Forecasters say a cold front clashed with high heat, causing Sunday's storm with gusts of more than 50 mph.
On Monday, Delmarva Power reported more than 3,600 customers still without power, mostly in Sussex and New Castle counties.
Authorities say a tree fell on a home near Talleyville, but no one was hurt. In addition, officials are investigating whether house fire in Bear that injured two firefighters was caused by lightning.
Some residents reported flooding in their homes.
Heat and humidity are expected to drop slightly until the middle of the week when more rain and thunderstorms are possible.