NEW YORK, Sept 23 (Reuters) - U.S. natural gas futures lost ground for the second straight session on Monday, pressured by moderating weather forecasts for much of the nation over the next two weeks that should slow overall demand. Front-month futures, which posted a two-month high of $3.82 on Thursday, finished last week nearly flat, gaining just 0.3 percent following a 4.2 percent rise the previous week. "The prompt contract is trading to its lowest level in five days as the outlook for milder weather that may limit demand for gas from power generators weighs on the market," Addison Armstrong, director of market research at Tradition Energy in Stamford, Connecticut, said in a report. At 9:05 a.m. EDT (1305 GMT), front-month gas futures on the New York Mercantile Exchange were down 4.3 cents, or 1.2 percent, at $3.644 per million British thermal units, after trading between $3.632 and $3.681. Some traders said the market seemed to be running out of steam after gaining ground in five of the last six weeks. Comfortable inventories, record-high gas production and fading heat as milder autumn weather sets in have stirred doubts among investors about further upside. Forecaster MDA Weather Services said warm temperatures across northern tier states and seasonal-to-cool readings in the South should continue to limit overall energy demand. Energy Information Administration data on Thursday showed total domestic gas inventories had stood at 3.299 trillion cubic feet. Stockpiles were 5 percent below last year's record highs at that time, but 0.5 percent above the five-year average. Traders said production shut-ins last week from flooding in Colorado might lead to another light storage build in next week's EIA report. The Thomson Reuters Analytics Group estimates that about 0.5 bcf per day may have been shut in last week. Early injection estimates for Thursday's EIA storage report range from 61 bcf to 71 bcf. Stocks gained 79 bcf during the year-earlier week, while the five-year average increase for that week is 75 bcf. Baker Hughes data on Friday showed the gas drilling rig count fell by 15 last week to 386 after posting a six-month high the previous week. Despite the weekly rig decline, the count has risen in eight of the last 13 weeks, stirring talk that new investment in pipelines and processing plants was allowing producers to pump more gas into an already well-supplied market. The EIA still expects U.S. gas production in 2013 to hit a record high for the third straight year. Nuclear plant outages on Monday totaled 10,405 megawatts, or 11 percent of U.S. capacity, versus 11,110 MW out on Friday, 17,062 MW out a year ago and a five-year average outage rate of 12,047 MW. Despite the rise in tropical activity this month, there were no significant threats to Gulf of Mexico gas production.