185
FXUS61 KPBZ 161137
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
737 AM EDT Fri Aug 16 2024

.SYNOPSIS...
An upper level disturbance will cross the region this afternoon
increasing the risk for showers and storms. A cold front will
cross the region late Saturday continuing the risk for showers
and storms. The weather pattern will remain unsettled into early
next week.

&&

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

- Two shortwave troughs will cross the area today. Chance for
  showers and storms, with the most likely time this afternoon.

- Temperatures remain near normal.

-------------------------------------------------------------------

A couple of shortwaves, moving eastward on the southeastern
flank of the large western Great Lakes 500mb low, will cross the
region today. The first will be this morning and the second
late this afternoon and evening. The first wave is weak and will
be banging into a upper level ridge currently located over the
region. The first wave will not be able to do much to the ridge
as the second wave will dig into western Ohio this morning and
cause additional height rises over the area. Any rainfall with
the morning wave has been light and on the order of a tenth or
two of an inch so far with plenty of dry air noted below 750 mb
on the 12z PIT sounding. As the low levels saturate some more
into this morning, some scattered showers will continue, but the
main theme will be cloudy skies.

The second, and stronger wave, will approach the western
reaches of the forecast area early this afternoon and will reach
the eastern edge as sunset approaches. This wave will be able to
eject the ridge eastward and cause minor height falls as it
crosses. The first wave will likely work to eliminate the drier
air in the lower levels, so the second wave will have a more
saturated atmosphere to work with. At least scattered showers
and storms look like a good bet this afternoon, but the severe
threat remains murky. Instability will be lacking with the HREF
showing CAPE values of less than 1000 j/kg over the entire area,
and less than 500 j/kg outside of Ohio. Shear will also be weak
with 20 to 25 kts possible this afternoon. Additionally, we
should have plenty of cloud cover today. Prob thunder on the NBM
is generally 30 to 40% with the highest late this afternoon and
evening. The latest CAMs do show a line of convection
developing just ahead of the shortwave late this afternoon, but
with a lack of shear, it struggles to remain organized as it
heads eastward. The current marginal risk looks reasonable as an
isolated strong to severe storm is not out of the question.
PWATS will rapidly increase today, so any stronger storms could
produce heavy downpours.

High temperatures will be a bit tricky today with the threat of
morning showers and ample cloud cover. Will go below NBM
guidance today, which will put highs very close to seasonal
averages.

&&

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

- Slow moving upper low to bring unsettled weather through the
  weekend.

- Strong to severe storms possible Saturday afternoon and
  evening with the passage of a cold front.

-------------------------------------------------------------------

The Great Lakes upper low will slowly drift eastward on
Saturday. Another short wave trough will cross the region late
in the day and drag a surface cold front with it. Instability
will be stronger on Saturday, but the shear remains marginal.
The surface boundary will provide a better forcing mechanism,
but we will still have to deal with ample cloud cover. The
severe threat looks better on Saturday, but this is nowhere near
a slam dunk. Models are hinting at some drier air moving in
ahead of the front which could allow for breaks in the cloud
cover, increasing the severe threat.

The upper low moves little on Sunday, across southwest Ontario
with upper trough in Ohio, so another day of diurnally driven
showers and thunderstorms is expected. Shear and instability
look to be weaker on Sunday which would decrease the severe
threat.

&&

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


- Ensembles favor Great Lakes troughing into the middle of next
  week but with a drying trend.

-------------------------------------------------------------------

Ensembles indicate that long wave trough will continue to
linger in the Great Lakes and Northeast States Monday, Tuesday,
and Wednesday. There are subtle indications that the trough
starts to lift out a bit on Thursday. The standard deviation of
500 mb heights in this area gradually increase with time Tuesday
and Wednesday as the details of short wave troughs become less
and less evident.

While diurnally driven showers are still a good bet under the
upper trough on Monday, confidence has increased that a large
Canadian high, and push of drier air, will move in for the
Tuesday to Thursday time frame with sunshine and seasonable
temperatures.

&&

.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
Mid and high clouds will stream across the area this morning as
a warm front lifts through with lowering, though still VFR,
CIGs in the 5-9kft range. Plenty of dry air is noted below 750
mb on the 12z PIT sounding, so only some light showers are
expected early this morning. As the low levels saturate
further, some restrictions to MVFR VIS are possible late morning
in any moderate showers.

Meager instability will build across the area by Friday
afternoon limited by high confidence thick cloud coverage
across most of the area save eastern OH where some breaks are
possible headed into this afternoon. An initial shortwave
trough, rotating around an upper low across the Midwest, will
cross the Upper Ohio Valley region this afternoon and early
evening. This should result in scattered showers and
thunderstorms developing, but confidence in coverage is
decreasing given the limited instability and lack of surface
forcing mechanism. Continued to carry only VCTS in the TAFs for
the most likely time of occurrence (after 18z for ZZV, 20z for
PIT, and 22z for LBE).

Instability will diminish Friday evening with a few showers
possible until the shortwave completes its passage. Ceilings
will then lower overnight with lingering low level moisture
resulting in restrictions to MVFR to IFR, primarily after 06z
Saturday, with probability for MVFR at 60-90% and IFR around
50-80%. Even at any sites that do receive rain today, fog
chances are lower with wind expected to remain elevated
overnight.

.Outlook...
Periodic restrictions and showers and thunderstorms are possible
through early Monday as low pressure slowly crosses the region.
The greatest coverage of showers/thunderstorms will be in the
afternoon and early evening. VFR returns by Tuesday under high
pressure.

&&

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

&&

$$

SYNOPSIS...22
NEAR TERM...22/MLB
SHORT TERM...22
LONG TERM...Craven
AVIATION...WM/Rackley/MLB