-- Discussion containing changed information from previous version are highlighted. --
856
FXUS61 KPBZ 211139
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
739 AM EDT Sat Sep 21 2024

.SYNOPSIS...
Very warm weather is expected through the weekend with a
marginal chance of severe storms today, mainly east of I-79. A
wetter pattern is becoming increasingly likely next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
KEY MESSAGES:

- Marginal risk of severe weather east of I-79. Primary threat
  is damaging winds from 1pm to 5pm.
-------------------------------------------------------------------

A batch of elevated showers and thunderstorms north of
Pittsburgh driven by shortwave energy and weak warm advection
will continue sliding east early this morning. Latest analysis
pegs around 500 J/kg of MUCAPE in this region. No organization
or threats are expected aside from some welcomed rain.

The main forcing for storms this afternoon will pass along a
shortwave, which will act to encourage ascent. The forecast
largely remains on track. Greater than 30kts of shear are
forecast in the 0-6km layer, most likely (60% to 90%) in western
PA this afternoon, though the majority of this is confined to
the 3-6km layer. Probabilities of greater than 1000 J/kg SBCAPE
are up to 60% to 80% for the same area, mainly after 2pm. DCAPE
is forecast around 800 to 1000 J/kg around the same time with
dry air making its way into the mid-levels. With these
ingredients together, the environment supports severe potential
east of I-79. Wind remains the primary threat in downdrafts,
though large hail could not be ruled out. With the ongoing
morning convection, trends may point more toward hail being the
primary threat as cooling in the mid-levels steepens lapse
rates and subsequently decreases DCAPE, but will continue to
monitor through the morning. Tornado threats are null with high
cloud bases and weak shear in the lowest levels.

Ensembles are starting to agree more on the timing of the main
wave passage between 1pm and 5pm east of I-79, though the trends
in forcing are a bit weaker for development. So the main
question is will forcing overcome the dry air. Overnight
guidance struggles to develop updrafts until after help from
topography. This may indicate the main threat to be just east of
the forecast area. Should the environment trend a bit more
unstable and the wave a bit stronger, probability of severe
weather east of I-79 would increase. At this time, severe
chances remain low probability with this uncertainty, highest
for the ridges.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
KEY MESSAGES:

- Lingering shower and storm chances for the ridges.
- Dry and very warm on Sunday.
---------------------------------------------------------------------

Development in the evening and early overnight period cannot be
completely ruled out with the mid-level wave still sagging,
though the environment starts to become more unfavorable after
6pm. Further, chances of severe weather remain quite low. Rain
and storm chances are the highest along the ridges with
topographic influences.

Chances of rainfall exceeding 0.25" are around 30% for the
ridges, and lower than 20% elsewhere. Unfortunately, it may not
even rain in portions of the D4 drought in eastern Ohio. This
means that drought concerns will persist through at least early next
week. See the long term for more in-depth information on
relief.

A brief return to hot and dry conditions is expected for Sunday
as temperatures ramp up to around 10 to 15 degrees above average
once again with ridging. The Zanesville metro even has a 50/50
shot at hitting 90F, though no records appear to be at stake at
this time.

&&

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

- Rain chances increase Monday and Tuesday, with a widespread,
  beneficial rainfall possible.
- Rain chances continue Wednesday through Friday, although
  model uncertainty throws timing and amounts in question.
- More moderate temperatures through the week.
-------------------------------------------------------------------

Ensembles are in decent agreement with a transition to quasi-zonal
flow aloft during the Monday/Tuesday period as the Sunday ridge axis
gets shunted into the Atlantic. Moisture flow will become more
favorable in the WSW flow aloft, and also ahead of surface low
pressure that should be in the vicinity of Michigan by Tuesday
morning. Chances of a beneficial rainfall during this period appear
good; the NBM depicts a 80 to 90% chance of at least a half-
inch of rain through 8 AM Wednesday, and a 50 to 70% chance of
an inch. This has increased since the last update.

Uncertainty increases greatly thereafter, as the ensembles start to
show more spread in handling the interaction of shortwaves from the
Rockies to the Upper Midwest. There remains general agreement that
the Upper Ohio Valley will remain in an upper trough, but the depth
of that trough and the position of its axis remain
questionable, as is the potential for the development of a
closed 500mb low. The solutions that keep a deeper trough axis
to our west would be more favorable for continued rainfall than
solutions with a weaker trough and those with the axis to our
east.

Despite this uncertainty, at least continued low precipitation
chances are appropriate through the end of the work week.  72-hour
totals between 8 AM Wednesday and 8 AM Saturday reach a quarter inch
in 25-40% of scenarios for eastern Ohio and 40-60% for northern
WV/southwest PA. A few high end scenarios exist pending tropical
influence. All told, the wetter pattern next week holds some
promise to at least somewhat mitigate the ongoing drought.
Temperatures moderating to seasonable levels (for daytime highs
at least) will help as well.

&&

.AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
-- Changed Discussion --
Elevated showers/thunderstorms driven by weak warm advection
will ride to the northeast of PIT this morning, with perhaps a
brief restriction possible at DUJ by 14Z. More robust showers
and thunderstorms are expected to arise after 17Z in a more
favorably unstable and sheared environment. The stronger storms
will be capable of producing gusty wind and hail, particularly
east of I-79. Expect locally lower cigs/vis in and around
showers and storms, while VFR is expected to prevail outside of
convection. PROB30 groups were used to highlight the most likely
period when a storm could impact terminals. ZZV is expected to
miss this threat altogether.

Southwest wind is forecast by later this morning, topping out at
around 10 knots away from thunderstorms this afternoon. A
frontal passage brings northwest winds to all airports by the
evening. ind speeds remain around 5-10 knots due to the weak
nature of the front.

Fog potential tonight will depend on where precipitation occurs
today and where the best clearing occurs tonight. Did include a
period of IFR fog after midnight where chances appear highest,
with refinement expected in future issuances.

.Outlook...
VFR returns Sunday afternoon under building high pressure
before restriction potential increases again next week as a more
unsettled weather pattern sets up.
-- End Changed Discussion --

&&

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

&&

$$

SYNOPSIS...Milcarek
NEAR TERM...MLB/Milcarek
SHORT TERM...Milcarek
LONG TERM...CL/Milcarek
AVIATION...CL