Periods of rain and thunderstorms are to persist into Tuesday evening, maintaining a flood threat across the entire metro area. Chicago lies on the warm side of a front extending from the southern Plains, to lower Michigan. Moisture-rich Gulf air is being focused along this boundary which separates polar air from unseasonable warmth, more typical of late April. Monday evening, while temperatures hovered near 60 degrees in Chicago, Minneapolis sat at a brisk 21 degrees. This clash between air masses will hold nearly stationary over the area until Tuesday evening, when colder, drier air to the northwest pushes through, ending the heavy rain threat. Rain totals of 2 to 4 inches are possible by Wednesday morning, with the higher totals more likely from the far south suburbs, into northwest Indiana.