U.S. natgas futures rise, remain under Monday's 20-month high

* Front month hit highest mark since July 2011 on Monday * Below-normal temperatures remain on tap for consuming regions * Nuclear plant outages still above five-year average By Eileen Houlihan NEW YORK, April 16 (Reuters) - U.S. natural gas futures edged higher early on Tuesday, lifted again by forecasts for continued below-normal temperatures in consuming regions of the nation, but remaining under Monday's highest mark in more than 20 months. A tightening supply picture an