Heavy rain and thunderstorms could bring flash flooding to the Chicago area overnight Sunday, with forecasters most concerned about the period from about 10 or 11 p.m. through early Monday.
A flood watch issued Sunday afternoon remains in effect until 7 a.m. Monday, according to the National Weather Service.
Showers and thunderstorms were developing across northwest Illinois and moving into the area Sunday evening, said Mike Bardou, a meteorologist with the National Weather Service.
The storms could bring multiple rounds of heavy rainfall to some areas, Bardou said. It remained unclear Sunday evening which areas would receive the most rain.
The heaviest rainfall in the Chicago area could begin between about 10 p.m. and midnight and continue for several hours overnight, Bardou said.
“Flash flooding is one of our biggest concerns here because the rain could fall very quickly, in the order of 1 to 2 inches per hour under some of these storms, or even perhaps briefly heavier than that,” Bardou said.
Some parts of the Chicago area, including many suburbs, had already received 1 to 3 inches of rain earlier Sunday, leaving some areas saturated and potentially with standing water, Bardou said.
The weather could also affect the Monday morning commute. Bardou said the bulk of the rainfall would probably occur before daybreak, but ponding or flooding could persist into the morning, depending on how much rain falls and where.
Bardou pointed to viaducts in the city, typical high-water areas and some expressways as places where flooding or ponding could potentially linger. Damaging winds are also a concern.
The weather service issued a severe thunderstorm watch at 8:44 p.m. Sunday for Cook, DuPage and several other Illinois counties. The watch remains in effect until 4 a.m. Monday.
Bardou said damaging straight-line winds and large hail were also concerns.
“The biggest thing is it’s a good evening and overnight to stay weather aware and have a way to get woken up by warnings if you’re going to be asleep,” Bardou said.
He also urged people who live near creeks that rise quickly or in areas prone to basement flooding to monitor conditions and have a way to receive warning notifications.
As of 9:40 p.m. Sunday, ComEd reported 288 active outages affecting more than 7,900 customers.