the calm before the storm

US /ðə kɑːm bɪˈfɔːr ðə stɔːrm/
UK /ðə kɑːm bɪˈfɔːr ðə stɔːrm/
"the calm before the storm" picture
1.

嵐の前の静けさ

a period of unusual quiet or inactivity before a period of great activity, excitement, or trouble

:
Everyone was quiet, but I knew it was just the calm before the storm.
みんな静かだったが、それがただの嵐の前の静けさだと分かっていた。
The quiet morning was the calm before the storm of the busy workday.
静かな朝は、忙しい仕事の日の嵐の前の静けさだった。