unseasonably cool weather during the rainy season
Refers to a chilly spell that occurs during the tsuyu (rainy season), typically in June or early July, when temperatures drop below normal.
This year's rainy-season chill is lasting longer than usual.
梅雨寒で体調を崩さないように気をつけて。
Be careful not to get sick from the unseasonable cold during the rainy season.
Compound of 梅雨 (tsuyu, 'rainy season') and 寒 (samu, 'cold'), with rendaku voicing of さむ to ざむ.