399
FXUS61 KPBZ 172026 AAA
AFDPBZ

Area Forecast Discussion...UPDATED
National Weather Service Pittsburgh PA
426 PM EDT Tue Jun 17 2025

.SYNOPSIS...
Showers and thunderstorms continue today, and the ongoing flood
threat has expanded to include areas farther north above the
I-80 corridor as a stalled front currently to our south lifts
north across the region. The ongoing active pattern should
break Friday, followed by drier conditions and area temperatures
climbing into the 90s by the end of the weekend.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
KEY MESSAGES:

- Moderate Risk of flash flooding for much of the region today
- Flood Watch in effect until 10pm
- Isolated instances of a damaging wind threat south of I-70
---------------------------------------------------------------

...Mesoscale Update...

Precipitable water has increased to 2.0 across SE OH into the
northern panhandle of WV. Moisture transport vectors indicate
this should continue pushing ENEwd over the next couple of
hours. A surface warm front was analyzed from NE OH into SW PA.
Expect additional showers and thunderstorms to contunue to
affect the region through at least this evening as this boundary
lifts slowly northward, and a shortwave crosses the area.
Continue to monitor for flash flooding across the region, with
overall trends in CAMS showing a higher probability of more
significant QPF generally N of PIT, though training etc will
still impact pockets of the entire area.

Previous discussion...
With further development, we face another day of a risk for
flash flooding, some potentially significant, with a primed
ground and ripe atmosphere.

Our morning sounding depicts a classic heavy rainfall environment.
Skinny MUCAPE with the bulk of the instability packed below -10C,
PWAT of 1.82" (daily max) and increasing in deep moist southwest
flow, warm cloud depths up to 14kft, and area dew points in the low
70s will be very supportive of dominant warm rain processes
with very efficient rainfall producers. In fact, even lightning
is lower probability today given the depth and width of the CAPE
profile. Will likely see very heavy rainfall with reflectivity
values not topping much higher than 30-40 dbz. While storm
motion vectors are a bit higher than previous days, PWAT values
are at record levels, so all we`ll need is a short period of
backbuilding over the same area, especially those that have
gotten hit hard in the past 48-72 hours, and flash flooding will
be likely. It`s also possible that heavier strips of rainfall
concentrate along the aforementioned boundary that will push
north as a warm front through the day today, but confidence in
placement is still on the lower side given the lack of stronger
surface convergence.

HREF probability for >1"/hour rates are high, highest up to 80% east
of Pittsburgh. Further, bumping up the neighborhood probability to
25 km over a window of 12 hours from 10am-10pm (accounting for
placement discontinuities among the CAMs) shows 10-20% probability
for up to 3"/hour rates. HREF max 24 hour QPF values through
Wednesday morning show strips of as high as 4", so while these types
of values are not expected across most of the area, this can give an
idea of the highest end, worst case scenario.

Given all of these factors in play, a Moderate Risk (3/4) is in
place for excessive rainfall for the WV panhandle and southwest PA
south of a line from Butler to Dubois, though it should be noted
that any location that gets hit repeatedly today will be at risk of
flash flooding... WPC Moderate Risks correlate to numerous flash
flooding events with significant flash flooding possible. Typically,
our area has only been placed in Moderate Risks or higher with
landfalling tropical systems. Never drive through flooded roadways
and remain vigilant for rapidly changing conditions in heavy
rainfall.

One other low end threat to be noted is a damaging wind/weak tornado
possibility. With weak low level shear, the threat should be
limited, but forecast soundings do have 100+ J/kg of 3CAPE with
low LCLs this afternoon. If a favorable interaction with remnant
boundaries or other showers/storms occurs, a quick spin up is a
low but not zero threat.

Coverage of rain will decrease with loss of daytime heating, but 850
mb convergence on the nose of an impinging low level jet is likely
to drive additional showers overnight, especially south of
Pittsburgh. A lower flooding threat will exist, but remnant MUCAPE
>250 J/kg and still high PWAT values will support additional heavy
rainfall producers.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH THURSDAY NIGHT/...
KEY MESSAGES:

- Some scattered showers Wednesday with lower coverage and flooding
  threat.
- Possible showers/storms overnight Wednesday and again Thursday
  with low end severe threat.
----------------------------------------------------------------

Wednesday could bring a relative lull in the action as ensemble 500
mb height changes show modest rises of a few mb in 12 hours. Some
scattered showers look more favored in the mid morning to early
afternoon hours before chances lessen headed into the afternoon as
shortwave support exits. Any of these showers could be brief heavy
rainfall producers, but at this time it looks like storm motion and
scattered nature should preclude significant flooding concerns.
Continued deep southerly flow will drive high temperatures into the
low to mid 80s with some afternoon breaks in the clouds possible.

Uncertainty then comes into play late Wednesday night with what will
likely be a line of storms moving across Indiana and Ohio along a
pre-frontal surface trough. Models notoriously struggle with
maintenance of decaying nocturnal convection, especially when there
is lingering MUCAPE, and ensembles all indicate 1000-1500 J/kg still
in place. Will have to see how this evolves and if the CAMs can shed
any better light over the next 24 hours, but if the line holds
together, we could see a damaging wind and hail threat favoring the
post-sunset hours. SPC has maintained the Marginal Risk (1/5)
covering most of our CWA.

The most notable height falls and upper support come Thursday as
surface low pressure drags a cold front through our area. Timing of
this boundary is still a bit uncertain and will be important for a
severe weather threat locally. Dew point SD spread shows the
boundary as far southeast as the ridges or as far northwest as the
Erie to Cleveland corridor by 18z. A slower progression (currently
less likely) would favor a higher severe threat in our area while a
quicker one would confine the threat farther east. Should the slower
one prove true, we`d again face a wind and hail threat primarily..
Machine learning favors closer to the I-95 corridor and a faster
moving boundary, and the SPC outlook is leaned that way with a
Slight Risk (2/5) over there and a Marginal Risk (1/5) locally. This
may change once timing becomes clearer.

&&

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

- Turning drier with excessive heat threat ramping up into next week.
-------------------------------------------------------------------

Friday should finally see a drier day in the wake of the boundary,
though a change in airmass doesn`t appear too likely with a brief
bout of northwest flow quickly flipping west to southwest again
as high pressure builds in across West Virginia. NBM MaxT
spread is pretty small with highs likely in the mid 80s.

The story of the long term period is going to turn from rain and
flooding to excessive heat. Longer range ensembles exhibit quite a
bit of certainty on a strong upper ridge building across the eastern
CONUS with a 90+% chance of 500 mb heights >590 dam. NBM probability
for highs >95F peak at 60-70% in the urban areas and valleys Monday
and Tuesday. Climatologic tools point toward heights 2-3+ SD above
normal for late June. While we often get plagued by diurnally
driven clouds and precipitation in these patterns, a rapid warm
up to well- above normal temperatures is increasingly likely,
and the threat may be longer-lived over the course of several
days with little relief at night. The takeaway message at this
point is that several days of highs into the 90s across most of
the area is increasing probability and compounding effects could
be significant.

&&

.AVIATION /20Z TUESDAY THROUGH SUNDAY/...
The combination of an abnormally moist environment, surface
heating, and approach of a mid-level shortwave will support
waves of heavy rain showers and isolated thunderstorms through
02z. Rainfall rates between 1-2"/hr will be common and can cause
rapid deterioration to 2SM or less and temporarily drop the
broadly rising cigs to fall back toward MVFR/IFR levels. Due to
the presence of a warm thermal profile, lightning potential will
be limited and thus removed from TAFs as terminal impact is too
low (less than 30%) for mention.

Though widespread convection should ease after 02z, continued
approach of the incoming shortwave may induce additional rounds
of more light to moderate rain showers with the axis of
precipitation potential shifting slightly east. Abundant
moisture and convergence as a surface trough nears should also
expedite cig falls toward widespread IFR by 12z Wednesday.

Isolated to scattered rain showers may resume after 15z
Wednesday with diurnal heating/mixing as the mid-level shortwave
finally crosses the region. Drier conditions and temporary
restriction reprieve is favored by Wednesday evening as
subsidence develops in between shortwave movement.

Outlook...
An approaching front and upper shortwave will increase
thunderstorm potential late Wednesday night through Thursday,
with timing dependent on eastward progression of upstream
convective systems. The broken line of thunderstorms may produce
gusty wind, lower cigs, and have high rainfall rates before
exiting the region by Thursday night.

High confidence in high pressure and rising heights aloft is
likely to support VFR conditions Friday into early next week.
There remains a low probability for late weekend thunderstorm
impact depending on how far east the upper ridge develops.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...Flood Watch until 10 PM EDT this evening for PAZ007>009-
     013>016-020>022-029-031-073>078.
OH...Flood Watch until 10 PM EDT this evening for OHZ039>041-
     048>050-057>059-068-069.
WV...Flood Watch until 10 PM EDT this evening for WVZ001>004-012-
     021-509>514.

&&

$$

SYNOPSIS...88
NEAR TERM...WM/MLB
SHORT TERM...MLB
LONG TERM...MLB
AVIATION...Frazier