Eventually, the run of 90 degree weather...and quite possibly the now-23 day stretch of days where Philadelphia hasn't dropped below 70 degrees will come to an end with a cold front that slides south from Canada late this week. The heat ridge that built overhead on Sunday and has slowly pushed west while expanding its grip over the eastern half of the country will gradually weaken its grip, move southwest into the Mississippi Valley, and allow for a trough to push across Canada by Friday.
The result of this will be a cool front that will ooze slowly southeast through the Great Lakes and Northeast for Friday and Saturday, bringing afternoon thunderstorms that could become severe each of those afternoons. For us, our highest risk of strong and severe storms will probably fall on Saturday afternoon and evening...with northern parts of the region probably having a higher risk of severe weather than the Shore and Delaware, depending on when storms ultimately fire up but odds would lean towards storms firing along I-80 and pushing southeast Saturday PM.
With the storms, it appears a broken or scattered line of severe weather will fire up in the afternoon and push southeast. Heat-busting thunderstorms can produce nasty winds...they are not always proficient on the rainfall and torrential rainfall side of things but the outflow and downbursts from these storms can yield pockets of damage. As always, pinpointing the "who gets what" several days out is inexact but be aware of the risk of strong to perhaps severe thunderstorms in parts of the region on Saturday afternoon and evening.