* U.S. weather expected to turn milder next week * Nuclear outages climb above last year and five-year average By Joe Silha NEW YORK, April 1 (Reuters) - U.S. natural gas futures ended mixed on Monday, with front months pressured by milder weather forecasts for late this week and next week that should slow demand, despite a brief cold shot expected for the next few days. Cold late-winter weather and above-average nuclear plant outages have helped put a huge dent in inventories and driven futures prices up nearly 30 percent since mid-February. But with milder spring temperatures just ahead, many traders expect further upside in prices to be difficult, particularly with production still flowing near a record peak. "The upcoming week is still expected to be much colder than normal east of Colorado, but after that last blast of cold, it looks like we're in for the start of the shoulder season," said Gelber & Associates analyst Aaron Calder, noting the milder turn in the weather likely prompted some longs to get out. Front-month gas futures on the New York Mercantile Exchange ended down 0.9 cent at $4.015 per million British thermal units after trading between $3.934 and $4.044. The front contract hit a 19-month high of $4.121 on Thursday before closing lower ahead of the Good Friday holiday. The nearby futures contract, which posted a sixth straight weekly gain last week, finished March about 15 percent higher and ended the first quarter up about 20 percent. Chart watchers noted near steady gains in open interest backed much of the recent price rise, a bullish sign indicating that new buying, not short covering, was fueling the upside. Futures-only open interest on Thursday hit a record high for the tenth straight session, climbing to 1,437,134 contracts. Traders noted that gas prices at $4 could slow demand by prompting utilities to use more coal to generate power and increase supply by encouraging producers to turn on more wells. After a brief cold spell this week, forecaster MDA Weather Services expects warmth to build eastward, with above-normal temperatures from the Midwest to the East seen starting late in the week and continuing at least through mid-April. OUTPUT STARTS TO SLOW? Energy Information Administration data on Friday showed that gross natural gas production in January fell nearly 1 percent from December levels, the second straight monthly decline. Output also dropped below year-ago levels for the first time in years, but it's still unclear if recent monthly declines were due to well freeze-offs from the cold or producers curbing dry gas flows because prices were not that attractive. Baker Hughes data Thursday showed the gas-directed rig count fell last week for the fourth time in five weeks, dropping by 29 to 389, its lowest since May 1999. EIA last month was still expecting marketed gas output in 2013 to hit a record high for the third straight year. ANOTHER STRONG STORAGE DRAW EXPECTED U.S. Energy Information Administration data last week showed total domestic gas inventories fell to 1.781 trillion cubic feet, just 61 billion cubic feet, or 3.5 percent, above the five-year average for that week. Most traders expect stocks to fall below the five-year norm in Thursday's EIA report, with withdrawal estimates ranging from 45 to 97 bcf versus a 43-bcf build during the same week last year and a five-year average increase for that week of 4 bcf. Stocks began winter at a record 3.929 tcf, but about 2.15 tcf of gas has been pulled from storage so far this heating season, or 45 percent more than last year at this time.