、。
It's not even the rainy season, so you don't need an umbrella, do you?
Literal
Rainy-season [even-でも] is-not [explanatory-んだから], umbrella [topic-は] is-not-needed [isn't-it-んじゃない].
Layered casual reasoning. 梅雨時 (つゆどき, 'rainy season period') with でもない ('it's not even') sets up the premise. んだから (= のだから) adds an explanatory-insistent 'so' — the speaker is making a logical argument. 要らない ('not needed') is the negative of 要る. んじゃない at the end is a casual tag question seeking confirmation — 'right? / don't you think?' The whole thing is a friendly, slightly teasing exchange about over-preparedness.