By Lauren Casey

PHILADELPHIA (CBS) — Some New Jersey counties are under a flash flood warning until early Sunday morning due to a heavy rain system sweeping across the region. Mercer County is under the warning until 2:45 a.m.

Other counties — Somerset, Middlesex, and Monmouth — are also included in the warning.

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The Jersey coast and parts of Delaware are under a coastal flood advisory until noon Sunday.

Periods of heavy rain and scattered thunderstorms Saturday night will wrap up Sunday morning. A few afternoon showers will linger.

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Temperatures will crash tomorrow evening, dropping between 20 and 30 degrees.

A secondary low could develop and bring a period of snow to parts of the area Sunday night and into Monday morning. The forecast is highly uncertain at this point.

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Wind chills will make Monday feel like it’s in the mid 20s as people return to work.