Extreme rain events increasing across United States

While much of Ohio through New England has had a dry summer with drought conditions, it may surprise you to learn that extreme rainfall events have been increasing across the United States and around the world.

You likely have seen the images of severe flooding from Louisiana over the last week. This is where over three times as much water fell on that state than what fell in Hurricane Katrina. Parts of Louisiana near Baton Rouge received more rainfall in 72 hours than Bakersfield, California has received in 5½ years. Even more concerning is while parts of the United States cannot seem to buy a drop of rain, other places like Baton Rouge are seeing unusual, extreme rain events.

According to the National Weather Service, the Louisiana flooding earlier this month was considered a 1-in-500-year rainfall event. However, that doesn’t necessarily mean it will be another 500 years before it happens again. Rather, it means that in any given year, there is a 0.2 percent chance of such a rainfall event occurring in that location. But that could be changing, and more rapidly than we may know. There have been nine 1-in-1,000-year rainfall events in the U.S. since 2010 alone, including a deadly flooding event in West Virginia in June and a devastating flash flood in Ellicott City, Maryland in late July, according to Steve Bowen, a meteorologist for the insurance company Aon Benfield.

A recent study conducted by the U.S. Global Change Research Project shows an increasing percentage of the contiguous 48 states receiving more than a normal portion of total annual precipitation coming from extreme single-day precipitation events. The question is, why? The National Academies of Sciences believes the most likely cause is from increasingly warm temperatures. The fact is warmer air can hold more moisture or water vapor. The atmosphere can hold 7 percent more moisture for every degree Celsius of temperature increase. This water vapor is readily available for storm systems to “use” in rainfall production. Basically, the more moisture in the air, the more rain that is likely to fall.

The warmer air can also support higher efficiency in rain producing clouds. The warmer air changes the cloudy physics involved that generate rain. The same, soupy air mass that was in place that generated extreme rainfall in Louisiana actually made conditions quite uncomfortable across the

south, Ohio Valley and New England as the humidity pushed heat index values over 100 degrees at times for multiple days in a row. A near-record warm Gulf of Mexico helped add water vapor to the air, with bathtub-like water temperatures near 90 degrees Fahrenheit in parts of the Gulf.

While we haven’t broken very many record high temperatures in Ohio this summer, it may surprise you to learn the average daily minimum for the lower 48 states was the warmest on record for June and July combined. Add that to the fact that according to NASA, in 136 years of modern record-keeping, this July was the warmest July according to a monthly analysis of global temperatures. The record-warm July continued a streak of 10 consecutive months (since October that have set new monthly high-temperature records.

Compared to previous years,the warmer global temperatures last month were most pronounced in the northern hemisphere, particularly near the Arctic. While a direct correlation of warming temperatures and the recent high rain events is difficult to pinpoint, it is likely a leading cause. Using that theory, it is likely that more extreme events will happen more regularly. And this would not just be extreme rain events, but could mean extreme snow events as well.

The good news is, the heat and humidity have subsided across the Ohio Valley, at least for now. And while warmer temperatures will return in the days ahead, the extreme humidity will hopefully stay away. But with the peak of hurricane season still to come next month, don’t be surprised to see reports of more extreme events.