068
FXUS61 KPBZ 162303
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
703 PM EDT Thu May 16 2024

.SYNOPSIS...
Other than isolated showers in the higher elevations, dry
weather will continue this evening. Unsettled weather returns
for Friday through the weekend, with showers and thunderstorms
possibly providing isolated heavy rainfall totals. Near to above
normal temperatures will continue.

&&

.NEAR TERM /THROUGH FRIDAY/...
KEY MESSAGES:

- Isolated higher-elevation showers will end this evening.
- After several quiet hours, more showers arrive from the west
  towards sunrise.
- Above-normal low temperatures are expected.
  _____________________________________________________________

Updated PoPs over the far east for this evening. Seeing popup
showers over portions of the eastern border, mainly north of
Indiana PA. This activity should dissipate after sunset. Rest of
the forecast is in good shape.

Previous discussion...

CAMs still depict a few isolated showers/storms popping up over
the terrain this afternoon, and this seems reasonable as weak
convergence and lingering moisture should aid the process.
Satellite shows some towering cumulus in the ridges east of
Pittsburgh, and expect some radar returns to start showing up
shortly. Elsewhere, scattered to broken cumulus will remain.
Clouds, and showers, will collapse somewhat by sunset.

The evening hours will be generally quiet as an upper ridge
departs. Towards morning, increasing vorticity advection and
moisture on WSW flow aloft will begin to bring some light rain
into the region, although the bulk will wait until after 12Z as
a shortwave approaches. Low temperatures will remain several
degrees above normal.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/...
KEY MESSAGES:

- A pair of upper disturbances will bring shower and
  thunderstorm chances to the region.
- Severe weather risk is low, but isolated heavy rainfall is
  possible.
- Near-normal high temperatures and above-normal low
  temperatures can be expected.
  ______________________________________________________________

A pair of shortwave troughs will impact the Upper Ohio Valley during
this period.  The first crosses the Great Lakes on Friday, with a
very weak to nearly nonexistent surface low reflection. Upper
jet support will be fading as the wave rotates across the
eastern lakes. HREF mean instability progs generally keep CAPE
at 750 J/kg or less. While 0-6km bulk shear of 35-40 knots is
possible, suggesting storm organization, the severe threat still
appears low, with CSU machine- learning guidance in agreement.
Localized heavy rainfall does appear to be possible with near-
saturated model soundings and precipitable water values of
around 1.4 inches progged, which is essentially at the top end
of the climatology. That said, NBM QPF expectations are not
excessive, with 24-hour probabilities of 0.50 inch or greater
through 8 AM Saturday in the 30-50 percent range, and chances of
an inch under 15 percent.

The second is a larger trough that will impact more of the eastern
CONUS through Saturday night. With an upper jet rotating through the
trough across the Tennessee Valley, favored left entrance region jet
dynamics lie closer to the Middle Ohio Valley and areas south of the
Mason-Dixon Line.  Thus, higher PoPs appear appropriate across the
southern CWA. Again, NBM 24-hour half-inch probabilities remain
below 50 percent with the same north-to-south gradient. Severe
threat remains very low as well.

Diurnal temperature spreads will be fairly low given the expected
clouds and rain, with near-normal maximums but above-normal
minimums.

&&

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

- Shower chances linger into Sunday as the second disturbance
  departs.
- Ridging should result in dry and warmer weather Monday and
  Tuesday.
- Rain chances return for Wednesday and Thursday.
  ____________________________________________________________

The earlier trend towards a more southerly track for the main
shortwave trough on Sunday continues in the recent guidance. This
does lead to generally low PoPs on that day, focused on areas south
of Pittsburgh where moist easterly flow lingers during the morning.

Ensembles then seem to be in decent agreement in keeping this
trough/upper low near or just off the southeast CONUS coast, while
slowly rising 500mb heights take control over the Upper Ohio Valley.
This points to mainly dry weather from Sunday night through Tuesday,
with increasingly above-normal temperatures through early next week.

The next shortwave trough then drives across the Upper
Mississippi Valley towards the Great Lakes during the midweek
period. Model cluster guidance shows strength and timing
differences with this, which point to various scenarios for
precipitation amounts and severe weather chances. In any case,
PoPs topping in in the likely range and muted, but still warm
high temperatures looks to be the forecast of least regret at
this time.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
VFR expected through at least 10z as ridging shifts east of the
area. The next low pressure system will approach the area Friday
morning, with shortwave activity initially lifting north of the
region. Showers, with low-probability thunder, are likely ahead
of this feature after sunrise, favoring eastern OH to northwest
PA where forcing/lift will be greatest.

Rain coverage should taper off and become more
isolated/scattered Friday afternoon, though the chance for
lightning will increase. For now, will continue to run with
PROB30 TSRA for the afternoon hours given lower spatial
confidence of impacts.

MVFR cig restrictions will favor FKL/DUJ after 18z, with
probabilities increasing areawide after 06z Saturday.

.Outlook...
Upper trough passage to the south Saturday will maintain
shra/tstm chances focused south of KPIT, with varying degrees of
restriction potential.

VFR and dry weather become high probabilities Sunday into Monday
under the influence of high pressure.

&&

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

&&

$$

SYNOPSIS...CL
NEAR TERM...CL
SHORT TERM...CL
LONG TERM...CL
AVIATION...Rackley/Frazier