It鈥檚 not just Texas and North Carolina. Intense rain is falling more frequently in many areas of the U.S. 鈥 though where it occurs and whether it causes catastrophic flooding is largely a matter of chance, according to experts.
More than 100 people died in over the weekend after 12 inches (30 centimeters) of rain fell in just hours. The by warm, moist air left over from Tropical Storm Barry and Hurricane Flossie that created conditions for repeated thunderstorms in the same location, said Texas Climatologist John Nielsen-Gammon.
Last year, dumped more than 30 inches (76 centimeters) of rain on western North Carolina, triggering catastrophic flooding that washed away roads and homes, killing more than 100 people in that state alone. This week, flooding from the remnants of prompted dozens of water rescues in other parts of North Carolina. And this spring, caused severe and deadly flooding.
Although it can be difficult to attribute a single weather event to 鈥 and hilly or mountainous terrain worsen flooding 鈥 experts say due to the burning of fossil fuels make catastrophic storms more likely.
That鈥檚 because the atmosphere can hold 7% more water for every degree Celsius (1.8 degrees Fahrenheit), creating a giant sponge of sorts that sucks up moisture from bodies of water and vegetation. The moisture later falls back to earth in increasingly intense, unpredictable and destructive downpours.
鈥淚t鈥檚 just loading the dice toward heavy rainfall when the situation is right,鈥 said Kenneth Kunkel, a climate scientist at North Carolina State University.
Intensifying rain storms
Going back through U.S. weather station records dating to 1955, Kunkel found that rain over the past 20 years has become more intense in the eastern two-thirds of the country, including the southern Great Plains, where Texas is located. Intensities have remained the same or declined in the West and southwest.
At the 700 stations that began collecting data in the late 19th and early 20th centuries, the highest proportion of two-day rainfall records have been set in just the past 10 years, Kunkel said, though that doesn鈥檛 fully reflect most Western stations, which were established later.
Nielsen-Gammon said the overall intensity of extreme rainfall in Texas has increased by 15% over the past 40-50 years.
Still, it鈥檚 almost impossible to predict where the most catastrophic rain will fall in any given year, Kunkel said.
鈥淭his month was the Texas Hill Country鈥檚 turn to get hit. Last fall ... in western North Carolina, it was our turn,鈥 Kunkel said, adding that just because an area was spared over the past 20 or 30 years, it 鈥渄oesn鈥檛 mean that they aren鈥檛 vulnerable. ... They got lucky.鈥
A 鈥榩erfect storm鈥 in Texas
The worst flooding and greatest loss of life in Texas occurred in Kerr County, in an area known as 鈥渇lash flood alley鈥 because of its steep terrain that funnels water to the Guadalupe River, a popular recreational area.
Though the county did not get the most rain from the storms, the 鈥渄istribution of rainfall was one of the worst possible patterns鈥 because the most intense downpours were over the headwaters of the south fork of the Guadalupe River, causing water to rush into areas where hundreds of people, including children, , said Nielsen-Gammon.
If the epicenter had been 10 miles (16 kilometers) north or south, the rain would have been divided among different river basins, he said. If it had been farther downstream, larger floodplains would have absorbed and slowed much of the water.
Years of drought also likely exacerbated the flooding.
Kerr County, for example, had been in extreme or exceptional drought for more than three years, aside from one four-week period last fall. That likely left the soil compacted, which caused water to run off instead of soaking into the ground, said Brad Rippey, a U.S. Department of Agriculture meteorologist.
Then, air from the warmer-than-normal Gulf of Mexico 鈥 a reflection of global warming 鈥 blew into the state with a higher water content than it would have had decades ago.
It all added up to 鈥渏ust a perfect storm of events鈥 that caused a catastrophe, said Rippey. 鈥淭here are things that had to come together to make this happen.鈥
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