How a spring rainstorm became a 500-year flood event in mid-Michigan

Rainstorms are a typical part of spring in Michigan. The rainstorm that hit mid-Michigan beginning on Sunday, May 17, though, proved one for the history books.




The flooding that resulted from up to 7 inches of rain in Midland, Saginaw and surrounding counties damaged homes and property, forced more than 10,000 people to evacuate and most spectacularly destroyed one power dam and severely damaged another.

It’s being called a 500-year flood event, meaning it is only likely to occur once every 500 years.

How did it unfold?

The story starts with the storm itself. One key ingredient fueled the downpour. MLive meteorologist Mark Torregrossa said the heaviest rain was produced by tropical moisture, as Tropical Storm Arthur was churning off the Mid-Atlantic coast.

In what he called a conveyor belt effect, a mass of moisture traveled from the East Coast westward into Michigan, allowing the ongoing storm to tap into that tropical moisture and cause a substantial increase in rainfall totals.

Mid-Michigan received a heavy amount of moisture from the storm, with Midland, Bay and Saginaw Counties floating around 3-4 inches of rain. The heaviest rainfall totals were found to the north, with Au Gres receiving 8.10 inches, East Tawas getting 7.97, and Sterling clocking in at 7.20 inches.

The rainfall itself caused headaches for various municipalities, with Bay City’s wastewater treatment plant requiring the use of an auxiliary pump to keep up with the sudden deluge of water the evening of Monday, May 18.

Numerous roads closed due to the heavy rain, especially in counties in northern mid-Michigan, with Arenac and Gladwin County beginning to see numerous road closures during the night on Monday.

“Many roads have washed away. What may appear to be standing water may be much deeper than you believe. As it gets darker, you will not be able to see the flooded areas. Unnecessary travel ties up resources if you end up stranded. Please stay home if you do not absolutely need to be traveling,” said an alert from the Arenac County Sheriff’s Department that night.

read more here