Thought Leadership

Eight Weeks and Four Floods: How understanding compound weather events can help you predict and manage emergency weather

August 23, 2022

By Rebecca Verity – Senior Climate Adaptation Scientist

My kids left school for summer break on May 27. By that evening, we had left drought-ridden California and were camping in Oregon, in the rain. We drove further north, through Washington (raining) and into British Columbia (also raining). After several sodden days, and faced with heading to a campsite that, in June, was still getting snow, we cried uncle and switched to a home rental. We were lucky – our family’s climate impacts this summer meant a few days with a little more luxury than we’d planned. Much of the country did not fare so well. By the time my kids were back in school, the U.S. would have weathered a “thousand-year flood” on an average of every other week.

Thousand-year floods are those so big that they would only be statistically expected once every thousand years. A decade ago, even one would have dominated the national news cycle for days. This summer, they ravaged communities from California to Missouri, Wyoming to Kentucky. Not a single one was caused by an exceptional hurricane, or even a named storm. Each of them was what we call a “compound event,” or a weather catastrophe caused by the collision of two or more weather patterns. This summer’s floods included three different types of compound event.

To better predict, avoid, and manage emergency weather, we need to be clear that devastating impacts come not just from individual huge weather events, like large hurricanes or tornadoes, but also from the interactions of much smaller weather patterns. Let’s review the compound impacts of the summer of 2022.

Rain-on-snow (Yellowstone National Park Floods, June 13, 2022)

Like our campsite in British Columbia, the northern Midwest had an unusually cold, wet spring. Winter snowmelt was delayed, and significant spring snowpack accumulated. Between June 10 – 13, temperatures spiked dramatically as a warm, drenching atmospheric river rolled into Wyoming and Montana, bringing two to three inches of rain, and speed-melting that late, deep snowpack. Unusually fast snowmelt, alone, is often enough to bring creeks and rivers to flood stage. Two to three inches of rain, alone, can do the same. Combined, it created a “catastrophic volume of rushing waters, which overtook rivers, created mudslides and rockslides, and overwhelmed infrastructure .” Roads in Yellowstone National Park were destroyed, and the park evacuated. Park Superintendent Cam Sholly described the floods as “a thousand-year event, whatever that means these days.”

Washed out road in Yellowstone National Park

This summer’s storms wrought catastrophic damage to roads in Yellowstone National Park. The massive flooding also destroyed homes, water and wastewater systems, power lines, bridges, and other critical infrastructure.

Rain-on-rain-on-rain (St. Louis, MO and Kentucky Floods, July 6-8, 2022)

Sholly’s skepticism of the term was prescient. Even as water levels were dropping in Yellowstone, exceptionally warm Gulf of Mexico water was pumping moist air up and around a heat dome developing over the southern U.S. The wet air was forced northward, where it stalled against a cold system hovering over the Great Lakes. Incredibly wet air accumulated day after day, until in the first week of July, a line of thunderstorms formed. High altitude winds pushed them out across Missouri and Kentucky in a single line track, drenching the same areas repeatedly. Both St. Louis, MO, and Hazard, KY were reporting 1,000-year return period rain. Nine inches of rain had fallen in Hazard in half a day: the same amount that typically falls over the town in 45 days. The first thunderstorms saturated soils, the next round filled creeks, and soon neither soil nor rivers could hold another drop, but the storms kept coming. Creeks and rivers burst their banks, destroying towns, homes, and lives. As of this drafting (Aug 22, 2022), two fatalities in St. Louis and 39 in Kentucky have been confirmed.

Rain-on-drought (Death Valley National Park Floods, August 5, 2022)

Meanwhile, back in California, one of the longest, deepest droughts in California history continued. Death Valley National Park, one of the driest, hottest places on earth, generally receives barely two inches of rain each year. While floods were raging in the Midwest, Death Valley had received only 0.04 inches of rain since the start of the year. As every gardener knows, parched soils do not absorb water quickly. The first rains following a drought frequently cause minor flooding as water runs over, rather than absorbs into, parched earth. In early August, Death Valley certainly qualified as parched. Then, on August 5,  nearly two inches of rain fell in only three hours. Like the rains in Missouri and Kentucky, this was a 1,000-year return period rain. Flash floods swept away cars, damaged roads, and stranded 500 visitors and 500 staff inside the park.

Each of these floods: Yellowstone, St. Louis, Kentucky, and Death Valley, were variations on a theme becoming much more common with climate change. The frequency of compound events is rising precipitously and driving some of the costliest disasters of our time. Many, if not most, of the worst of these were compound weather impacts.

**Even as I get ready to publish, news reports are pouring in on flash flooding across Arizona and Texas. While the images are dramatic and the risks are real, the numbers so far indicate these floods to be in the range of 25 -year to 500-year average events. These numbers may change as rain is still falling. These floods also fall in the category of rain-on-drought, or, in the case of Texas with multiple days of thunderstorms forecast, rain-on-rain-on-drought. **

The U.S. has managed flood risk for well over a century using a relatively concise toolbox of structural solutions, such as levees and dams, which have generally worked well. But as billion dollar disasters add up, it is increasingly clear that safeguards designed for the weather of our past must adapt to the weather of our present, and to our projected future, as we learn to live with much more frequent inundation. We can do this by both strengthening traditional safeguards, while also drastically expanding flood risk management tools to include non-structural solutions, nature-based solutions, and behavioral solutions, each of which are designed to help us safely live within regularly flooding landscapes. From wetland and aquifer restoration to better flood warning systems, to rethinking how and where we build, U.S. agencies such as the U.S Army Corps of Engineers and FEMA are getting creative with climate adaptation while supporting public and private projects that do the same. GEI is doing this too, here to help communities with creative, forward looking approaches to flood resilience and risk management. Please contact me anytime to discuss your ideas for managing flood risk, so we can build stronger communities, together. The best time is now.

And heeding the warning sounded by four 1,000-year floods in the space between a child’s leaving 7th grade and starting 8th? That is the most urgent action of all.

Revisit my previous blog posts on