A severe storm packing winds as high as 70 mph and golf-ball size hail are expected Chicago early Wednesday night, but storm warning already have triggered air and rail travel cancelations and delays.
Flash flooding and even tornados are possible before the storm exits the area late Wednesday.
"Be inside tonight," said National Weather Service meteorologist Amy Seeley. "And pay attention to all the latest weather updates."
As of 5:30 p.m., METRA had halted inbound and outbound service on its Union Pacific Northwest and West Lines "due to severe weather related conditions," according to a service alert posted on the agency's web site.
As of 3:30 p.m., airlines have canceled more than 120 flights at O'Hare Airport, but only some of those flights were called off proactively because of the impending storms, Dept. of Aviation spokeswoman Karen Pride said.
Airlines at both O'Hare and Midway anticipate the weather will cause delays later Wednesday evening, Pride said.
The storms had moved into Kane and McHenry Counties late Wednesday afternoon before moving into Chicago. City officials activated Aurora's severe weather sirens due to the tornado warning that was issued for Kendall County. There have been no tornadoes spotted within the City limits, authorities said.
"We haven't had any wind speed reports yet, but trees and power lines are down." said Seeley. "And we're still worried about the golf-ball size hail it dropped on Rockford."
Fellow meteorologist Andrew Krein also sounded the alarm.
"We're talking a major threat here. We're talking damaging winds, large hail, a chance of tornadoes and also very heavy rainfall," Krein said. "There's a flash flood watch out for basically all of Northern Illinois and the Chicago metro area and into Lake and Porter counties in Indiana."
Chicago should see two to three inches of rainfall, with some areas expected to get a bit more rain. Flooding will occur in low-lining areas, river streams, viaducts and Chicago's streets: "It doesn't take as much rainfall in the short amount to cause flooding in Chicago."
Klein said the threat of tornadoes is very real.
"These storms are going to develop rapidly,..." he said. "The threat [of tornadoes] is here, and I would emphasize the threat is over the whole area."
On Tuesday, meteorologists worried that Chicago would see "derecho" winds, which pack sustained winds of at least 75 mph, but Kein said Chicago likely won't be seeing those winds Wednesday. Derecho storms imply a damaging widespread wind threat that covers 200 to 300 miles, at least, Klein said. He said Iowa might see derecho winds, but in Chicago could still expect "destructive, damaging winds, possibly in excess of 70 mph."
On Tuesday afternoon, the Chicago White Sox tweeted that its home-turf game against the Toronto Blue Jays "has been postponed due to a forecast of severe weather conditions."
The last derecho hit Chicago in July 2011, when weather stations at Midway Airport and in Crystal Lake recorded 75 mph winds. Nearly 90,000 ComEd customers lost power.