475
FXUS61 KPBZ 141805
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
205 PM EDT Tue May 14 2024

.SYNOPSIS...
Scattered showers, and a thunderstorm or two, remain possible
through Wednesday as an upper trough approaches and crosses. Dry
weather is forecast Wednesday evening through Thursday evening,
before unsettled weather returns to end the week. Temperatures
will remain slightly above normal.

&&

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

- Scattered light rain showers will remain possible through the
  night
- Thunderstorm chances remain low, with no severe threat.
- Above-normal low temperatures are forecast.
  ______________________________________________________________

Light rain showers/sprinkles continue to lift northward across
the region in a pattern of weak vorticity advection. Although a
thunderstorm or two remains possible, near-term mesoanalysis
shows CAPE values peaking in the 500-600 J/kg range, with ACARS
soundings showing generally unfavorable lapse rates.

A better round of vorticity advection exists upstream in the
Middle Ohio Valley, where a more robust area of showers
currently exists. This forcing should arrive in the southwestern
portion of the CWA after 00Z, with higher PoPs appropriate.
Forcing overall appears to weaken as a Missouri upper low fills
somewhat, so more scattered activity is indicated overnight for
the rest of the region. QPF overall remains muted, and will
remain below a quarter-inch in most cases. The abundant moisture
and cloud cover will help to keep temperatures above normal,
with most lows in the upper 50s and lower 60s.

&&

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

- Shower/thunderstorm chances continue through Wednesday, with
  little risk of strong thunderstorms or heavy rainfall.
- Ridging provides drier weather Wednesday night into Thursday
  night.
- Near to above normal temperature expected.
  ______________________________________________________________

Most guidance is in good agreement that the main upper trough,
centered over the Middle Ohio Valley at 12Z Wednesday, will track
slowly eastward through Wednesday night. This will lead to better
coverage of showers and a few thunderstorms during this period as
compared to today. This will be especially true to the northwest
of Pittsburgh, where a slow-approaching surface boundary
associated with Hudson Bay low pressure will exert a bit of
influence. This is also where thunderstorms will be a bit more
likely, as HREF progs show better surface-based instability of
500-750 J/kg during the afternoon. However, severe risk appears
minimal given this low level of CAPE, as well as unimpressive
shear.

Precipitation coverage should wane Wednesday evening as the
trough departs and as the surface boundary continues to weaken
as it crosses. Ridging will then take hold, with minimal
precipitation chances late Wednesday night through most of
Thursday night, although some showers may begin to show up in
the western CWA prior to 12Z Friday ahead of the next shortwave.

Chances of heavy QPF values remain low, with probabilities of an
inch or more remaining generally 25 percent or less according to
the NBM. Some localized higher totals may be possible in
thunderstorms, but in general, 0.25 to 0.50 inch can be expected
from tonight through Wednesday evening.

&&

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

- Forecast uncertainty increases during the extended period.
- Another trough is likely to provide unsettled weather at least
  for Friday and Saturday, although rain amounts are iffy given
  the uncertainty.
- Temperatures near or above normal are expected.
  ______________________________________________________________

Uncertainty creeps upward in the extended period.  The mid-level
ridge will be departing the Appalachians Friday morning, with
moisture increasing once again ahead of the next trough, which
ensembles have crossing the Mississippi River Friday evening and
reaching the Upper Ohio Valley by Saturday.

Thereafter, speed and timing differences arise in the models with
the handling of this trough. GEFS/GEPS members are favoring a
quicker departure and a weaker trough, while ENS favors the opposite
outcomes. Differences then continue to get larger from there, with
cluster analysis indicating anything from strong ridging to the
presence of a closed upper low is on the table by early next week.

For now, will roll with the NBM suggestions for the extended period
and wait for increased model clarity. This would suggest a return of
unsettled, but likely not overly impactful weather Friday and
Saturday, with lower rain chances during the early part of next
week. Near to slightly above normal temperatures are currently
forecast, but again, there is a sizable range of outcomes here.

&&

.AVIATION /17Z TUESDAY THROUGH SATURDAY/...
Moist advection ahead of and eventually around an upper level
low that will traverse south of the region is expected to
promote widespread low VFR to MVFR conditions through the
period. Lack of strong forcing will limit probability of
terminal impact from isolated showers through 06z, before a more
noteworthy wave promotes greater shower coverage between 06z and
21z Wednesday. Of note, models highlight potential frontogenetic
banding that would support a swath of showers and low
probability thunderstorms across eastern OH into northwest PA.
If these do develop, be prepared for locally lower cigs/vsbys in
heavier rain.

Ensemble model guidance has struggled with cig heights and
timing of lowering restrictions, meaning lesser confidence in
the timing of TAF restriction impacts.

.Outlook...
Restrictions are likely to linger or see only brief improvements
Wednesday afternoon while shower and thunderstorm chances favor
eastern OH to northwest PA.

Exiting of the upper trough and high pressure will promote VFR
conditions by Thursday afternoon. The next low pressure system
is likely to arrive Friday into the weekend.

&&

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

&&

$$

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