383
FXUS61 KPBZ 160534
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
134 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 /THROUGH TODAY/...
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.

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

Only a minor change to the PoPs overnight. Returns on the radar
are currently not reaching the ground due to a very dry layer
of air below the cloud deck. Have decreased PoPs overnight but
increased cloud cover.

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. Interestingly, 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 should be light, with much of
it struggling to reach the surface.

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 CAM models 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 /TONIGHT 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 /06Z FRIDAY THROUGH TUESDAY/...
High pressure will slide east through the overnight, maintaining
VFR through much of Friday. Mid and high clouds will increase
overnight ahead of an approaching warm front. The front will
lift NE across the region Friday morning, with lowering, though
still VFR, cigs. Models are a bit more robust with low-
probability late-morning light showers, so have included Prob30
groups.

Instability should build across the area by Friday afternoon,
with HREF MU CAPE 500-1000 j/kg. An initial shortwave trough,
rotating around an upper low across the Midwest, will also cross
the Upper Ohio Valley region Friday afternoon and early evening.
This should result in scattered showers and thunderstorms
developing, but confidence in coverage is decreasing. Continued
to carry only VCTS in the tafs for the most likely time of
occurrence. The main uncertainty in the coverage of
thunderstorms will be existing cloud cover through the day, and
the amount of destabilization that occurs.

Instability will diminish Friday evening, with a few showers
possible until the shortwave completes its passage. Ceilings
will then lower overnight, primarily after 06z Saturday.

.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
SHORT TERM...22
LONG TERM...Craven
AVIATION...WM/Rackley