U.S. natgas futures hover near 18-month high on lingering cold

* Front month near $4 level, highest since September 2011 * Nuclear outages still running above normal * Cold weather on tap in long-term forecasts * Coming Up: Baker Hughes gas drilling rig data on Friday By Eileen Houlihan NEW YORK, March 22 (Reuters) - U.S. natural gas futures rose 1 percent early on Friday, boosted to just under Thursday's 18-month spot chart high as cold weather lingered in most of the United States. An extremely cold late winter, a string of suppor