Mesoscale Discussion 1555
NWS Storm Prediction Center Norman OK
0229 PM CDT Wed Aug 18 2021
Areas affected...Central/eastern PA...northern
VA...MD...DC...eastern WV Panhandle
Concerning...Tornado Watch 447...
Valid 181929Z - 182100Z
The severe weather threat for Tornado Watch 447 continues.
SUMMARY...The tornado and damaging-wind threat will continue to
gradually increase this afternoon.
DISCUSSION...While cells with occasional weak rotation have been
noted recently across central PA and northern MD, supercell
structures have this far struggled to be sustained across WW 447.
Earlier discrete cells may have struggled to mature in the midlevel
dry slot (noted in WV imagery), but renewed development may be more
vigorous this afternoon as the dry slot shifts eastward and ascent
associated with the primary midlevel vorticity maximum spreads over
Low-level shear/SRH remains sufficient to support some tornado
threat with any sustained supercell. The greatest relative tornado
risk may reside across central PA in the vicinity of the
increasingly diffuse warm front, as convection deepens to the south
and moves through the baroclinic zone this afternoon. However, any
sustained cell within the warm sector will also pose some threat.
Locally damaging wind gusts will also remain possible, especially in
areas where some steepening of low-level lapse rates has occurred.
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