement in the next 24 to 36 hours.

Long range suggests a shift to a cooler, dry pattern by the
middle of next week as upper troughing develops over the Great
Lakes. Expect temperature to fall below the seasonal average,
with Tuesday potentially being the transition day from shower
activity to dry weather.

&&

.AVIATION /01Z FRIDAY THROUGH TUESDAY/...
VFR and light wind are favored through the TAF period under the
influence of high pressure to the north.

A weak boundary sitting near the I-70 corridor plus weak
shortwave movement may foster isolated southeast-moving
convection between 02z-08z. Lightning is possible, but terminal
impact remains low with hi-res model trends bearish on storm
intensity. If heavy rains develops, pockets of fog may form as
the near-surface layer moistens. The overall threat for fog is
lowering from prior outputs due to likely cirrus coverage
limiting radiational cooling overnight as convection continues
well south of the region.

.Outlook...
Shower and thunderstorm chances return Saturday afternoon and
evening, but may be dependent on evolution of storms that occur
across the western Great Lakes. Any thunderstorms poses a risk
for gusty, erratic winds and lowered visibilities.

Thunderstorm probabilities are highest late Sunday into Monday
as a more robust low pressure system moves through the Great
Lake region. A pattern shift thereafter will introduce periodic
precipitation chances and potential for more prolonged cig
restrictions.

&&

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

&&

$$

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