-- Discussion containing changed information from previous version are highlighted. --
749
FXUS61 KPBZ 202347
AFDPBZ

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

.SYNOPSIS...
Mostly dry weather and continued well-above normal temperatures
are forecast through Tuesday under upper ridging. Better shower
and thunderstorm chances return Wednesday and continue through
Thursday, with severe weather a possibility.

&&

.NEAR TERM /THROUGH TUESDAY/...
-- Changed Discussion --
KEY MESSAGES:

- Mostly dry through tonight.
- Temperatures of around 10 degrees above normal are forecast.
  _____________________________________________________________

No appreciable changes needed to the forecast this evening as a
few showers remain over the ridges where orographic lift has
acted as the only source of lift, but any such activity is
struggling with copious dry air noted on the 00z PIT sounding
and will end with sunset. The overnight period is expected to be
quiet underneath an upper ridge with just some high clouds
spilling over the axis. As far as fog potential, given another
day of drying and mild overnight lows (10 degrees above normal),
do not expect much development tonight. Latest ensemble
probability holds at less than a 20% chance of visibility less
than 5 miles through daybreak Tuesday.

Lows will be well above average tonight in the low 60s for
most.
-- End Changed Discussion --

&&

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

- Isolated showers/storms are possible Tuesday afternoon, with
  well-above normal temperatures that may approach record levels
  in a couple of cases.
- The risk for severe storms increases for later Wednesday
  afternoon and evening, especially north and west of
  Pittsburgh.
  _____________________________________________________________

Models still indicate that a weak shortwave trough will knock down
the ridge somewhat and help to push the axis off of the Atlantic
coast on Tuesday. This could lead to some isolated to scattered
shower/storm development Tuesday afternoon, as HREF progs
indicate 750-1250 J/kg of surface-based CAPE will develop.
However, there may be an issue with the amount of dry air in the
mid-levels potentially stunting development. For now, will go
above the NBM PoP suggestions and add isolated PoPs to areas
north and west of Pittsburgh, where CAPE will be highest. With
shear low and with the downburst threat appearing minimal,
severe chances are quite low Tuesday.

The bigger story Tuesday remains the well-above normal
temperatures, which should climb well into the upper 80s in many
locations. Think that the NBM depiction of chances of 90-or-
above highs may be a bit overdone, but some localized high
temperatures to that level are certainly possible, and went a
tick above the blended guidance mean values. As far as the
potential for record high temperatures, the May 21st record high
at DuBois (86, from 2022) still looks to be in greatest
jeopardy. However, if temperatures overperform tomorrow, other
records at Pitt International (92, from 1911), Morgantown (92,
from 1934) and New Philadelphia (91, from 1962) could at least
be approached.

After a quiet and warm Tuesday night, there is decent model
agreement that a compact but potent shortwave trough will ride
across the Upper Mississippi Valley Tuesday night, closing off
into an upper low near the western Great Lakes and drifting into
Ontario Wednesday night. The surface reflection of this system
will push a cold front across the lakes and into our region by
Wednesday night, with a possible prefrontal trough out ahead of
the main boundary. This will provide the basis for increased
shower and thunderstorm chances during the afternoon and evening
period on Wednesday.

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.

&&

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


- Scattered showers/storms on Thursday as a cold front crosses.
- 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.
  ____________________________________________________________

The main cold front will likely be crossing the area Thursday
morning with at least scattered showers and possibly a few
thunderstorms accompanying it.  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 through Thursday night.

Meanwhile, the Ontario upper low will likely slowly 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 /00Z TUESDAY THROUGH SATURDAY/...
VFR conditions are expected through the TAF period under the
influence of high pressure, with light and variable winds
tonight, but increasing Tuesday afternoon out the southwest to
around 10 knots at most.

Temperature moderation will limit fog potential (current
probabilities are less than 15% for reductions below 5SM) prior
to sunrise Tuesday morning. Afternoon heating will result in
scattered afternoon cumulus.

.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.

&&

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

&&

$$

SYNOPSIS...CL
NEAR TERM...MLB/CL
SHORT TERM...CL
LONG TERM...CL
AVIATION...Shallenberger