Updated 04/10/13 – 10:51 a.m.

CHICAGO (CBS) — Keep your umbrella handy today. Heavy rain and thunderstorms are moving through the Chicago area.

CBS 2 Meteorologist Megan Glaros reports the greatest chance for severe storms is Wednesday afternoon and evening, south of I-80, although there is a chance for strong storms most of the day.

Heavy storms were already moving through the northwestern suburbs Wednesday morning.

In the morning, the storm could bring penny-sized hail and wind gusts of up to 50 mph, with frequent lightning and heavy downpours, according to the National Weather Service.

Rainfall of up to three-quarters of an inch is possible on Wednesday, with up to an additional inch overnight.

The rain was prompting delays and cancellations at the airports. As of 10:30 a.m., 170 flights had been cancelled at O’Hare International Airport, and some airlines were experiencing delays of up to an hour. At Midway International Airport, only a few cancellations had been reported, and some flights were being delayed up to 30 minutes.

More rain is likely on Thursday, and by Thursday night, falling temperatures could bring a mix of rain and snow to the area.