644
FXUS61 KPBZ 211502
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
1102 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 (1/5) of severe weather east of I-79. Primary threat
  is damaging winds from 1pm to 5pm.
-------------------------------------------------------------------

In the wake of departing morning showers and thunderstorms,
scattered cloud coverage should allow for the atmosphere to recover
by early afternoon with highs forecast to rise to the mid 80s ahead
of another round of development expected with some strong to severe
storms possible east of Pittsburgh.

The main forcing will be from a shortwave and placement in the left
exit region of an upper jet which will both act to encourage ascent
on the synoptic scale. Environment-wise, the PIT morning sounding
shows the bulk of the dry air above 600 mb or so with >1000 J/kg of
DCAPE present, plenty for a downburst wind threat. Deep layer shear
>30 knots will support storm organization with most of that
shear confined to the 3-6 km layer with a modest jet and weaker
low-level flow. Highest probabilities of both SBCAPE >1000 J/kg
and deep layer shear >25 knots this afternoon sit between 60-70%
across western PA/northern WV. Despite some curvature in the
lower levels of the hodographs, weak speed shear and high dew
point depressions will preclude a tornado threat.

Wind remains the primary threat though large hail remains a
possibility as well. With the morning convection, cooling in the mid-
levels may locally steepen lapse rates at the expense of some DCAPE.
However, a mode of failure may stem from a couple warm noses in the
mid-levels around 500 mb. If we can`t erode that warm layer, likely
with the shortwave, updrafts may have a tough time punching through
that level which would greatly reduce the wind and hail threats as
cores wouldn`t be able to reach the optimal hail growth zone.

Most of the 12z CAMs are still suggesting reliance on orographic
processes to initiate the best coverage of storms with the absence
of a surface forcing mechanism, and indicating that the best threat
is pointing toward the ridges and east of our forecast area. The
primary window will be between 1pm-5pm coincident with shortwave
passage. Unfortunately for drought concerns, this isn`t going to be
the widespread rain we`re longing for (but check the short and long
term for better chances).

&&

.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. Some areas of fog tonight are possible, especially where rain
falls, with clearing skies, a stagnant airmass, and achievement
of crossover temperatures.

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 /15Z SATURDAY THROUGH WEDNESDAY/...
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.

&&

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

&&

$$

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