SUMMARY...Thunderstorms are expected to develop from northern
Virginia into northern Maryland and southeast Pennsylvania by
21-22Z. A few supercells capable of damaging wind gusts and isolated
tornadoes will be the main threats. A tornado watch will likely be
needed prior to 21Z.
DISCUSSION...As of mid afternoon a progressive vorticity maximum was
situated over southwestern PA and will continue east into the Middle
Atlantic region during the late afternoon and evening. A band of
low-topped convection has already developed in association with this
feature extending into northern WV where instability is limited.
Farther downstream, a lee trough resides just east of the central
and southern Appalachian mountains. East of this feature
temperatures have warmed to the upper 80s F with dewpoints in the
low to mid 70s contributing to 1500-2000 J/kg MLCAPE despite very
weak mid-level lapse rates resulting from warm temperatures aloft.
As the corridor of deeper ascent attending the shortwave trough
continues east and begins to overtake the western edge of the moist
warm sector, additional storms will likely develop from northern VA
into southeast PA. Winds in the 850-500 mb layer will undergo a
modest increase with approach of the impulse, resulting in 35-40 kt
effective bulk shear and 100-200 m2/s2 storm relative helicity. This
environment should be sufficient for some organized storm structures
including a few supercells capable of locally strong to damaging
gusts and a couple of tornadoes through early evening.