The flood watch is in effect until 7 a.m. Friday for Cook County, the collar counties and the majority of northern Illinois, according to the National Weather Service.

The quick-falling rain led some parts of Chicago and Cook County to get upwards of five inches of rain in less than three hours.

Straighter, smoother streams flow more violently than those that meander due to a reduction in friction. Areas with steeper terrain can see water accelerate downhill at a much faster rate causing a more destructive flash flood than flatter, open areas.

The National Weather Service issued a flash flood warning at 9:46 p.m. Tuesday night for Chicago and Cicero and the situation quickly escalated.

Friday, temperatures are expected to reach the mid 80s with a 40 percent chance of heavy rain. Saturday, the chance of thunderstorms is 50 percent with highs in the upper 80s.

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