Tornado Warnings (IMAGE) Northern Illinois University Caption The 757 tornado warnings (red polygons) issued by NOAA's National Weather Service from May 17 to May 30 of this year. Credit Northern Illinois University Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.