344
FXUS61 KPBZ 281729
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
129 PM EDT Tue May 28 2024

.SYNOPSIS...
Two passing shortwaves over the Great Lakes will keep rain and
thunderstorm chances elevated today and tomorrow. Cooler than
average temperatures are expected through Friday.

&&

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

- Shower and thunderstorms chances increase after 12pm with a
  passing shortwave from the northwest. A few storms may have
  small hail (pea sized).
-------------------------------------------------------------------

1:30pm: Sporadic shower activity has increased north and west
of Pittsburgh over the last hour. Lightning chances are
currently low given updrafts being confided near or just below
the freezing level. However, thunder chances will increase due
to the peak surface heating time frame and cold advection
aloft.

Previous Discussion:

A deep upper-level trough is currently tracking trough the Great
Lakes and Midwest region. Embedded with this large scale
pattern is numerous shortwaves that will impact the Ohio River
Valley. The first shortwave, currently tracking trough northwest
Ohio, will reach western Pennsylvania in the next 2 to 3 hours.
With diurnal heating from breaks in the clouds and cooling
aloft, elevated instability will build across the region;
Hi-Res guidance suggests 300J/kg to 700J/kg of MUCAPE. Therefore,
thunderstorms and showers will initiate easily with the noted
low to mid-lvl ascent associated with the shortwave. A few
storms may have small hail given a lower freezing level (below
9kft) and drier conditions in the mid-levels. Updrafts that can
climb above 11kft have the best chance at producing hail.

Besides convection, cooler than average temperatures are
expected under northwest flow, an increase in cloud coverage,
and lingering convection. Afternoon high temperatures will trend
5 degrees below average.

Rain intensity will decrease after 10pm with the loss of surface
heating, but remnant showers may linger passed midnight with
the exiting shortwave and a new disturbance entering central
Ohio.

&&

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

- Rain chances are in the forecast for Wednesday.

- Temperatures will drop below normal, as much as 10 degrees by
  Wednesday.
-------------------------------------------------------------------

The parent low mentioned in the near term will continue to be
the main driver of weather across the area heading into the day
on Wednesday. This low will further track east into the New
England region as it further pushes a cold front through the
region. Rain chances will be higher with the stronger wave
timed for a Wednesday evening passage, when rain totals of 0.25
to 0.50 inch appear most likely. NBM probabilities of greater
than a quarter inch are in the 50 to 70 percent range over a
good portion of the region on that day. The chance for greater
instability reaching over 1000 J/Kg MU CAPE for Wednesday may
mean a better chance for strong to at least sub severe storms.
Given the better parameters, there may be a marginal severe
storm during the afternoon and poses something to monitor. The
saving grace this day, is that the shear is weaker than on
Tuesday. The loss of heating for Wednesday during the day will
allow convection to wane just after sunset. Given the cooler air
infiltrating behind the front, Wednesday night will feature
temperatures below normal and fog development unlikely.

Behind the cold front, temperatures will trend below normal,
potentially 10 degrees on the cooler side by Wednesday.

&&

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

- Dry weather and cooler temperatures continue into Friday,
  with some warming starting on Saturday.
- Low rain chances return for Sunday and Monday in a more
  uncertain pattern.
-------------------------------------------------------------------

Surface high pressure builds into the area on Thursday, albeit under
a still-present upper trough, with decent agreement on this scenario
according to cluster analysis.  Will continue with a dry forecast
for now, but would not be surprised if a few isolated showers occur
during the day along the ridges before the trough axis works east of
the area. Below-normal temperatures are likely Thursday/Thursday
night, with lows perhaps in the lower and mid 40s.

Thereafter, an upper ridge axis builds into the region with fairly
high confidence, leading to dry weather from Thursday night into
Saturday at least. Temperatures will moderate back to seasonable
levels by Saturday.

Thereafter, spread in the models increases regarding how quickly the
upper ridge moves off/breaks down and how quickly deeper moisture
returns. Will keep the NBM suggestion of low PoPs for Sunday and
Monday with temperature expectations of near to slightly above
seasonal levels. Stout ridging in place as this next disturbance
arrives may lead to a difficult forecast given the subsidence
and drier air in place to start the new week.

&&

.AVIATION /17Z TUESDAY THROUGH SATURDAY/...
A broken cu field with VFR cigs (save for FKL/DUJ where MVFR
continues) has formed across the area as well as scattered showers,
which so far have been mostly confined to areas along and north of
the I-80 corridor. Expect this shower activity to continue becoming
more numerous across the rest of the area as we head into the
afternoon hours, with a few thunderstorms also possibly mixing in.
This activity wanes some after 02Z, though a stray lingering shower
overnight cannot be ruled out. Widespread cig restrictions settle in
early Wednesday morning with lingering low-level moisture and cold
advection dominating. Hi-res ensemble guidance is suggesting high
probabilities (70%+) for widespread MVFR/IFR and even medium
probabilities (40-60%) of at least some instances of LIFR occurring
at all area terminals. Another round of scattered to numerous
showers also develops Wednesday as the upper trough axis finally
rotates through the region. Minimal lightning is expected, even
during the afternoon, due to limited instability. Light and variable
winds tonight become north-northwest around 5-10 knots on Wednesday.


.Outlook...
Pivoting of the upper trough through the region Wednesday will
generate widespread showers and an off chance at a rumble of
thunder, along with widespread cig restrictions. High pressure
builds into the area Wednesday night through the end of the week,
bringing VFR conditions back to the area through that time.

&&

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

&&

$$

SYNOPSIS...Hefferan
NEAR TERM...Hefferan
SHORT TERM...CL/Shallenberger
LONG TERM...CL
AVIATION...Cermak