Even though it's January, it feels as warm as early spring.
Literal
January [quotative-と言うのに] early-spring [like-のような] warmth is.
A short weather observation. ~と言うのに ('even though one says, despite being') expresses contrast between what something 'is' and what it's actually like — stronger than plain ~のに. 春先 (はるさき) is a set noun meaning 'early spring, the very beginning of spring'. ~のような functions as a comparative modifier, 'like X'.