Strong squall lines with occasional tornadoes are most frequent during these months. August Rainfall amounts decline slightly from the previous month during these summer months in most areas of the State. Topography is still the main controlling influence with the northern highlands and the hills of the southwest receiving about 4 inches per month. In contrast the Lake Michigan shorelands generally receive 3 inches or less. The tendency for mature thunderstorms to cross the State from northwest to southeast during the mid-summer months, in contrast to the southwest to northeast flow of the other months of the year, cause the precipitation pattern to take on a slightly different appearance.
Thunderstorms are rather localized and occasionally severe with hail and strong winds.
Pressure patterns associated with the shower activity tend to be weak with the main activity occurring near frontal zones. As autumn approaches, low pressure systems intensify and more frequently pass close to the State with their trailing cold fronts and squall lines. Fronts again have a tendency to stall in the western Great Lakes region, often lying diagonally southwest to northeast across the State for several days with the associated heavy shower activity. On rare occasions, a dissipating hurricane from the Gulf Coast region may spread rain across the southeastern portions of the State. September, therefore, in the mean, is as wet or wetter than August in many areas of the State.