UPDATE 1-U.S. natural gas futures fall to 13-year low on weak heating demand

(Adds latest prices) Dec 14 (Reuters) - U.S. natural gas futures fell to a 13-year low in intraday trade on Monday on forecasts of continued unseasonably mild weather that is expected to keep heating demand low through late December. Front-month gas futures on the New York Mercantile Exchange fell for a fourth day in a row on Monday, closing down 9.6 cents, or 4.8 percent, at $1.894 per million British thermal units. That kept the front-month in oversold territory for a sixth day