Understanding Why Flash Flooding Events Have Been So Rampant Lately

It has been a summer of deadly flooding across the U.S. What is causing the constant waves of heavy rain and the resulting flash flooding? Here is a look at what scientists are saying.

Summer is Peak Time for Flash Flooding Concerns

Every week seems to bring a new threat of flooding to the U.S. Few states have been spared from the persistent rain and storms that unleash these flooding events.

The most significant flooding catastrophe thus far unfolded in the early morning hours of the Fourth of July when a surge of heavy rain and slow-moving thunderstorms sent the Guadalupe River overflowing its banks in central Texas. At least 130 fatalities have been blamed on this natural disaster.

Just a few days later, Tropical Storm Chantal sent almost a foot of rain across parts of North Carolina. The Eno River hit a crest of over 25 feet, easily beating the previous record set in 1996 by Hurricane Fran. At least six people died during this flooding event.

Chicago experienced what historians are calling a 1-in-1,000-year rainfall later in the week, drenching the city and flooding several streets. That week also saw a deadly flooding event in the mountain town of Ruidoso, New Mexico, killing three people. More recently, Kansas City experienced a flash flooding event that turned roadways into rivers.

It was not just the intensity of these flooding events but the fact that they happened in quick succession that raised alarm bells. While part of the blame lies simply with the fact that summer is the flash flood season, experts are also putting the onus on climate change for the increasing nature of these disasters.

Flash flooding is naturally more common during the summer months. The increasing temperatures this time of the year fuel the development of more intense thunderstorms. This is because warmer air is capable of holding increasing amounts of moisture, amplifying rainfall rates when compared to the cooler months of the year.

For instance, the Texas flooding came at the hands of lingering moisture left from the former Tropical Storm Barry. Likewise, North Carolina's flooding was also supported by a tropical weather event.

Forecasters also note that thunderstorms that ignite during the summer tend to move at a slower speed. The weakening of upper-level winds in the atmosphere as the summer approaches creates a smaller temperature difference between the Earth's opposing poles and the equator. This sends the jet stream farther to the north.

It is easy to understand that storms that slow down or stall generate heavier amounts of rain over one particular area. The greater rainfall rates are often too much for the ground to absorb or for drainage systems to work efficiently. The end result is a higher possibility of a flash flood.

The tragedy in Texas demonstrates what happens when a stationary storm trains over one area for a long period of time. Over 7 inches of rain fell across the Guadalupe River region in just a few hours, triggering the deadly flooding.

Similarly, a stalled storm was also responsible for the 5 inches of rain that fell in just 90 minutes in Chicago. The city's drainage systems could not keep up with rainfall of this magnitude. Chicago is an example of how urban areas are often the most vulnerable to flooding, as water quickly runs off pavement and overwhelms storm drains. This problem is even worse if the drains become clogged with debris created by gusty storms.

Recent wildfire burn scar areas are also more susceptible to flooding. This was the case with the recent flooding event in Ruidoso. Last summer's South Fork and Salt fires had decimated the terrain in this part of southern New Mexico. Scorched vegetation has a more difficult time absorbing rain, producing a water-repellent layer below the soil.

As a result, rain that falls over burn scars is more likely to see a higher rate of runoff. This threat is elevated across steep terrain. The area surrounding Ruidoso has seen at least 12 flash flood emergencies since last year's wildfires.

Drought conditions behave similarly. Soil that is too dry for too long is more likely to be increasingly compacted, lowering its capacity to efficiently absorb water. Climatologists note that the preexisting drought conditions that have been present over Texas and New Mexico led to an elevated risk of flooding when the heavy bands of rain hit the region.

Impacts of Climate Change on Flooding Risks

Mother Nature is not the only one to blame for the increase in flash flooding events across the U.S. Climate change is also creating a bigger threat of flash flooding.

Scientists have been warning for years that the atmosphere can contain about 7% more water vapor for every 2-degree increase in temperature. As such, it makes sense that a warming planet will also increase the chances of greater intrusions of moisture-rich air that can extend well beyond the tropics.

The increase in global temperatures is also raising the threat of wildfires and drought conditions. This means that there is a greater prevalence of widespread burn scar areas more prone to flash flooding concerns.

A recent study out of the research group Climate Central demonstrated that rainfall rates have increased in nearly 9 out of 10 American cities dating back to 1970. Because so many of the drainage systems of these metro areas were built before this time, the nation's infrastructure is not equipped to handle the uptick in rainfall rates.

The bottom line is that while flash flooding is inherently going to increase in the summer months, the threat is growing more concerning due to the impacts of global warming.

Update on Texas Flooding Disaster

The most devastating flooding to hit the U.S. this year happened on the morning of July Fourth. Over 130 have been confirmed dead in this incident, including several children who were swept away at summer camp.

Local officials released a bit of good news on Saturday when they drastically revised the number of people missing downward. The City of Kerrville in Kerr County brought the number of known missing people down from 97 to three. The news release noted that the rest of the individuals previously believed to be missing have been confirmed as safe.

Recovery efforts for the three people still reported as missing will continue along the Guadalupe River watershed.

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