UPDATE 1-U.S. natgas futures at 18-mth high near $4/mmBtu

* Front month hits highest level since September 2011 * Nuclear outages still running above normal * Cold weather remains on tap in long-term outlooks * Coming Up: EIA natgas storage data on Thursday By Eileen Houlihan NEW YORK, March 21 (Reuters) - U.S. natural gas futures rose to their highest mark in 18 months early Thursday, boosted to just under $4 per million British thermal units by continued cold weather and expectations for a fifth straight supportive weekly storag