Simplify the following text for a Grade 12 native English-speaking person.
Original: It was downgraded to a tropical depression near Clarksville , Tennessee , but its remnants were last distinguishable in the eastern Great Lakes region on August 31 , when it was absorbed by a frontal boundary .
Simplified: Katrina weakened to a tropical depression near Clarksville , Tennessee , but its remnants were last seen around the eastern Great Lakes on August 31 when it became extratropical .