NEW YORK, Sept 23 (Reuters) - Front-month U.S. natural gas futures, up early on warm midweek weather and firmer physical prices despite milder U.S. forecasts for next week, trimmed gains but remained higher Thursday morning after a government report showed a weekly inventory build below market expectations. At about 10:31 a.m. EDT (1431 GMT), the front month October contract NGV0 shot up about 16 cents, or 4 percent, to $4.133 per million British thermal units, before slipping back to about the $4 area by 10:37 a.m., still up about 4 cents. Just prior to release of the weekly storage data at 10:30 a.m., the front month was trading in the $4.07 area. The U.S. Energy Information Administration report showed total domestic gas inventories for the week ended Sept. 17 climbed by 73 billion cubic feet to 3.340 trillion cubic feet, a level not normally reached until the first week of October. [ID:nPRWPI54] The build was below the Reuters survey estimate for an 80 bcf gain but slightly above the year-ago rise of 66 bcf and the five-year average increase for that week of 70 bcf. (Reporting by Joe Silha; Editing by Marguerita Choy)

Source