Yes, WARMEST is a Scrabble word!

Letter values in Scrabble US: W 4, A 1, R 1, M 3, E 1, S 1, T 1.

Is it a valid word?

WARMEST is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

having or producing a comfortable and agreeable degree of heat or imparting or maintaining heat
psychologically warm; friendly and responsive
(color) inducing the impression of warmth; used especially of reds and oranges and yellows
having or displaying warmth or affection
freshly made or left
easily aroused or excited
characterized by strong enthusiasm
characterized by liveliness or excitement or disagreement
uncomfortable because of possible danger or trouble
of a seeker; near to the object sought