Meteorological Factors Contributing to an Urban Flooding Event in Detroit, MI - August 23-24, 2023

Within the last two decades, Metropolitan Detroit has experienced three major flooding events that have each resulted in a considerable amount of property damage, as well as a number of injuries to local residents. A common characteristic shared by these events was the unexpected magnitude of rainfall, and subsequent extent of the flooding, across the area. In an effort to obtain a better understanding of the factors leading to the extreme nature of these events, the National Weather Service Forecast Office (Pontiac/Detroit) initiated a research study to determine potential key, common, factors leading to the severity of these events. It is hoped that the results of this overall work will lead to the improved forecast and warning of such events in the future.

This research project focuses on the most recent of these events, 23 to 24 August 2023, during which some areas of southeast Michigan/Metropolitan Detroit received localized rainfall totals in excess of 7.0 inches over the two day period. The most significant rainfall came during the period of 00 UTC to 12 UTC on 24 August 2023. A range of factors commonly associated with extreme rainfall and flooding were considered through the analysis of WSR-88D radar reflectivity, GOES-East Infrared Imagery, upper-air soundings, and Rapid Refresh (RAP) model reanalysis depictions of precipitable water (PWAT), 850-mb low level jet (LLJ), 850-mb dew point, 850-mb equivalent potential temperature, and 500-mb winds.

Our research results suggest that synoptic scale flow in the days preceding, and during, the event helped to transport significant moisture from the remnants of Tropical Storm Harold and Hurricane Hillary into the Great Lakes Region, with resulting PWAT values across the region exceeding 2.0 inches at times. Important mesoscale factors included a pattern of large warm cloud layers exceeding >10,000 feet present throughout the event, as well as a quasi-stationary frontal boundary across the Great Lakes, and localized areas of low pressure moving along the front, that often aligned nicely with areas of new convection. Several mesoscale convective systems (MCSs) impacted southeast Michigan during the event period, the apparent merger of which contributed to one of the more defining characteristics of the storm: the transformation into a mesoscale convective complex (MCC) who’s propagation became a significant contributor to excessive rainfall and severe weather.

The unique components prior to and during the 23 to 24 August 2023 event make it difficult to compare this system with the two other events being evaluated by the National Weather Service Forecast Office (Pontiac/Detroit) as part of this collaborative effort. However, we believe that these results are still valuable as they highlight potential factors that could be overlooked in the real time analysis of a potentially severe flooding system within the Metro Detroit area.