CHICAGO (Reuters) - An unstable mix of cool autumn air and warm lake waters spawned dozens of tornadic funnel clouds over Lake Michigan on Saturday, prompting a marine warning, the National Weather Service said.
The National Weather Service issued the warning after receiving reports of waterspouts from weather spotters up and down the vast lake's western shore, which stretches from Indiana to Wisconsin. There were no immediate reports of any injuries or damage.
None of the waterspouts, which can pose a danger to any planes or boats that travel through them, had moved onshore. But the National Weather Service warned that if any waterspouts did make landfall, residents needed to "find shelter immediately."
"We've had confirmed sightings from Green Bay all the way south to Chicago shoreline," said Samuel Shea, a meteorologist with the National Weather Service in Chicago.
"Everything has been happening over the lake itself, generally two to five miles out from the shoreline."
Shea said the climatological culprit was a big upper level low pressure area that has parked itself over the region, bringing cold air into contact with lake water that has retained the warmth it picked up over the hot summer.
He said the result was "a very unstable lower atmosphere that was just prime weather" for waterspout formation.
Lake Michigan is the second largest of the five Great Lakes and the only one located entirely within the United States.
(Reporting by James B. Kelleher; Editing by Cynthia Johnston)