The best site for a lot of information on the storms is wunderground.com. Look at the historical maps for storms like Karl. If Karl follows the projections (which all take the storm due north in a few days), it will not hit the U.S. Actually, the historical maps show that only one Sept. storm that has formed near Karl has ever hit Florida. That one did not take a north turn, but headed due west.
If it turns north, as all of the computer models say it will, The U.S. should be fine.