Sep Nymex natural gas (NGU24) on Tuesday closed down by -0.041 (-1.87%). Sep nat-gas prices on Tuesday fell on a mixed US weather forecast, which may curb nat-gas demand. According to the Commodity Weather Group, forecasts shifted hotter in the eastern and southwestern parts of the US but cooler in parts of the Midwest to the Plains for August 17-21. On Monday, nat gas prices rallied to a 3-week high on forecasts for hotter US temperatures that would boost nat-gas demand from electricity providers to run air conditioning. Last Monday, nat-gas prices tumbled to a 3-1/2 month nearest futures low…