* Warmer weather expected next week seen slowing demand * Nuclear power plant outages still slightly above average * Stock-building season off to a slow start * Coming up: Reuters natural gas storage poll Wednesday By Joe Silha NEW YORK, April 23 (Reuters) - Front-month U.S. natural gas futures ended lower on Tuesday for a second straight day as investors focused on forecasts for milder weather next week that should finally slow heating demand and shrugged off some chilly weather this week. Traders said lingering cold and prospects for light storage builds this month have supported prices, but after nine straight weeks of gains some see the upside stalling, at least until hotter weather kicks up air conditioning demand. Cold late-winter weather, a chilly spring and above-average nuclear plant outages put a huge dent in record gas inventories and helped drive prices up 40 percent since mid-February. "Winter showed up late this year and the market doesn't want to pull back much. We're going to get light (storage) injections this week and next week, but it looks like the forecasts are finally calling for spring," a Texas-based trader said. Front-month gas futures on the New York Mercantile Exchange ended down 2.9 cents at $4.238 per million British thermal units after trading between $4.218 and $4.319. The front contract, which has lost nearly 4 percent in the last two sessions, hit a 21-month high of $4.429 last Thursday. Traders noted that gas prices have climbed to levels that could dampen demand by making gas less competitive with coal for power generation. High prices could also tempt producers to turn on more wells, increasing supply. Record growth in futures open interest that has accompanied recent price gains has also raised concerns investors with long positions may rush to take profits when moderating spring temperatures finally slow space heating needs. After the cold shot this week, Commodity Weather Group noted that computer models mainly show seasonal to warm anomalies for the six- to 15-day outlook. But the forecaster added "Since it is early May, we would need to see stronger anomalies to generate a (cooling) demand response." ANOTHER LIGHT INVENTORY BUILD EXPECTED U.S. Energy Information Administration data last week showed total domestic gas inventories rose by 31 billion cubic feet to 1.704 trillion cubic feet. Most traders viewed the build as supportive for prices, noting it came in below the Reuters poll estimate of 34 bcf and below the five-year average increase for that week of 39 bcf. The season's first injection, which came about three weeks later than usual, widened the storage deficit relative to the five-year average by 8 bcf, leaving stocks at 74 bcf, or 4 percent, below that benchmark. Chilly weather this month is expected to continue to slow inventory builds and drive stocks further below average for the next couple of weeks. Injection estimates for Thursday's EIA report range from 19 to 48 bcf, with most in the mid-30s. Stocks rose 43-bcf during the same week in 2012, while the five-year average build for that week is 50 bcf. OUTPUT NOT SLOWING MUCH YET Baker Hughes data on Friday showed the gas-directed rig count rose slightly last week for the second straight week, stirring expectations that higher gas prices may be tempting producers to hook up more wells. While the gas rig count is hovering just above a 14-year low of 375 posted two weeks ago, production so far has not slowed much from the record high hit last year.