* WHAT...Thunderstorm activity through late this afternoon will create varying currents, rapid water level fluctuations, and dangerous swimming conditions with waves 3 to 5 feet expected. Northerly winds will develop tonight and create high wave action and dangerous swimming conditions with waves 6 to 9 feet expected. * WHERE...In Illinois, Lake, Northern Cook and Central Cook Counties. In Indiana, Lake and Porter Counties. * WHEN...Through Wednesday morning. * IMPACTS...Swimming conditions will be life threatening, especially for inexperienced swimmers. For information on swim advisories or bans at Chicago beaches: www. cpdbeaches. com. Remain out of the water to avoid dangerous swimming conditions and do not venture out onto piers, jetties, breakwalls, or other shoreline structures.