* Front month hits highest mark since late July 2011 * Cool weather back on tap in consuming regions * Nuclear plant outages back above five-year average By Eileen Houlihan NEW YORK, April 15 (Reuters) - U.S. natural gas futures seesawed on either side of positive territory early on Monday, after rising in electronic trade to their highest mark in more than 20 months. Traders said a tightening supply picture, more below-normal temperatures for consuming regions and a slew of nuclear power plant outages helped boost prices. Gas futures are up about 37 percent since mid-February, lifted by cold late-winter weather that put a huge dent in inventories, above-normal nuclear plant outages and stronger price expectations. But most expect further upside to be difficult, with milder, spring-like temperatures likely to curb heating demand by late-month before warmer weather sparks any early cooling demand. As of 9:23 a.m. EDT (1323 GMT), front-month May natural gas futures on the New York Mercantile Exchange were at $4.242 per million British thermal units, up 2 cents, or less than 1 percent, after rising as high as $4.29, the highest for a nearby contact since late July, 2011. The latest National Weather Service six to 10-day forecast issued on Sunday called for below-normal temperatures for a little more than the eastern half of the nation and above-normal readings in the West. Nuclear outages totaled 27,500 megawatts, or 27 percent of U.S. capacity, up from 22,300 MW out on Friday, 27,300 MW out a year ago and a five-year average outage rate of 24,100 MW. INVENTORY DRAW BELOW EXPECTATIONS, ABOVE AVERAGE Last week's gas storage report from the U.S. Energy Information Administration showed domestic inventories fell the prior week by 14 billion cubic feet, below Reuters poll estimates for a 21 bcf draw but above the year-ago gain of 11 bcf and the five-year average build of 15 bcf for that week. Domestic gas inventories of 1.673 trillion cubic feet are nearly 33 percent below last year's record high and nearly 4 percent below the five-year average. Inventories started the heating season at record highs, but two weeks ago stocks slid below the five-year norm for the first time since September 2011. But last week's decline should be the last of the heating season, with estimates for this week's report all looking for a modest build. Early injection estimates for this week's report range from 16 bcf to 55 bcf versus a 21-bcf build during the same week last year and a five-year average rise for that week of 39 bcf. Total gas pulled from storage this winter is about 2.25 tcf, roughly 770 bcf, or 52 percent, more than last year and about 15 percent more than the normal heating-season draw. Baker Hughes data released on Friday showed the gas-directed drilling rig count rose by 2 from the prior week's 14-year low, to 377.