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general::
Phrase(s): the lull before the storm [and] the calm before the storm
a quiet period just before a period of great activity or excitement. (Literal in reference to weather.) • It was very quiet in the cafeteria just before the students came in for lunch. It was the lull before the storm. • In the brief calm before the storm, the clerks prepared themselves for the doors to open and bring in thousands of shoppers.
McGrawhill's American Idioms And Phrasal Verbs