Thunderstorm chances will increase over the next 30 hours.
These storm chances will come in three distinct waves. The first
chance is most likely this afternoon over northern West
Virginia and southwest PA (this chance has decreased). The
second will pass early Friday morning, most likely for Ohio and
western PA. The third will come on a cold front late-Friday
morning into early afternoon, most likely for western PA.
The first wave will be along a surface warm front. This will have a
medium probability of ignition and very low probability of
impacts. The main uncertainty of ignition is ties to instability
generation. Median SB/MU CAPE shows near-zero values, with HREF
ensemble max showing up to 200 J/kg possible. In order to
ascertain any impacts, instability >100 J/kg is needed which is
only 10% likely at this point. Current radar trends reflect
low-to-no instability. Prevalent cloud cover will continue to
be unfavorable for sustained updrafts.
Into tonight, a low level jet is forecast to ride up and through the
area, increasing low-level convergence / upper-level
divergence. This will provide a high probability of initiation
and a low-to-medium probability of impacts. Given the high
probability of decoupling, and intense updrafts being mostly
shear-driven, and any limit instability being elevated, the main
risk with these storms will be hail. This round is represented
by the SPC marginal risk outlook. While widespread small hail is
most likely, achieving large hail >1" will be reliant on
maintaining dry air aloft and mature updrafts. Given limited
elevated instability, both of these will be difficult to achieve
concurrently. So, there is a higher probability of higher
coverages of small hail, but hail >1" cannot be ruled out.
The third threat will come on a passing cold front tomorrow. This
will bring a low probability of initiation, but a conditional
low-to-medium probability of impacts. The cold front will be
accompanied by mean 0-3km mean winds of 50kts. The main
uncertainty will again be instability. There remains a 25%
chance of >100J/kg of CAPE for now, but in the event of
clearing, we may top this threshold. Combined with the forcing
from the front, this would bring the highest conditional chance
of damaging winds should updrafts initiate on the front,
primarily tied to fast storm motion and linear mode.