NEW YORK, Oct 14 - U.S. chain store sales rose modestly as consumers started their holiday shopping early, though warmer weather tempered further gains, Instinet Research said in its Redbook report on Tuesday. Sales at major retailers were up 3.6 percent in the week ended Oct. 11 compared with the same week a year ago, after rising 3.1 percent in the preceding week, the report said. Sales grew 1.2 percent in October compared with September, the report said. Retailers expect falling temperatures to trigger demand in seasonal clothing and winter sportswear, especially as the holidays approach. The Redbook data are compiled from a sample of same-store sales at general merchandise retailers representing about 9,000 stores. Same-store sales measure revenue at stores open at least one year. The index is released weekly by Instinet Research, a division of Instinet, a -owned electronic brokerage.//