noun
return of the rainy season; rainy season returning after clear weather
Refers to the rainy period that sometimes occurs in late summer or early autumn after the end of the main 梅雨 (tsuyu) season, often after a stretch of clear, hot days.
今年は戻り梅雨が長くて、洗濯物が乾かない。
This year the return of the rainy season is long, and the laundry won't dry.
戻り梅雨の影響で、週末の天気はぐずつきそうです。
Due to the return of the rainy season, the weekend weather looks unsettled.
秋雨 is the autumn rain in general, whereas 戻り梅雨 specifically refers to a return of rainy season-like weather after a break.
Compound of 戻り (modori, 'return') and 梅雨 (tsuyu, 'rainy season'), literally 'returning rainy season'.