250
FXUS61 KPBZ 062009
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
409 PM EDT Mon May 6 2024

.SYNOPSIS...
Probability of rain showers and isolated thunderstorms increase
for areas south of Pittsburgh after 3pm this evening with an
approaching disturbance. Isolated fog returns early Tuesday
morning for portions of the region. Severe weather potential
increases Tuesday evening, after 6pm and could last into the
overnight hours.

&&

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

- Rain and thunderstorm chances increases this evening.
  Lightning and areas of heavy rainfall rates will be the main
  concern

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4pm Update: Showers and thunderstorms have developed south of
I-70 over the last 2 hours. With high rainfall rates in noted back-
building thunderstorms over Wetzel/Marion county (2 to 3 inches
per hour), a Flash Flood Warning was issued. Urban areas and
small creeks and streams will likely be impacted for the next 2
hours.

Previous Discussion:

A passing shortwave has initiated showers and thunderstorms for
portions of the region. Storms are not expected to be organized
with marginal shear (effective shear less than 30kts) despite
MLCAPE ranging between 900J/kg to 1100J/kg on sampled soundings
near MGW.

Like yesterday, with very saturated environment (PWATS values
ranging between 1.30 to 1.45), isolated storms can prompt heavy
rainfall rates. Urban areas will likely be the places of
interest today for any lingering or back- building storm under
marginal shear.

Probability of fog (less than a half a mile in visibility)
ranges from 45% to 65% for areas south of I-70 during the
overnight time period. Weak moisture boundary layer convergence
will likely cause probabilities to increase and isolated areas,
and could create visibility issues for travelers. A Special
Weather Statement or Dense Fog Advisory may need to be issued
for forecast zones south of Pittsburgh in the next 12 to 18
hours.

&&

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

- A passing warm front from the west will increase the risk of
  severe storms late Tuesday evening into early Wednesday
  morning
- Hail, damaging wind, tornadoes, and areas of flooding could
  occur with the passing disturbance
- Portions of eastern Ohio have a higher probability of severe
  storms
- Ridging over the Great Lakes is expected to reduce
  precipitation chances briefly Wednesday afternoon/evening

-------------------------------------------------------------------

A strong low pressure system, that will impact the Great Plains
later today, is expected to eject into the northern
Plains/Dakotas early Tuesday morning through Tuesday afternoon.
With a closed upper-lvl low positioned north over Newfoundland
and the low to our northwest, ridging is expected to build
briefly over the Great Lakes. Therefore, Tuesday is expected to
stay mostly dry in the morning/early afternoon. A few stray
showers may develop after 3pm Tuesday as the ridge axis quickly
breaks down as new surface low forms along a warm front in the
Midwest.

The timing of severe weather is expected to be between 7pm
Tuesday to 2am Wednesday. Despite the loss of sfc heating, the
surge of warm, moist air from the south from a developing and
progressing low-level jet will keep the region very unstable.
Little to no CIN and MLCAPE values ranging 1000J/kg to 1500J/kg
are expected during the overnight time period. Effective shear
will range from 45kt to 50kts and will support well-organized
storms. Hail, tornadoes, and damaging winds are all possible
with this passing disturbance. Very large hail (+2 inches, larger
than a golf ball) may form in storms that has strong
reflectivity above 25kft to 30kft, based on model soundings
given a deep layer of drier air above 500mb to 300mb.

The Storm Prediction Center (SPC) made minor adjustments to the
Slight/Marginal Risk threat for our region Tuesday night into
Wednesday. Changes today were to expand the Marginal threat into
western Pennsylvania and to add at 15% hatch area for hail in
Muskingum/Coshocton County for 2+ hail potential. An Enhanced
risk was also added just west of our County Warning Area (CWA)
due to a combination of higher probabilities of hail and
tornadoes.

The potential of severe storms decreases between 2am and 4am
Wednesday morning as mid-level ridging builds.

&&

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


- A new disturbance is expect to return severe storms early
  Thursday morning
- Ensemble models suggest a very wet pattern into the weekend
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Our region will continue to remain in an active weather pattern
as a new low pressure system forms over the Great Plains early
Wednesday, tracks east, and enters the Ohio River Valley early
Thursday morning. Severe storms could produce damaging winds,
and/or tornadoes due to high shear and MUCAPE values ranging
between 500J/kg to 1200J/kg. The low-level jet will likely be
stronger than the Tuesday night disturbance, Hi-Res guidance
suggesting 850mb winds ranging from 55kts to 65kts.

A passing cold front may stir additional severe storms Thursday
afternoon southeast of Pittsburgh. However, the main threat
will likely reside over the East Coast (noted in SPC`s Day 4
Outlook).

Ensemble models project a pattern shift late week as an
elongated trough builds over the Great Lakes. Below average
temperatures are likely Friday through Sunday under northwest
flow and 850mb temperatures could range from 2C to -1C. At the
moment, the potential for frost/freeze concerns is considered low.



&&

.AVIATION /19Z MONDAY THROUGH SATURDAY/...
Any MVFR cigs are expected to improve to VFR this afternoon amid
dry advection. A crossing shortwave will trigger additional
showers and possibly a thunderstorm primarily south of I-80 later
this afternoon and evening.

Winds remain light through the period.

.Outlook...
An active pattern will keep periods of precipitation and related
restrictions through much of the week.

&&

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

&&

$$

SYNOPSIS...Hefferan
NEAR TERM...Hefferan
SHORT TERM...Hefferan
LONG TERM...Hefferan
AVIATION...88