-- Discussion containing changed information from previous version are highlighted. --
483
FXUS61 KPBZ 201714
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
114 PM EDT Mon May 20 2024

.SYNOPSIS...
Mostly dry weather and a warming trend is expected through
Tuesday. Shower and thunderstorm chances return Wednesday and
continue through Thursday, with severe weather a possibility.

&&

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

- Mostly dry and warm day with plenty of sunshine.
  _____________________________________________________________

The main change to the forecast for today was to add around a
15% PoP for the extreme northern CWA. While the abundant
sunshine will allow surface-based CAPE of 500-1000 J/kg to build
this afternoon, the present upper-level ridge and weak flow
should allowed this instability to become unrealized in most
cases. The lone exception could be isolated convection that
could occur along a Lake Erie-induced breeze boundary this
afternoon. A couple of the CAMs hint at this possibility, and
although confidence is very low, did think that a minimal PoP
addition north of I-80 was prudent. No other changes to the
forecast other than minor tweaks.

Previous discussion...

Eastern ridge will amplify today and the area will lie beneath
the ridge axis. This will keep the area dry, allow for lots of
sun, and support well above normal temperatures.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY/...
KEY MESSAGES:

- Mostly dry weather is forecast through Tuesday night.
- Temperatures continue to climb to 10-15 degrees above normal, with
  a few record high values possibly approached on Tuesday.
- Risk for storms increases Wednesday.
  _____________________________________________________________

A weak shortwave trough will drift across Ohio on Tuesday. This
will flatten the ridge and shove it over the Atlantic coast.
Moisture will be limited with this feature and the stronger lift
within the trough will swing south of the area. For now, models
are just showing an increase in high clouds on Tuesday.

The bigger story during this period remains the well-above
normal temperatures. Plenty of sun, a drying ground, and 850 mb
temps climbing into the 14-16C range will contribute to highs
reaching well into the 80s Tuesday. Tuesday may be the warmest
day on an areawide basis. NBM probabilities of 90 degree or
higher temperatures are 50 to 70 percent range in river valleys
and metro areas on that day. The May 21st record high at DuBois
(86, from 2022) looks to be in greatest jeopardy. However, if
the warmer end of guidance proves to be correct, other records
at Pitt International (92, from 1911), Morgantown (92, from
1934) and New Philadelphia (91, from 1962) could at least be
approached.

Model guidance still favors an upper low/trough in or near the
northwestern Great Lakes by Wednesday afternoon. Given the
increased shear expected with the system, as well as the
potential for decent instability in the warm and humid
conditions ahead of the front Wednesday, the afternoon/evening
period of that day appears to hold the best potential for severe
weather. CIPS/CSU guidance, NBM-based CWASP progs, and SPC all
seem to suggest this possibility, with areas west of Pittsburgh
showing the best potential. This will of course be given
increased scrutiny as we approach the midweek period.
Temperatures will likely be muted a bit by the approaching
clouds and rain Wednesday afternoon, especially west of
Pittsburgh, but will still remain well above normal.

&&

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


- Storm threat continue into Thursday, with some potential for
  strong thunderstorms.
- Near to just above seasonable temperatures and lower rain chances
  are expected Friday and into the weekend.
  ____________________________________________________________

A second shortwave trough, will cross the Michigan area
Wednesday night and get absorbed into the larger trough. Minor
strength and timing issues remain, but overall, the pattern
still favors higher PoPs with the second shortwave, bridging the
Wednesday night/Thursday period, which will likely also involve
a cold frontal passage at the surface during the day on
Thursday.

It appears that the Thursday night/Friday period will be relatively
quiet with weak surface ridging indicated, along with temperatures
closer to climatology. Model details become more unclear over the
weekend, but with the possibility of weak shortwaves crossing the
Middle/Upper Ohio Valley in WSW flow aloft, some lower-end PoPs in
the 20 to 40 percent range are necessary for both Saturday and
Sunday. Temperatures may nudge up to a few degrees above normal.

&&

.AVIATION /17Z MONDAY THROUGH FRIDAY/...
-- Changed Discussion --
VFR are expected through the TAF period under the influence of
high pressure, with light southerly winds increasing Wednesday
afternoon out the southwest. Weak shortwave movement may be
enough to generate isolated convection at the end of the TAF
period, but probabilities of terminal impact are too low for
mention (less than 20%). The more likely result is an uptick in
sct to bkn afternoon cu field.

Temperature moderation will limit fog potential (current
probabilities are less than 15% for reductions below 5SM) prior
to sunrise Wednesday.

.Outlook...
Shower and thunderstorm chances increase Wednesday 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.
-- End Changed Discussion --

&&

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

&&

$$

SYNOPSIS...22/CL
NEAR TERM...22/CL
SHORT TERM...22/CL
LONG TERM...CL
AVIATION...Frazier