If you're wondering just how rare this past weekend's deluge was − it was extremely rare. In fact, it's classified as a thousand-year event.
The National Weather Service told the Milwaukee Journal Sentinel the Aug. 9-10 storm ranks second for a two-day rain event, according to information dating back to 1871. And the Milwaukee area has only seen that much rain one other time before, in 2008, NWS said.
So what exactly is a 1,000-year event? Here's what to know.
What is a 1000-year flood?
When flooding is referred to a 200-, 500- or 1,000-year event, it doesn't actually refer to time. Instead, it refers to probability.
In turn, a 1,000-year flood event doesn't mean that it happens once every 1,000 years. What that means is a flood has a 1 in 1,000 chance of occurring in any given year.
Technically, multiple big storms like this could happen within the same week, although statistically it would be very unlikely.
How much rain is a 1,000-year flood in Milwaukee County?
The amount of rainfall that will incite a 1,000-year flood event depends on the specific location and the duration of time that the rain is falling.
Ironically, the Milwaukee Metropolitan Sewerage District is in the process of modeling and developing maps to see what it would look like if a bigger storm, like a 1,000-year flood event, hit Milwaukee County. On average across Milwaukee County, 9.18 inches of rainfall over 24 hours would be considered a 1,000-year event, according to the NOAA that the sewerage district used to model these events.
However, this is not a hard and fast number. For instance, according to meteorologist Sarah Marquardt, the magic number to classify as a 1,000-year flood event at Mitchell Airport is 8.98 inches of rain in 24 hours.
Marquardt said that many areas throughout the city crossed the threshold to make this past weekend's storm considered a 1,000-year event.
Did climate change cause this flood?
It's difficult to attribute individual events to climate change. However, since 1950, Wisconsin has become on average 17% wetter − or roughly 5 inches, according to the latest Wisconsin Initiative on Climate Change Impacts report. The southern two-thirds of Wisconsin has experienced the biggest increase in rainfall.
Climate change is fueling more extreme rainstorms, as warmer air can hold more moisture that eventually comes down as rain.
As extreme rainfall intensifies, experts warn that current flood control systems are already strained and won’t be able to protect communities in the future. The American Society of Civil Engineers' annual report card gave failing marks to the nation’s dams, levees and stormwater systems this year, with none scoring above a D.
Billions of dollars in repairs are needed to upgrade these systems to stand up to the added stress of climate change, many of which received funding through the Inflation Reduction Act. However, this year, the Trump administration pulled funding from these projects as it rolled back the climate goals of the previous administration.
According to a recent analysis by the Center for Public Integrity, extreme weather events will cost Wisconsin up to $16 billion over the next 15 years.
Rain totals in some Milwaukee County spots top 10 inches
Caitlin Looby covers the Great Lakes and the environment for the Milwaukee Journal Sentinel. Reach her at [email protected], and follow her on X @caitlooby.
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This article originally appeared on Milwaukee Journal Sentinel: Milwaukee rain levels caused a 1,000-year flood event
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