May 15, 2019

In the spring of 2019, floodwaters overwhelmed levees across the Midwest, drenching towns and causing billions of dollars in infrastructure and crop damage. Record downpours in Tennessee provoked a state of emergency and led to mudslides. In California, heavy precipitation damaged thousands of buildings. Extreme rain events have devastated communities around the nation — and the frequency and severity of such events are expected to worsen in a warming world.

Human-caused climate change intensifies the heaviest downpours. More than 70% of the planet’s surface is water, and as the world warms, more water evaporates from oceans, lakes, and soils. Every 1°F rise also allows the atmosphere to hold 4% more water vapor. So when weather patterns lead to heavy rain, there is even more moisture available for stronger downpours, increasing the risk and severity of flooding.

Floods often happen on the rainiest day of the year — the single calendar day with the most precipitation. Climate Central tracked how these wet days are trending over time, analyzing data for 244 cities around the country. In most areas, rainfall extremes have intensified as the climate has warmed.

Since 1950, the wettest day of the year has gotten wetter in 79% of the cities analyzed. For 32 cities, that day’s extreme precipitation has increased by an inch or more, based on an analysis of the linear trend (detailed methodology below). Houston, Texas tops the list with an additional 2.78 inches of rain, followed by Greenville, North Carolina (2.44 inches), Hattiesburg, Mississippi (1.94 inches), and Baton Rouge, Louisiana (1.91 inches). All four are located in the Southeast, the wettest part of the nation, where cities average 50-70 inches of rainfall annually.

In addition to getting stronger, extreme downpours are happening more frequently than in the past. In 80% of the cities analyzed, the top 1% of rain events have been recorded disproportionately recently. This recent swell is strongest in Clarksburg, West Virginia; Louisville, Kentucky; and Marquette, Michigan.

All-time records for rainfall have also surged recently. Nearly all the cities analyzed have data since 1950; 78% have data going back at least a century. If there were no trend in extreme rainfall, the all-time records would be evenly distributed across that century or more. However, 35% of cities have set their rainfall records since 1990. The most recent records in our analysis come from Hurricane Harvey, which dumped one-day totals of 16 inches in Houston and an astounding 26 inches in Beaumont, Texas.

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