The historic flooding from over the weekend is still top of mind for many of us. Some of us still have water seeping into our basements and others lost nearly everything. As our rain event was ending over the weekend, I put up a post stating that the flooding was "historic" and that it was likely a "500-year rainfall event" and possibly even a "1,000-year rainfall event". That post received hundreds of comments; many from people who were confused how we could call something a "500-year rainfall event" when we don't have 500 years of weather records and others were confused at how we could have a "500-year rainfall event" when we had similar flooding back in 2010 and 2008 and 1997 and 1986, etc. So here is my attempt to explain what the term "500-year rainfall event" really means and to answer some of the other frequent questions I have received.

Key Points:

• You don't need 500 years of weather data to determine the frequency of a "500-year rainfall event"
• You can have more than one "500-year rainfall event" in that 500 year window. It's just a 0.2% probability of seeing this much rain each year.
• We flooded because the "low level jet" kept sparking storms over the same area along a stalled surface front. Also, the heaviest rain fell in a highly developed area with lots of concrete.
• Climate change likely played a role in the flooding.
• Cloud seeding or other weather modification did NOT play a role in the flooding.

"500-Year Rainfall Event"

First of all, the phrase "500-year rainfall event" or even 100-year, 50-year, etc. is not just a term we throw out there haphazardly. The terms are correlated to exact thresholds. Those thresholds come from a document from NOAA called "Atlas 14". The goal of "Atlas 14" in its own words is to "provide precipitation frequency estimates for durations of 5-minutes through 60-days at average recurrence intervals of 1-year through 1,000-year."

By using "Atlas 14" we can determine if a heavy precipitation event has the chance to occur every year, five years, ten years, 50, 100, 200, 500 or 1,000 years. The phrase "500-year rainfall event" can also be a bit misleading. The phrase means to act as a guide to give us a probability of how frequently something like this could occur. Another way of looking at it would be to say that in any given year there is a 0.2% chance of a rainfall of this magnitude happening. That doesn't mean that it couldn't have also happened ten years ago or could again happen five years from now but there is only a 0.2% chance of it happening in any given year, or once every 500 years.

In the specific case of August 9-10, 2025, we used the 6-hour rainfall total in Milwaukee from Saturday evening through early Sunday morning of 6.59". That total comes very close to the threshold for a 500-year rainfall event of 6.73" and since the data from Milwaukee, which officially comes from the airport, was a little lower than locations on the northwest side of Milwaukee which may have received over 12" of rain, it seems safe to confidently say this was, "at least a 500-year rainfall event."

The 1,000-year rainfall event threshold was also attainable with a threshold of 7.55" in six hours. Some locations in southeast Wisconsin likely received that amount of rain, or more, in those six hours.

Atlas-14 precipitation frequency estimates. Milwaukee reached the threshold for a 500-year rainfall event and some other areas likely saw a 1,000-year rainfall event. NOAA - Atlas 14

Do we need 500 years of weather data?

We do not need 500 years or 1,000 years of precise weather data to say with confidence that this is a rainfall whose frequency of occurring is once every 500 or 1,000 years. We can use math, formulas and science to make that probability determination. Try thinking about it like the lottery. The odds of winning the Powerball jackpot are approximately 1 in 292.2 million. That doesn't mean that we need to see 292.2 million Powerball drawings to determine the chances are only 0.00000000342% of winning. You also can have more than one winning Powerball jackpot within those 292.2 million drawings. If you are interested in reading more about the methodology that "Atlas 14" uses, you can read about that by clicking here.

Is the 500-year rainfall guidance outdated?

Given that we seem to be having so many high precipitation events over the last few decades, some of you have questioned whether the guidance in "Atlas 14" is outdated and the answer is likely yes. In fact, work on "Atlas 15" is currently being done and will likely be released in 2026. It will be interesting to see how the thresholds for 500-year and 1,000-year rainfall events change.

What about a "500-year flood plain"?

It's also worth noting that a "500-year rainfall event" could be different than a "500-year flood plain". The flood plain is determined by FEMA and is used by engineers, insurance agencies, realtors, etc. to determine the risk of any given location to flood. Similar to a "500-year rainfall event", a "500-year flood plain" means there is a 0.2% chance of flooding in that flood plain any given year.

Why did we flood?

Any time we deal with devastation like this historic flooding it is natural to ask questions like, "Why did this happen?" and "Who can we blame?".

In this particular incident, the "why" might be easier for me to answer from a meteorological perspective. A frontal boundary stalled over southeast Wisconsin and was the focus for thunderstorm development. In addition, a strong low level jet was able to give storms enough forcing to keep developing over that same stalled front. If you look at a radar loop from Saturday evening into Sunday morning you would see storms developing along the frontal boundary and moving over the same area over, and over, and over again. It wasn't until the low level jet started to lose some intensity Sunday morning that the rain began to taper off.

This rain was also focused on a part of southeast Wisconsin that is extremely developed. Eastern Waukesha County and western/northern Milwaukee County saw the worst of the flooding. This area is highly developed with neighborhoods, business districts and manufacturing. That means a lot of concrete and not a lot of green spaces and the storm drainage systems quickly became overwhelmed by the intensity and magnitude of the rain.

There were a few examples across affected areas where purposefully created green spaces and retention/detention ponds really did their job. Hart Part in Wauwatosa and Cahill Park in Whitefish Bay are a few examples of smart green infrastructure that likely saved even more basements from being flooded. MMSD (Milwaukee Metropolitan Sewerage District) has been working very hard to create more of these spaces over the last few decades.

Was climate change to blame?

Likely yes. It is hard to pinpoint any one weather event, such as the weekend flooding, on climate change. But when we look at trends, all signs point to a changing climate. Out of the Top 10 rainfall events in Milwaukee, six of them have occurred in the last 25 years out of 150 years of data. We also know that climate change causes an increase in more extreme weather. That means hotter temperature records but also a high frequency of high magnitude rain events.

Milwaukee rankings for 1-Day rainfall. The rain we picked up Saturday was the second most on records since 1871. xmACIS2

Weather Modification?

I have received a lot of questions about cloud seeding and other "weather modification" and whether or not that played a role in the historic flooding. The simple answer is - no. Cloud seeding or other weather modification is not currently being done in Wisconsin and there is no known recording of cloud seeding or other weather modification ever being done in the state. According to the Government Accountability Office, cloud seeding is only being done in nine states and most of them are in the southwest or the west coast. The state closest to Wisconsin that does some cloud seeding is North Dakota.

Flooding Resources:

Hopefully you found this dissertation helpful in understanding how the flooding happened, why it was so historic and some of the factors that did and did not go into the heavy rain. If you or someone you know is affected by the flooding there are many local resources for you to access. Go here for an updated list of ways to get help.