Trim on a warped window frame

I am enclosing a side porch to make a TV room, and installed 3 windows in the room. One of the windows is a triple-wide awning window measuring 72"x17" (wxh). At one point in the building process, after I had installed the window, I replaced part of the roof. While the roof was open, a sudden rain shower dumped a good bit of water in the room. I didn't notice at the time, but some water settled in the window frame (I'm not sure if that's the right term, but what I mean is the 1x6 wood that extends from the actual window and sits in the framed-out jamb). Now the window frame is warped-- the middle of the bottom piece is higher than the ends of the bottom piece, like this guy's mouth . I have installed drywall around the window, but as I think about installing trim, it looks like I'm going to have to try to flatten out the bottom of the window frame. because the trim will look funny otherwise Can I just try to sand the middle of the bottom piece down until it's flat, or can I somehow use some kind of filler on the sides of the bottom piece so it looks flat?? Do I need to try to put some more screws in to see if I can bring the middle down?
Thanks for your help!

 

predrill the 1*6 and use a screw which is driven in with a square bit drive ...It will enable you to get alot more force into the screw....