Mesoscale Discussion 1557
NWS Storm Prediction Center Norman OK
0357 PM CDT Wed Aug 18 2021
Areas affected...MD...DC...northern VA...central/eastern PA
Concerning...Tornado Watch 447...
Valid 182057Z - 182230Z
The severe weather threat for Tornado Watch 447 continues.
SUMMARY...The threat for a couple tornadoes and isolated damaging
wind gusts will continue into the early evening.
DISCUSSION...At 2045Z, robust convection has recently developed
across south-central PA, in conjunction with the primary midlevel
vorticity maximum associated with post-tropical cyclone Fred.
Further east, convection continues to occasionally flare up across
southeast PA and northern MD. The westernmost convective band now
moving across south-central PA should be maintained as it moves
eastward this evening. While the strongest low-level shear/SRH may
remain east of this band, occasional rotating cells will be possible
into this evening with the band, posing a threat of isolated
damaging wind gusts and a couple tornadoes.=20
Additional rotating cells will remain possible further east,
potentially evolving out of showers ongoing across southeast PA, or
along the southern edge of a cluster moving across central PA, where
a diffuse baroclinic zone remains in place. Some tornado threat may
still evolve with any sustained cells in this region, where
low-level shear/SRH remain somewhat enhanced.
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