* Front month ends down for third straight week * Weather expected to moderate, slow demand in next 2 weeks By Joe Silha NEW YORK, May 10 (Reuters) - U.S. natural gas futures ended lower on Friday as bearish weekly inventory data and moderating weather forecasts, that should slow gas usage, drove the front-month contract to its third straight weekly decline. Gas prices have been on the defensive for much of the last three weeks, sliding more than 11 percent during that period, as milder spring weather curbed space heating needs. The milder turn in the weather followed a cold winter and chilly spring that whittled down record high storage levels and drove gas prices up more than 40 percent from mid-February lows. But expectations now are that the milder trend may lead to above-average weekly inventory builds and possibly leave a high level of stocks heading into the next heating season. "The market didn't come unglued after Thursday's big storage build, but it did close near the lows today, which could point to more downside," said Steve Mosley at The SMC Report, noting bearish weather for the next two weeks should favor more big inventory builds. Front-month gas futures on the New York Mercantile Exchange ended down 7.3 cents, or 1.8 percent, at $3.91 per million British thermal units after trading between $3.907 and $4.010. The front contract, which hit a 21-month high of $4.444 last week and a five-week low of $3.883 this week, lost 3.2 percent in the last five sessions, its third straight weekly decline following nine consecutive weeks of gains. Some chart traders, noting the front month posted a double top above $4.40 in late April and early May, expect any upside to be limited, at least until hotter weather arrives and forces homeowners and businesses to crank up air conditioners. Commodity Weather Group noted the latest six- to 10-day and 11- to 15-day forecasts turned a bit warmer overall for the Midwest and East, but the forecaster said it did not expect temperature anomalies to generate much load. ANOTHER BIG INVENTORY BUILD Data from the U.S. Energy Information Administration on Thursday showed that total domestic gas inventories rose last week by 88 billion cubic feet to 1.865 trillion cubic feet. The weekly build was above the Reuters poll estimate of 83 bcf and well above the five-year average increase for that week of 69 bcf, and initially pressured prices. The gain, which was the fourth of the stock building season and exceeded market expectations for the second straight week, trimmed the deficit versus the five-year average by 19 bcf, leaving stocks at 99 bcf, or 5 percent, below that benchmark. Early injection estimates for next week's report range from 87 to 108 bcf versus a 56-bcf build during the same week last year and a five-year average rise for that week of 83 bcf. PRODUCTION CLIMBS DESPITE FEWER RIGS Baker Hughes data Friday showed the gas-directed rig count fell this week by four to 350, its lowest since June 1995. Despite the steep decline in dry gas drilling, production has not slowed much, if at all, from 2012's record highs. The EIA on Tuesday raised its estimate for domestic natural gas production in 2013, expecting output this year to be up about 1 percent from last year. If realized, it would be the third straight year of record production.