-- Discussion containing changed information from previous version are highlighted. --
870
FXUS61 KPBZ 210139
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
939 PM EDT Fri Sep 20 2024

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

&&

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

- Quite this evening, then scattered showers possible northwest
  of Pittsburgh towards morning.
-------------------------------------------------------------------

Rain chances will start to increase to the northwest of
Pittsburgh tonight. Warm, moist advection associated with an
advancing warm front may create a few showers and perhaps a
thunderstorm prior to sunrise. Current QPF associated with this
activity is quite light. Another warm night is on tap, with lows
in the 60s for much of the region. Patchy valley fog should be
less of a concern with a cloud increase and a bit better
temp/dewpoint spread.
-- End Changed Discussion --

&&

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

- Marginal severe storm potential Saturday afternoon, mainly
  east of I-79 with the primary threat being damaging winds.
- Dry and very warm on Sunday.
---------------------------------------------------------------------

Precipitation chances continue into Saturday morning thanks to
the warm front and associated warm/moist advection. This morning
activity will lend an uncertainty factor to afternoon activity -
the main question remaining the potential for severe weather.

The 12Z HREF still suggests a favorable environment for low-end
severe threats. Joint probabilities of 1000 J/kg of surface-based
CAPE and 30 knots of 0-6sm bulk shear are between 40% and 70% across
central portions of the CWA. The higher CAPE values would be
dependent on enough clearing in the wake of morning activity to
allow for sufficient destabilization. Dry air in the mid-
levels will support DCAPE levels of around 1000 J/kg during the
afternoon as well. However, this dry layer will need further
assessment as the event approaches, as too much mid-level dry
air may hinder the development of strong updrafts. Assuming
vigorous convection occurs, the wind threat still appears to be
the primary concern. Shear aloft may be sufficient for a
secondary threat of hail, although CAPE in the hail growth zone
may not be enough for a more significant threat. Weak low-
level shear will also largely negate the tornado threat.

The main uncertainty will be the timing of the passage of the
associated vort max and its impact on convective initiation. The
various CAMS advertise initiation anywhere from the PA/OH border to
just east of the CWA.  The solutions with a slower passage and thus
further west initiation during peak heating would be more favorable
for a larger potential area of severe risk. The current SPC day 2
marginal risk, aligned from PIT on eastward, appears well-placed at
this time.

While these storms may produce a needed wetting rain on a
localized basis, the HREF shows only small pockets of 30-50%
probability of more than a quarter-inch of rain. This activity
is not very likely to produce widespread benefit to the
drought...but if you want hope on that front, refer to the Long
Term section.

Most activity will move east of the region Saturday evening,
with skies clearing as mid-level ridging approaches. Along with
calming wind, valley fog will again be a possibility, especially
in locations blessed with daytime rain.

A brief return to dry and hot conditions is expected Sunday as
ridging crosses overhead. Widespread reading in the 80s are
expected, perhaps near 90 once again in eastern Ohio.

&&

.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 60-80% chance of at least a half-inch of
rain through 8 AM Wednesday, and a 35-55% chance of an inch.

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. 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 /01Z SATURDAY THROUGH WEDNESDAY/...
A passing disturbance Saturday will bring some isolated to
scattered showers to the area after 12Z, with probabilities for
a few thunderstorms also increasing after 18Z. If storms do
form, a few could become strong enough to produce gusty winds
and small hail, primarily east of PIT. Expect locally lower
cigs/vis in and around the heavier showers and storms, while
VFR is generally expected to prevail outside of convection.

A weak frontal passage brings northwest winds to all airports
Saturday afternoon/evening, though still only around 5-10 knots
(becoming light and variable Saturday night).

.Outlook...
The potential for restrictions Saturday night through Sunday
morning will depend on where precipitation forms Saturday and
where clearing occurs Saturday night. Latest hi-res ensemble
guidance shows the highest probabilities for fog (~50%) residing
west of PIT where there is a greater chance for clearing skies.
Meanwhile, the highest probabilities for low cigs (50-70%)
reside north and east of PIT, closer to the higher terrain.

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...Rackley/CL
SHORT TERM...CL
LONG TERM...CL
AVIATION...Cermak/CL