明日(あした) ashitaは wa晴(は)れ()る() hareruに()決(き)ま()っ()て()る() nikimatteruよ yo。 It's definitely going to be sunny tomorrow. Literal Tomorrow [topic-は] clear-up [surely-に決まってる] [assertion-よ]. ~に決まっている expresses strong conviction — 'it's certain that, it's bound to.' 決まってる is the casual contraction of 決まっている. The よ adds assertion, making this a confident declaration. weather~に決まっている (bound to be)
明日(あした) ashitaは wa晴(は)れ()る() hareruに()決(き)ま()っ()て()る() nikimatteruよ yo。 It's definitely going to be sunny tomorrow. Literal Tomorrow [topic-は] clear-up [surely-に決まってる] [assertion-よ]. ~に決まっている expresses strong conviction — 'it's certain that, it's bound to.' 決まってる is the casual contraction of 決まっている. The よ adds assertion, making this a confident declaration. weather~に決まっている (bound to be)