-- Discussion containing changed information from previous version are highlighted. --
186
FXUS61 KPBZ 270607
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
207 AM EDT Mon May 27 2024

.SYNOPSIS...
An additional round of showers and storms will cross the Upper
Ohio Valley late tonight through early morning, with spotty
showers possible again during the afternoon Monday as a cold
front crosses the region. A few cooler days are anticipated thru
mid-week as low pressure aloft becomes stronger in the eastern
part of the country.

&&

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

- An additional round of showers/storms will cross the region
  late tonight, but is not expected to bring severe weather
-------------------------------------------------------------------

10pm Update: Severe weather threat has waned, as storms have
outpaced lingering buoyancy and the atmosphere stabilizes.
Lingering showers/storms near the ridges will advance newd. An
additional round of showers/storms is anticipated late tonight,
with no severe threat expected.

7pm Update: A squall line will continue to advance into western
PA this evening. Damaging wind gusts have been reported in a
number of counties in eastern Ohio. Surges within the line
segment may prompt a tornado warning. With the fast track of the
line, flash flood will not be a concern through midnight.

Previous Discussion:

Flat upper ridging and surface high pressure make a brief
appearance in the wake of Saturday`s cold front and provide a
dry day through this afternoon. This will be the best day of
the holiday weekend with little noticeable difference in the
airmass behind the cold front as NBM probability for >85F is
40-60% mainly from Pittsburgh south. It will feel on the muggier
side too with dew points in the low to mid 60s. The only
wrinkle will be increasing moisture in developing southwest flow
aloft that will overspread mid/upper level clouds this
afternoon ahead of a line of showers and thunderstorms that will
move across the area.

Latest CAMs have been consistent advancing an already developed squall
line across the Tennessee Valley into our area with the support
of an upper shortwave. Most likely timing to reach our western-
most counties being around 5pm. The highest threat for severe
will be in eastern Ohio where a Slight Risk (2/5) is in place;
elsewhere the Marginal (1/5) remains. Despite the line`s arrival
into western PA around sunset, it may be able to maintain its
strength aided by a developing 30-50 knot 850 mb jet and hi res
ensemble mean CAPE of 1000-1500 J/kg. Of concern with this line
will be primarily damaging wind, but a conditional QLCS tornado
threat may exist as well. Forecast soundings depict a veering
wind profile ahead of the line (resultant of the increasing
south/southwest LLJ atop a southerly/southeasterly surface
wind), 20-30 kt of 0-3 km shear oriented nearly line-normal to
the northwest/southeast oriented squall line, and up to 100 J/kg
of 3CAPE to promote stretching. Thus, if the line remains
balanced/shear dominant, in addition to a damaging wind threat,
any part of the line that surges with an orientation
perpendicular to the 0-3 km shear vector may support the risk
for QLCS tornadogenesis. The one question, and potential
limiting factor for tornadoes, will be DCAPE values >750 J/kg
which may aid in making the line cold pool dominant and thus
limiting the tornado threat. The 2% tornado risk has been
expanded further east because of this concern. Heavy rain may
accompany the line as well which could pose a localized flooding
concern especially across urban areas and locations that got
the most rain the past 24 hours.

The line will push out of our area around midnight and give us brief
break before additional energy pivots around an upper low across the
Great Lakes and redevelopment of rain showers is expected south and
west of Pittsburgh. Hi res ensemble 90th percentile (reasonable
worst case scenario) doesn`t paint much more than 0.30"/hr rates, so
despite this area seeing the most rain in the past 24 hours, don`t
expect to see widespread flooding concerns outside of perhaps some
minor issues in flood prone areas.

&&

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

- Another round of potentially strong to severe thunderstorms Monday
  afternoon
- Daily rain chances and below normal temperatures take over
-------------------------------------------------------------------

Ensembles indicate only slight deviations from the mean in regard to
the depth of an upper trough and additional shortwave energy
traversing an upper low spinning across the Great Lakes through
mid week. Aided by daytime heating and slow passage of a cold
front, yet another round of strong to severe thunderstorms is
possible on Monday with most of our area again outlined in a
Marginal Risk save for the higher elevations of PA/WV which are
in a Slight Risk. With the threat for any lingering boundaries
from Sunday or other focuses of development not being resolved
by current CAMs, confidence in coverage and development is low.
That said, initiation along the ridges by late morning quickly
moving off to our east is possible. Along and ahead of the cold
front, we`ll likely see additional convection across eastern OH
and western PA, but destabilization remains in question owing to
breaks of cloud coverage as southwest flow aloft advects in
moisture ahead of the front. Large spreads in NBM low end vs.
high end coverage lend lower confidence to this aspect of the
forecast. Mean CAPE is progged around 800 J/kg and effective
shear near 30 knots, so some strong to severe storms are
possible with primary threats of damaging wind and hail, but
trends will need to be monitored. Highs Monday will be thus be
cooler than those over the weekend with mid 70s most likely but
dependent on the cloud cover.

The upper low will swing more shortwaves around its base and aid in
daily, diurnally driven scattered precipitation chances as well as
below normal temperatures. Late Tuesday into Wednesday morning may
feature the highest rain chances as a more potent shortwave and
surface trough cross the region with 24 hour totals ranging from a
trace to 0.75" at most.

&&

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

- Cool temperatures continue through the end of the week
- Dry weather favored to take over by Thursday
-------------------------------------------------------------------

By Thursday, ensemble disagreement increases with the axis and
strength of the upper trough, but any flavor of solutions doesn`t
kick it out of the eastern CONUS until toward next weekend. Broad
surface high pressure should build on Thursday and turn the area
dry, but it`s possible that it`s slower to build and shower chances
continue into Thursday especially as a potent upper wave makes one
final pass through the area before upper ridging finally builds with
decent confidence among the long range ensembles. Lows may dip as
cool as the low to mid 40s come mid to late week as the cooler
airmass settles in with highs 5-8 degrees below average.

&&

.AVIATION /06Z MONDAY THROUGH FRIDAY/...
-- Changed Discussion --
An additional round of showers/storms will advance west to east
across all terminals from 07Z-12Z with moderate confidence, but
should pose no concern beyond moderate rain and reduced
vsby/cigs.

Most of Mon will remain MVFR-VFR at all terminals until a cold
front crosses the region in the afternoon. Sufficient heating of
the boundary layer may occur to yield modest buoyancy as the
cold front crosses, helping invigorate a cu field and scattered
showers after 18Z. Confidence is highest in convection east of
PIT, so have included PROB30s in for DUJ/LBE/MGE. Elsewhere,
timing and placement of showers is too unclear to include more
than VCSH at the moment. These should clear by early evening.
Gusty wind to 20-25 kts will slowly diminish overnight.

.Outlook...
Periodic restrictions and rain chances continue through mid-
week as a series of disturbances move through the region within
broad ern-CONUS low pressure aloft.
-- End Changed Discussion --

&&

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

&&

$$

SYNOPSIS...MLB
NEAR TERM...MLB
SHORT TERM...MLB
LONG TERM...MLB
AVIATION...Kramar/Rackley