A FLOOD WATCH remains in effect for most of northern Illinois and southern Wisconsin until early Monday morning, while a FLOOD WARNING remains in effect for Winnebago, Boone and McHenry counties until 10:30pm Sunday.

Steady and heavy rain has come down for most of Sunday morning and afternoon pushing rainfall totals across north-central Illinois and southern Wisconsin to near four, to five, inches! As of 6:30pm, the Rockford Airport has received 4.45 inches of rain, more than what the entire month of September averages!

The heavier bands of rain are now beginning to shift north into southern Wisconsin, giving most across northern Illinois a break in the rain. Low pressure spinning south of Lake Michigan will begin to pivot north, and then slightly back west Sunday night as a cut-off low pressure system moves across the Stateline. This will allow another band of heavier rain to rotate back in, this time from the north, late Sunday night and Monday morning. Additional rainfall totals of half an inch to an inch are possible during that time. Scattered rain showers and a blustery northwest wind can then be expected for the rest of Monday afternoon. High temperatures will only reach the low 60s.

The reason for the heavy and extended period of rain has to do with both the low at the surface and above, being driven by mostly a cut-off low to our west over Iowa. While the cold front has already passed through the Stateline, passing through late Saturday evening, the upper level low – cut-off low – remains to our west.

High amounts of moisture in the atmosphere have been pulled up and ahead of the cut-off low, getting wrapped back in to the north and west of the surface low pressure system. The greatest lift and forcing in the atmosphere and around low pressure systems can be found to the north and west, which is where most of the Stateline has been Sunday afternoon. As a result, the continuous feed of moisture combined with the slower movement of the storm system has allowed very high rainfall rates to occur, resulting in high rainfall totals.

That heavy band of rain is now moving north into Wisconsin, but will rotate back around to the west and south into northern Illinois by Monday morning. This will result in a period, just before sunrise, of heavier rainfall – perhaps with a few embedded thunderstorms – Monday morning. Scattered rain showers are then expected throughout the afternoon under mostly cloudy skies.