Thu Feb 27, 2014 2:23pm EST *Yellen says some data weak, weather impact uncertain * J.C. Penney, Best Buy shares rally after results * Dow up 0.5 pct; S&P 500 up 0.5 pct; Nasdaq up 0.7 pct By Caroline Valetkevitch NEW YORK, Feb 27 (Reuters) – U.S. stocks rose on Thursday after comments from Federal Reserve Chair Janet Yellen offered some relief…