The seasonal rain front will become active starting tomorrow.
Literal
Tomorrow [from-から] seasonal-rain-front [subject-が] active become [polite-ます].
梅雨前線 (つゆぜんせん) is 'the rainy-season front' — the meteorological term for the stationary front that brings Japan's June rainy season (梅雨). A domain-specific term that newscasts and weather reports use frequently. 活発 is a な-adjective ('active/lively') + になる for 'become active.'