Flash Flood Warning 9, NWS Pittsburgh, PA

Area Forecast Discussion
National Weather Service Pittsburgh PA
213 PM EDT Wed May 21 2025

.SYNOPSIS...
Shower and thunderstorm chances continue today as low pressure
lifts north toward the lower Great Lakes. There remains a
flooding threat for portions of the upper Ohio Valley and a
low-end severe risk today. Rain chances will continue into
Thursday with lesser chances by Saturday. However, the upper
trough will keep temperatures below average.

&&

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

- Flood watch remains in effect through Thursday morning.
- Unsettled pattern with heavy rain and low end severe potential
  today.
- Temperatures below normal.
-------------------------------------------------------------------

Occluding surface low analyzed over northeast Ohio will slowly
move east-northeast today. First round of moderate-heavy rain
has lifted out of the region with a break ahead of a line of
showers moving through eastern Ohio tied to a cold front. Warm
front remains situated across the forecast area in the vicinity
of KPIT. Surface heating with some scattering of the cloud deck
has allowed for temperatures to get into the mid-upper 60's
with dewpoints starting the creep near 60 degrees. With the
heating we are experiencing, we are starting to see some CAPE
values exceeding 300j/kg as noted in the ACARs soundings and
RAP analysis. With 30-40kts of shear, think that there could be
a few strong-severe storms as noted by the marginal risk for
severe by SPC. With the placement of the warm front, there is
some concern that interactions with that boundary and the
increase of directional low level shear could result in rotating
storms. This supports the non-zero probability for a tornado
this afternoon.

In addition to the low-end severe threat, PWATs >1.00 inch will
support heavy rain, with some training possible given the
general storm track expected through the evening. Since where
we are likely to see storm total >2.00" of rain lines up with
some of our more urban areas or those locations that saw heavy
rain last week, still think the areas highlighted in the flood
watch are appropriate.

Vorticity lobes rotating around the upper low will maintain
chances for rain overnight and cloud cover will likely keep
temperatures near seasonal values or just a few degrees under as
cold advection ensues.

&&

.SHORT TERM /THURSDAY THROUGH SATURDAY/...
KEY MESSAGES:

-Rain showers will persist on through Thursday and Friday.
----------------------------------------------------------------

Low pressure will remain parked over the Great Lakes through the
start of the weekend, gradually shifting eastward into Saturday.
This will keep unsettled weather across the upper Ohio Valley
through the time period with rain chances continuing. Expect
higher coverage in showers Thursday afternoon with the help of
diurnal heating and a general decrease in precipitation into
Friday with the possibility of short-wave ridging drying out the
region outside of the I-80 counties and the ridges where
upslope may allow for showers to persist.

Subsidence increases area wide on Saturday but northwest flow
may still lead to a few showers lingering over the northern
counties, though chances are lowest by this time.

Cold advection will keep temperatures well below seasonal
averages through the period - with highs barely hitting 60 when
we should be in the 70's in late May.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
KEY MESSAGES:

- Continued daily rain chances into the weekend.
- Moderating temperatures but still below normal.
-------------------------------------------------------------------

A series of shortwaves will traverse the broad upper low bringing
enhanced periods of rainfall into the weekend. Ensembles continue to
show slight differences in speed of low pressure departure,
impacting shower chances on Saturday. Building high pressure to our
south may begin to win out on Sunday and some solutions suggest just
some upslope driven showers, but confidence still remains low.
Despite some moderation, temperatures are still favored to hold
below normal. Continued low confidence for Monday for precip
potential is expected as most models stall the exiting front and
some push the front through. Thus will leave low chance precip probs
in from Monday and most of Tuesday as well as temperatures try
to make a return to normal.

&&

.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
A cold front will cross the area with ample forcing for storm
development. The instability gets more questionable north and
east of Pittsburgh, hence, PROB30s were issued for FKL and DUJ.
Tempos were issued for the other terminals based on the timing
of the stroms. These storms could put down temporarily strong
gusts, generally from a southwesterly direction, but should be
confined to the front itself.

Drier air aloft makes its way in behind the front temporarily
in upper subsidence allowing ZZV, HLG, and MGW a brief break
from ceiling restrictions during that time. PIT and AGC have a
moderate chance to see a brief window of VFR ceilings. Another
shortwave of shallow instability and northwest flow aloft
invigorates more showers into the evening and overnight hours as
surface winds veer more southwesterly. Chances of IFR cigs Wed
night are generally >40% for now across a majority of the region
aside from ZZV/MGW.

Periods of rain and restrictions (mostly IFR) are expected to
continue through Thursday.

Outlook...
Periods of rain and restrictions (mostly MVFR) are expected to
continue on Friday as surface low pressure moves across the
region. Conditions are expected to become drier on Saturday with
the chance of ceiling restrictions lifting mainly south of PIT.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...Flood Watch through late tonight for PAZ021-029-031-073>076.
OH...None.
WV...Flood Watch through late tonight for WVZ021-509>514.

&&

SYNOPSIS...34
NEAR TERM...34
SHORT TERM...34
LONG TERM...Shallenberger
AVIATION...Lupo

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