* Front month well below recent 21-month high * Weather expected to moderate over next two weeks * Nuclear power plant outages still below normal By Eileen Houlihan NEW YORK, May 6 (Reuters) - U.S. natural gas futures sank to a one-month spot chart low early on Monday, under $4 per million British thermal units again, as forecasts called for moderating temperatures over the next two weeks. Last week's unexpectedly large weekly inventory build led to a 7 percent selloff on Thursday before prices rose slightly on Friday. But the milder forecasts have pushed prices lower again, and they remain well below last week's 21-month high hit 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 the onset of milder spring weather soon is expected to curb any late-season heating demand before heavy cooling loads kick in. As of 9:17 a.m. EDT (1317 GMT), front-month June natural gas futures on the New York Mercantile Exchange were at $3.988 per mmBtu, down 5.3 cents, after sliding as low as $3.971, the lowest mark since early April. The contract rose to $4.444 last Wednesday, its highest mark since late July 2011. The latest National Weather Service eight-to-14-day forecast, issued on Sunday, called for above-normal temperatures for about the western half of the nation and in the Northeast, with below-normal readings in Texas and normal readings in much of the mid-Continent and in the Southeast. Nuclear plant outages totaled 18,200 megawatts, or 18 percent of U.S. capacity, down from 18,500 MW out on Friday, 23,700 MW out a year ago, and a five-year average outage rate of 21,100 MW. LARGER-THAN-EXPECTED BUILD BUT STOCKS BELOW NORMAL Last week's gas storage report from the U.S. Energy Information Administration showed domestic inventories rose in the prior 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 cold spring. 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 this week's EIA gas storage report range from 58 bcf to 91 bcf, versus a 30 bcf build in the same week last year and a five-year average rise for that week of 69 bcf. Baker Hughes data on Friday showed the number of rigs drilling for natural gas in the United States fell by 12 to an 18-year low of 353.