UPDATE 3-U.S. natgas futures close down, end week up 2.8 percent

* Tropical activity stirs in Atlantic basin * Northeast, Midwest heat this week turns milder next week By Joe Silha and Eileen Houlihan NEW YORK, Aug 30 (Reuters) - U.S. natural gas futures ended lower on Friday, their first loss of the week as traders took profits ahead of a long holiday weekend, and ahead of milder Northeast and Midwest weather next week that should slow overall demand. New York Mercantile Exchange floor trading will be closed on Monday for the U.S. Labor Day