McCormack saw his opening two goals cancelled out by a stunning strike from Cameron Stewart and then Charlton skipper Johnnie Jackson 20 minutes from time.
However, a brilliant volley from a tight angle and then a superb free-kick in added time handed Leeds a first win on the road since mid-September.
"It was a strange game because for 70 minutes we were well in there," Powell said.
"But McCormack showed his quality. Conditions play a part but you have to be very diligent when you are defending in your own box.
"We showed some character to come back but McCormack was the major difference in the end between two well-matched teams.
"It's disappointing to be on the end of a result like that after the run of clean sheets we've had. We'll have to dust ourselves down and move on."
- Sports & Recreation