* Front month gains for second day * Chilly weather seen continuing through April * Nuclear plant outages still above five-year average * Coming Up: EIA, Enerdata natgas storage reports Thursday By Joe Silha and Eileen Houlihan NEW YORK, April 17 (Reuters) - U.S. natural gas futures ended higher on Wednesday for a second straight day, backed by chilly weather forecasts for the next two weeks that should underpin heating demand despite prospects for milder temperatures next month. Front futures have mostly been in an uptrend since mid-February, rising nearly 35 percent during eight straight weeks of gains. Chilly March and April weather and above-average nuclear plant outages helped whittle down record high inventories and tighten overall supplies. Nuclear plant outages have mostly been running above average and have increased demand for gas-fired replacement power. "Natural gas prices are holding the recent range on a temperature outlook that appears quite similar to yesterday's run," Citi Futures analyst Tim Evans said in a report, also noting some buying ahead of Thursday's weekly inventory report. Front-month gas futures on the New York Mercantile Exchange ended up 5.4 cents, 1.3 percent, at $4.214 per million British thermal units after trading between $4.146 and $4.244. But chart traders noted the market seemed to be struggling here, with the front contract unable to break above Monday's 20-1/2-month high of $4.29. Buyers may be growing cautious as prices near levels that could slow demand by prompting more utilities to switch from gas back to coal for power generation and increase supply by tempting producers to hook up more wells. There are also concerns that record growth in futures open interest that has accompanied recent price gains means there are a lot of new longs in the market that could rush to take profits and sell out once temperatures turn milder. In its six-to-10-day outlook, MDA Weather Services said strong cold remains on track to drop into the central United States, with below or much below normal temperatures expected in the Plains, Midwest and South. Northeast states will see mostly seasonal readings during the period. INJECTION SEASON SET FOR SLOW START The stock-building season is set to get under way, albeit three weeks later than usual, with Thursday's U.S. Energy Information Administration storage report expected to show a build of 34 billion cubic feet, according to a Reuters poll on Wednesday. Stocks rose 21 bcf during the same week last year, while the five-year average rise for that week is 39 bcf. Persistent cold in April, particularly in the Midwest, is likely to keep weekly injections below average for the next few weeks and drive overall inventories further below the norm for that time of year. EIA data last week showed that total domestic gas inventories had fallen to 1.673 trillion cubic feet, 32 percent below last year's record highs for the period and 4 percent below average. Winter withdrawals this year totaled 2.25 tcf, 770 bcf, or 52 percent, more than last year, and 15 percent more than the normal draw during the heating season. RIG COUNT CLIMBS, WHEN WILL OUTPUT SLOW? Baker Hughes data on Friday showed the gas-directed rig count rose slightly last week but is still hovering just above the 14-year low of 375 posted two weeks ago. Drilling for natural gas has mostly been in decline for the past 18 months. The count is down about 60 percent since peaking in 2011 at 936, but so far production has not slowed much from the record high hit last year. EIA recently trimmed its estimate for domestic gas production growth in 2013, but still expects output to rise 0.3 percent from 2012's record levels.