Climate predictions say there will be an increase in heavy rain events. In Missouri the average yearly precipitation may or may not change significantly (see series of posts beginning here). But even if the yearly average doesn’t change, more of it will come during heavy precipitation events, between which there will be less precipitation.

Figure 1. Source: GlobalChange.gov.

It is already happening. Figure 1 shows the change in the frequency of heavy rain events in the contiguous 48 states. As you can see, the trend shows a clear increase over time. They occur at least 50% more frequently than they did in the 1920s and 1930s.

Here’s how they measured “heavy precipitation event” for this chart: for the years 1901-1960 they computed the two-day precipitation total that is only exceeded once every 5 years. They then looked at each decade and counted how many times there had been a two-day precipitation total that exceeded that amount. They presented the results as a percentage deviation from the expected number. During the 1910s, 1920s, and 1930s, there were fewer than expected heavy rain events. During the 1990s and 2000s, there were 30-40% more than expected.

Figure 2. Source: GlobalChange.gov.

Figure 2 shows something a little different. Here, they compared the amount of precipitation falling in the heaviest 1% of precipitation events from 1958-2012. The figure shows that it increased 71% in the Northeast, 37% in the Midwest, and 27% in the Southeast. The only region where it decreased was Hawaii.

These charts show that not only are heavy precipitation events occurring more frequently, but more precipitation is falling during them.

“Heavy Precipitation Event” is a term that doesn’t convey much. Let’s look at two examples. One occurred in Missouri almost a year ago, in late December, 2016. Storms occurred over all of Missouri, with the heaviest precipitation roughly paralleling Interstate 44’s route through the state. Over 3 days these storms dumped up as much as 10 inches of rain in a band running from Branson to St. Louis. The result was record flooding along the Meramec River (Figures 3 and 4).

Figure 3. Source: U.S. Weather Service.

Figure 4. Flooding in Valley Park, December, 2016. Source: Unknown photographer.

In August, 2016, similar flooding occurred in Louisiana, but this was even worse (See Figure 5).

Figure 5. Source: Department of Agriculture.

Up to 31 inches of rain fell over a period of 4 days, and record flooding followed. It was the worst U.S. disaster since Hurricane Sandy in 2012, with 146,000 homes damaged. The dollar amount of damage represented is still being figured out.

Climate change modelers, of course, predicted an increase in heavy precipitation events. It’s already happening, and these floods shows just what it means.

Sources:

“2016 Louisiana Floods.” Wikipedia. Viewed online 11/13/2016 at https://en.wikipedia.org/wiki/2016Louisiana_floods.

National Weather Service. December Historic Rainfall and Flooding Event Review. St. Louis Forecast Office. Downloaded 2016-11-13 from http://www.weather.gov/lsx/12_26_2015.

Unknown photographer. Meramec River at Valley Park. Advanced Hydrologic Prediction Service. Downloaded 11/13/2016 from http://water.weather.gov/ahps2/hydrograph.php?gage=vllm7$wfo=lsx.

GlobalChange.gov. Broadcast_Heavy_Precipitation-map_V2. National Climate Assessment 2014. Downloads, Graphics (Broadcast). Downloaded 11/13/2016 from http://nca2014.globalchange.gov/downloads.

GlobalChange.gov. Broadcase_Trends-in-heavy-precip_V2. National Climate Assessment 2014. Downloads, Graphics (Broadcast). Downloaded 11/13/2016 from http://nca2014.globalchange.gov/downloads.