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354
FXUS61 KPBZ 210756
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
356 AM EDT Tue May 21 2024

.SYNOPSIS...
Another very warm day with plenty of sunshine and mainly dry
conditions. The risk for showers and storms returns Wednesday
and Thursday, with severe weather a possibility.

&&

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

- Isolated showers/storms are possible this afternoon, mainly
  north and over the ridges.
- Well-above normal temperatures today that may approach record
  levels at a couple of climate sites.
  _____________________________________________________________

Upper level ridge will flatten and be pushed eastward this
morning as a weak shortwave trough slowly travels across the
region today. The best lift within the trough will sink well
south of the area. A popup afternoon shower or storm cannot be
completely ruled out as the trough drifts across PA, but
moisture will be limited. The main impact from the wave will
likely be an increase in cirrus clouds and scattered cu this
afternoon. Temperatures will again be well above normal.
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&&

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

- The risk for severe storms increases Wednesday afternoon and
  evening, especially north and west of Pittsburgh.
- Scattered showers/storms on Thursday as a cold front crosses.
  _____________________________________________________________

A quiet and warm night, under brief ridging aloft.

A strong shortwave trough will swing through Ohio Wednesday
morning and then clear western PA by early evening. This wave
will increase the risk for showers and storms as it moves
through. A cold front will trail behind the upper level wave,
but it likely won`t begin to impact the area until sometime
after dark.

The degree and placement of the severe risk will depend on timing of
the boundary/trough mentioned above.  At this time, western portions
of the CWA would seem to have higher risk given the likely lean
towards evening arrival, with perhaps some weakening as storms moved
east across the CWA.  SPC and NBM CWASP guidance seem to agree with
this general thinking, leaning towards areas north and west of
Pittsburgh. Deep shear in the 0-6km layer appears fairly
marginal for a supercell threat at this distance, but given the
ongoing heat and humidity ahead of the boundary, instability
should be sufficient for some threat of damaging wind and hail.
Storm strength will likely wane after midnight as noted above,
but at least scattered showers and storms will linger as the
cold front enters the forecast area. High temperatures on
Wednesday will depend greatly on how soon clouds and rain
arrive, but they still will be well above normal.

The cold front will very slowly cross the region on Thursday as
zonal to southwest flow aloft will work against its southward
movement. Showers and storms are likely to continue on Thursday
with the crossing boundary. The severe weather risk on Thursday
should be tempered by the clouds and rain limiting daytime heating,
and thus the buildup of instability.  This will also keep
temperatures a bit closer to climatology.

&&

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

- Relatively dry with seasonable temperature behind the front
  Thursday night into Friday.
- Near to just above seasonable temperatures chances of
  scattered showers/storms return for the holiday weekend.
  ____________________________________________________________

Meanwhile, an upper low over Ontario will likely weaken and
drift into or near New England by Friday evening. While this
happens, upper flow over our region will transition through
zonal to weak ridging, according to ensembles/cluster analysis.
The cold front should also be able to clear our CWA to the south
as well, but there is a bit of uncertainty with this. Thus,
while the Thursday night into Friday night period should be
relatively dry, some chance-level PoPs will need to be
maintained, especially in areas south of Pittsburgh closer to
the potential position of the front.

There are indications that a weak shortwave will attempt to
undercut the ridging by later Friday night into Saturday, which
could push moisture back northward and lead to a bit better
precipitation coverage. The forecast for the remainder of the
holiday weekend is muddled, as it will depend on the possibility
of the old frontal boundary moving back northward, as well as
shortwave details in the general WSW flow aloft. As such, a dry
day cannot be guaranteed from Saturday through Memorial Day.
Temperatures overall from Friday on will be a bit above normal
for the most part, given the overall flow pattern.

&&

.AVIATION /08Z TUESDAY THROUGH SATURDAY/...
VFR is expected through the TAF period under continued high
pressure. Light and variable winds are expected through the
overnight, increasing to around 5-10 kts from the southwest by
Tuesday afternoon.

Fog is unlikely this morning due to temperature moderation.
Afternoon heating will result in scattered cumulus.

.Outlook...
Shower and thunderstorm chances increase Wednesday afternoon
into Thursday with the passage of a low pressure system; timing
on the arrival of the first wave of storms remains varied.
Thunderstorms will be capable of producing gusty and erratic
wind.

Periodic thunderstorm chances and associated restrictions are
likely through the weekend as an active weather pattern
develops.

&&

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

&&

$$

SYNOPSIS...22/CL
NEAR TERM...22
SHORT TERM...22/CL
LONG TERM...CL
AVIATION...Rackley/Shallenberger