* Front month well below Wednesday's 21-month high * Weather outlooks remain mixed for May * Nuclear power plant outages still below normal * Coming Up: Baker Hughes gas drilling rig data Friday By Eileen Houlihan NEW YORK, May 3 (Reuters) - U.S. natural gas futures slipped early on Friday, falling under $4 per million British thermal units for the first time in about a month in continuation of Thursday's 7-percent selloff. Traders said Thursday's unexpectedly large weekly inventory build and forecasts for milder weather later this month have added pressure to prices, after they rose to their highest level in 21 months on Wednesday. A long cold winter put a huge dent in inventories, and lingering cool weather this spring led to a slow start to the injection season. But most traders said the expected onset of milder spring weather soon will finally curb any late-season heating demand before heavy cooling loads kick in. As of 9:23 a.m. EDT (1323 GMT), front-month June natural gas futures on the New York Mercantile Exchange were at $4.017 per mmBtu, down 0.8 cent, after sliding as low as $3.977. The contract sank 7 percent on Thursday, its biggest one-day loss in nine months, after rising to $4.444 on Wednesday, its highest mark since late July 2011. Goldman Sachs said in a report on Friday that the market reaction to the large build was "likely overdone" and that they were maintaining their near-term $4.25/mmBtu price forecast with a $4.50 average for the second half of 2013. The latest National Weather Service six to 10-day forecast issued on Thursday again called for above-normal temperatures for about the western third of the country and in New England and below-normal readings for most of the remainder of the nation. Nuclear plant outages totaled 18,500 megawatts, or 18 percent of U.S. capacity, down from 20,400 MW out on Thursday, 21,700 MW out a year ago and a five-year average outage rate of 21,700 MW. A LARGER THAN EXPECTED INVENTORY BUILD Thursday's gas storage report from the U.S. Energy Information Administration showed domestic inventories rose last week by 43 billion cubic feet, above Reuters poll estimates for a 28 bcf build and the year-ago gain of 31 bcf. But inventories started the injection season about three weeks later than expected due to the unusually cold spring and stocks, at 1.777 trillion cubic feet are nearly 31 percent below last year and more than 6 percent below the five-year average. Early injection estimates for next week's report range from 43 bcf to 84 bcf versus a 30-bcf build during the same week last year and a five-year average rise for that week of 69 bcf. EIA data earlier this week showed gross natural gas production in February climbed for the first time in three months. Output rose to about 1.27 bcf per day, or 1.8 percent above the same month last year, after dropping below year-ago levels in January for the first time since 2010. The report dimmed prospects that record high production would slow anytime soon despite Baker Hughes gas drilling rig data last week that showed the rig count dropped to a 14-year low. Traders awaited the next Baker Hughes drilling rig report that will be released later Friday.