Total rainfall between July 5 and July 12, 2016. Much of the heavy rain in northern Minnesota and Wisconsin fell in the span of 24 hours. Image credit: Dennis Mersereau

Residents of the Upper Midwest didn’t need to hop on a plane to experience the tropics on July 11, as a slice of evaporated paradise streamed northward and dumped an incredible amount of rain on Minnesota and Wisconsin in just one day. The crushing surge of water pushed local waterways to record levels of flooding, leading to what the National Weather Service called a “potentially catastrophic flooding” event.

Communities north of Minneapolis, Minnesota, measured double-digit rain totals in just over a day’s time. Brainerd, Minnesota, near the epicenter of the severe storms, reported 8.78 inches of rain between the night of July 10 and the early morning of July 12. To put that kind of deluge in perspective, Brainerd only averages 7.66 inches of rain during the months of May and June combined. That total in Brainerd is the most rain they’ve ever seen in such a short period of time since the station there began keeping records back in 1908. Today, July 13, Minnesota Governor Mark Dayton and Lieutenant Governor Tina Smith plan to tour the storm damage.

Neither natural nor man-made water systems are meant to handle that much water that quickly. The Bad River near Ondanah, Wisconsin, broke its record-high flood mark when its depth peaked at 27.28 feet around noon on Tuesday, July 12. The previous flooding record at that river gauge site was 22.20 feet. Portions of the Mississippi River north of St. Cloud, Minnesota, will reach moderate flood stage over the next couple of days as the influx of water slowly filters downstream. While it doesn’t appear there was significant damage beyond a few damaged buildings and some washed-out roads, two men in Wisconsin died in the flooding.

A weather model graphic showing precipitable water, or moisture streaming north from the Gulf of Mexico into the Upper Midwest during the evening on Monday, July 11, 2016. Image credit: Pivotal Weather

Though this region isn't known as a toasty and humid region of the country, the tropics played a key role in dumping so much rain in so little time. We can measure the amount of moisture in the atmosphere using a metric called precipitable water, or PWAT for short. Measured in inches, the PWAT tells you how much rain would fall if you were to wring out all of the moisture in the atmosphere above one particular spot. Higher PWAT values equate to higher moisture, and values that exceed 2 inches are excessive outside of a place like the southeastern U.S.

More than half of the central United States’ yearly precipitation occurs during the warm months as storms come to life in the heat and humidity. The farther inland you go, however, the less moisture thunderstorms typically have to work with. As shown in the weather model graphic above, a low-pressure system developing over the northern Plains dragged tropical moisture north from the Gulf of Mexico, allowing thunderstorms over Minnesota and Wisconsin to tap into a deep reserve of tropical air and produce prolific rain totals in short order.

An animated radar loop showing thunderstorms over the Upper Midwest between 10:00 PM CDT on July 10, 2016, and 1:00 AM CDT on July 12, 2016. Image credit: Dennis Mersereau

Normally you would only get a quick inch or two of rain from a storm that forms in this kind of soggy environment, but this was not a normal situation. A squall line charged toward the rainy bullseye on the evening of June 9, setting the scene for the flooding to come. This squall line served as a focus for more thunderstorms to develop over the affected areas in the hours after it passed through. These storms north of Minneapolis began "training," or moving over the same areas again and again like train cars over railroad tracks. The thunderstorms, influenced by the remnants of the squall line and a nearby warm front, produced continuous heavy rain that lasted for more than 24 hours in some spots.

Events like this can be hard to forecast. Flood watches were not announced for the flooded areas before the rain started falling. The best models and the best forecasters can have trouble picking up on dangerous localized situations until they’re already unfolding. An unexpected 10 inches of rain in one day is a great argument for keeping up with weather forecasts and listening for watches and warnings—no matter where you are across the country.