-- Discussion containing changed information from previous version are highlighted. --
982
FXUS61 KPBZ 211806
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
206 PM 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 /THROUGH TONIGHT/...
KEY MESSAGES:

- Marginal risk (1/5) of severe weather east of I-79. Primary threat
  is damaging winds from 1pm to 5pm.
- Clearing skies late with areas of fog overnight.
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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 primarily 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
>40 knots in the 0-6 km layer 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).

Some areas of dense fog tonight are possible with hi res
ensemble probabilities up to 70%, especially where rain falls
combined with clearing skies, a stagnant airmass, and
achievement of crossover temperatures.

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/...
KEY MESSAGES:

- Dry with above average temperatures on Sunday.
- Rain chances (yes, rain chances) increase starting Monday.
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A shortwave ridge axis will slide overhead in the wake of the
departing disturbance from Saturday with height rises and warm
advection supporting temperatures again into the 80s, warmest in
eastern Ohio, with 90 not out of the question there. High clouds
will increase in the evening from the west ahead of approaching low
pressure.

The main takeaway headed into the next workweek is that we are
finally seeing appreciable chances for beneficial rain. The upper
ridge axis will quickly be shunted east by an approaching trough,
and a shift to southwest flow aloft on its backside will open the
door for better moisture to work its way into the region. Clustered
ensembles exhibit pretty good agreement through Tuesday night with
just some strength and timing differences of the progression of the
responsible upper trough. Rain chances will increase Monday morning
as surface low pressure progged to be across Michigan pulls a trough
through the area. Ensemble probability for >0.5" of rain through
Tuesday night is still high (up to 80%) and a 50-70% chance of >1".

&&

.LONG TERM /WEDNESDAY THROUGH FRIDAY/...
KEY MESSAGES:

- Rain chances increase Monday and Tuesday with a widespread,
  beneficial rainfall possible.
- Rain chances continue for the latter half of the week but
  confidence in amounts is low.
- Temperatures closer to average through the week.
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Uncertainty then increases significantly with ensembles
becoming much more spread on how the upper pattern evolves. The
majority consensus is that the Ohio Valley remains in a
troughing setup with significant differences on placement and
amplitude of the trough, but even one solution is attempting to
establish a ridging pattern. For continued beneficial rain, we
would want the solution(s) that sets up a trough deeper and off
to our west. Long story short, scenarios exist where additional
beneficial rain sticks around for the latter half of the week,
and scenarios exist for a drier period.

Despite this uncertainty, at least continued low precipitation
chances are appropriate through the end of the work week. 72 hour
precip totals have seen a slight eastward nudge in the past several
ensemble runs with probability for another >0.5" from Wednesday
through Saturday morning at 40-50% east of Pittsburgh and 20-40%
further west. A few high end, low probability 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 /18Z SATURDAY THROUGH THURSDAY/...
-- Changed Discussion --
Scattered showers and storms have formed ahead of an advancing
cold front, supported by a shortwave trough aloft. These are
already east of a rough FKL/AGC/HLG line, and will continue to
move east, largely exiting the region by 22Z. Along with brief
downpours creating potential drops to IFR visibility, the
stronger storms will be capable of producing gusty wind and
hail. Expect locally lower cigs/vis in and around showers and
storms, while VFR is expected to prevail outside of convection.

Southwest wind around 10 knots away from thunderstorms this
afternoon with a shift to west/northwest expected behind the
boundary, with speeds of 5 to 10 knots.

Skies will largely clear after sunset behind the front. Fog
potential tonight will depend on where precipitation occurs/has
occurred today and where the best clearing occurs tonight. DUJ,
LBE, and MGW appear to have the highest threat of IFR to LIFR
visibility after 06Z. Fog will lift by 14Z, with VFR conditions
for the remainder of Sunday.

.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
SHORT TERM...MLB
LONG TERM...MLB
AVIATION...CL