sunny spell during the rainy season
Refers to a break in the 梅雨 (tsuyu) rains, when the sun comes out temporarily.
Today is a sunny spell during the rainy season, so I can hang laundry outside for the first time in a while.
Compound of 梅雨 (tsuyu, 'rainy season') and 晴れ (hare, 'clear weather').