A stalled boundary draped across northwest Illinois and southwest Wisconsin Monday afternoon produced a few isolated thunderstorms. While the storms remained below severe limits, a few of the stronger ones did pose a risk for gusty winds and had torrential downpours – especially over parts of Stephenson and Jo Daviess counties.
Most of the storm activity through Monday evening will be confined to the northwest of Rockford, fading as we near sunset. Isolated showers and thunderstorms will remain possible overnight.
Temperatures Monday afternoon warmed into the middle to upper 70s thanks sunny skies, but cooler temperatures can be expected Tuesday and Wednesday with an increase in cloud cover. Highs on Tuesday will only warm to the low 70s, and then upper 60s and low 70s Wednesday.
Low pressure currently centered over central Minnesota will move closer to the Stateline Tuesday. As it does, thunderstorm activity will begin to increase in coverage – first in the morning and then again during the afternoon and evening. With high amounts of moisture in the atmosphere, heavy downpours remain a threat along with the risk for hail. Funnel clouds may also occur with any of the stronger storms that occur. The colder temperatures aloft with the low will help increase instability and lift within the atmosphere. With the presence of a boundary under the low at the surface, low level wind shear will increase slightly, causing weak rotation with some of the stronger storms. If this occurs, funnel clouds may be possible.
The threat for strong storms will end with sunset, but heavy rainfall will remain possible overnight Tuesday and into Wednesday. Over the next couple of days rainfall totals up to two inches could occur, with areas favored from far northern and northwest Illinois to southern Wisconsin to receive the highest totals.