* Front month trades just below Thursday's 18-month high * Nuclear outages still running above normal * Cold to continue into early April By Joe Silha NEW YORK, March 22 (Reuters) - Front-month U.S. natural gas futures ended slightly lower on Friday for a third straight day, but cold weather this week and forecasts for more of the same next week still allowed the spot contract to eke out a slim net gain for the week. Cold late-winter weather has put a huge dent in inventories and helped drive futures prices up about 25 percent in the last five weeks. Above-average nuclear plant outages have also increased demand for gas-fired replacement power and underpinned recent price gains. "It still looks pretty cold for the next two weeks, so prices are probably going to stay near these levels until warmer weather moves in," a Pennsylvania-based trader said. Front-month gas futures on the New York Mercantile Exchange ended down 0.8 cent at $3.927 per million British thermal units after trading between $3.913 and $4.006. The nearby contract climbed to $4.025 on Thursday, its highest mark since September 2011. The nearby contract, which posted an 18-month high of $4.025 on Thursday, gained 1.4 percent this week following a 6.7 percent rise last week. But despite recent gains, some traders see only limited upside from here, noting production was still flowing at or near an all-time peak and milder spring weather was likely to slow demand later next month. In addition, gas prices above $4 could curb demand by prompting some utilities to use more coal to generate power and increase supply by encouraging producers to turn on more wells. Commodity Weather Group still expects cold to dominate for the next 10 days, with some days predicted much below normal temperatures for the Midwest and South. The forecaster expects early April to remain cool for the Midwest, East and South. STORAGE DRAW FALLS BELOW EXPECTATIONS U.S. Energy Information Administration data on Thursday showed total domestic gas inventories fell last week by 62 billion cubic feet to 1.876 trillion cubic feet. The draw included a 4 bcf reclassification from base gas to working gas in the Producing Region. Most traders viewed the decline as bearish, noting it fell short of market expectations for the first time in five weeks. The Reuters poll was looking for a 70 bcf draw. The drop did cut 36 bcf from the surplus versus the five-year average, but storage is 162 bcf, or 9 percent, above that benchmark. Most traders expect that surplus to shrink sharply in next week's inventory report, with early withdrawal estimates ranging from 59 to 94 bcf. Stocks rose 45 bcf during the same week last year, while storage normally builds 6 bcf that week. Stocks will end the heating season below 1.8 tcf, or about 3 percent above average. A Reuters poll in mid-January showed most analysts had expected stocks to finish winter at about 2 tcf. Total gas pulled from storage so far this winter is about 2.050 tcf, about 580 bcf, or 39 percent, more than the same time last year and nearly 5 percent above normal. RIGS CLIMB, OUTPUT NOT SLOWING MUCH Baker Hughes data on Friday showed the gas-directed drilling rig count fell this week for the third time in four weeks, dropping by 13 to 418. The count is hovering just above the 14-year low of 407 posted two weeks ago, but production has not slowed much, if at all, from the record high posted last year. EIA still expects marketed gas production in 2013 to hit a record high for the third straight year.