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.

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.

Texas. North Carolina. Illinois. New Mexico. Flash flooding has surged into the spotlight in the last week with record-breaking flood events occurring nearly back-to-back across the country. Here’s why they happened now and why floods are becoming more intense.

Follow for live updates in the Texas flooding as more than 160 are missing as rescuers continue a desperate search

Destructive flash floods hit New Mexico and Chicago on Tuesday, prompting water rescues. Here’s where they may hit next.

A "considerable" flash flood warning has been issued in Boston as heavy, life-threatening rain pounds the city and areas to the south. Two to 4 inches of rain has fallen so far. Rainfall rates are reaching up to 2 inches per hour.