Recent flash flood events in the Upper Midwest: Causes and common characteristics

In the Upper Midwest, heavy flood-producing rains result from an abundant supply of tropical (or subtropical) moisture and a quasi-stationary source of lift. These conditions are met through strong northward advection of warm, humid unstable air from the Gulf of Mexico or remnant tropical moisture from a former tropical storm system. Subsequently, this moisture is typically lifted in an overrunning pattern along and north of a slow-moving or nearly stationary frontal boundary. Waves of low pressure traversing along the boundary, high instability, the presence of a low-level jet (typically a nocturnal feature), and upper level short-wave troughs can enhance this type of event. Recurrent thunderstorm development occurs, with individual storm cells following a similar track (training) and dumping heavy rain onto already saturated soils, contributing to greater runoff and subsequent flooding. If these conditions are maintained over a period of weeks or even months, persistent heavy rainfall can lead to widespread catastrophic flooding such as occurred during the great Midwestern floods of summers 1993 and 2008. The focal point of this research, however, is on flash floods which occur on smaller time scales, usually over several hours or up to a day in prolonged events. Over the past three seasons, areas in the upper Mississippi Valley (primarily Illinois and the adjacent states of Iowa, Minnesota, Missouri, and Wisconsin) have experienced significant heavy rain episodes and subsequent flash flooding. The purpose of this study is to investigate the synoptic weather patterns in each of these cases to determine characteristics common to flash floods.

The first case occurred in August 2007 when areas in southeast Minnesota and southwest Wisconsin received 10-12 inches of rainfall overnight (with localized higher amounts), leading to flooding of historic proportions. Copious amounts of moisture rode northward up and over a warm front, leading to thunderstorms, which continued to redevelop and move E/SE along the same axis. The heavy rains persisted hour after hour with high rainfall rates of 1 to 2 inches per hour common. In September 2008, tropical moisture from the remnants of a former Eastern Pacific tropical storm (Lowell) was transported into Illinois by strong southwesterly flow. Strong forcing associated with slow-moving frontal boundaries acted to produce widespread rainfall of 6-8 inches over a multi-day period across the northern half of the state, with single day totals of 4-5 inches not uncommon. Flash flooding was widespread and significant river flooding affected the Chicago area. In mid-May 2009, areas in the Midwest were subjected to a strong spring storm system with multiple frontal boundaries interacting with seasonally high atmospheric moisture levels, resulting in heavy rainfall and severe weather. In northern Missouri, a very warm and humid air mass combined with an upper level disturbance and stalled warm front to produce nearly 18 hours of rain. Slow-moving thunderstorms with rainfall rates of more than 2 inches per hour caused flash flooding. A similar set up further to the east in central and eastern Illinois also led to very heavy rainfall rates of 2 to 4 inches per hour. Widespread flooding occurred, with numerous road closures in both rural and urban areas (such as Champaign/Urbana), while a significant levee breech occurred on the Spoon River near London Mills, Illinois.

Each flood event featured abundant moisture, strong atmospheric forcing, and upper level support. The presence of slow-moving frontal boundaries typically served as both a focusing and forcing mechanism for recurrent thunderstorm development. In the days prior to the flash flood episodes, the affected areas received additional rainfall which prepared the ground so that it was more quickly saturated once rainfall began. Advection of warm, moist air and high instability combined with a persistent pattern that resulted in high precipitation efficiency, high rainfall rates, and much greater than normal runoff.