* Front month still well below recent 21-month high * Weather forecasts mixed throughout the nation * Nuclear power plant outages remain above normal * Coming Up: EIA natgas storage data on Thursday By Eileen Houlihan NEW YORK, May 15 (Reuters) - U.S. natural gas futures rose about 1 percent early on Wednesday, extending gains for a third straight session after a cool start to the week in the Northeast was expected to further slow inventory builds. Above-average nuclear power plant outages also helped to firm near-term demand, traders said. A long, cold winter put a huge dent in inventories and a cool spring led to a slow start to the injection season, but forecasts for milder weather later this month should finally curb any lingering heating loads before heavy cooling demand kicks in. As of 9:15 a.m. EDT (1315 GMT), front-month June natural gas futures on the New York Mercantile Exchange were at $4.063 per million British thermal units, up 3.9 cents, or just under 1 percent. The contract hit a one-month low of $3.883 last week after climbing to a 21-month high of $4.444 on May 1. The latest National Weather Service six to 10-day forecast issued on Tuesday called for above-normal temperatures in the Northeast and Southwest and below-normal readings along the Gulf Coast and in the Northwest and other northern-tier states. Nuclear plant outages totaled 19,500 megawatts, or 19 percent of U.S. capacity, down from 20,100 MW out on Tuesday, but up from 16,200 MW out a year ago and a five-year average outage rate of 19,100 MW. Early injection estimates for Thursday's EIA gas storage report range from 90 bcf to 106 bcf, versus a 56-bcf build in the same week last year and a five-year average rise for that week of 83 bcf.