Flash floods can happen anywhere, but factors such as heavy precipitation, geography and soil conditions can put some areas at greater risk. Russ Schumacher at Colorado State University is studying these factors to make more accurate forecasts. "When Nature Strikes" is produced by NBC Learn in partnership with the National Science Foundation and The Weather Channel. For a classroom activity related to this video, please click the Links section below.
When Nature Strikes -- Flash Floods
MARSHALL SHEPHERD reporting:
Flash floods are one of the most common and deadly severe weather events in the United States. Part of the danger arises when fast-moving flood waters from rainfall suddenly appear sometimes miles away from the downpour. Russ Schumacher is an atmospheric scientist at Colorado State University. He's part of a research effort that's funded by the National Science Foundation to study large thunderstorm systems that can trigger severe flash floods.
During May, 2015, atmospheric conditions over Texas and Oklahoma were just right for stormy weather-- and historic amounts of rain fell. Flash flood warnings went out repeatedly to communities in the flood zones. But even with advanced warnings, more than 30 people died in the floods, more than a thousand homes washed away, and first responders performed more than 500 dangerous water rescues. Flooding is usually caused by heavy rain, but it's hard to predict when, where, and how much rain is coming.
RUSS SCHUMACHER (Colorado State University): Flash floods are a really difficult forecasting problem. The first part of it is how much water is going to hit the ground and where. And then the next step is what's going to happen to it after it hits the ground, where are the areas that are vulnerable to flooding? Is the ground already really wet, such that it just can't hold any more water?
SHEPHERD: Russ Schumacher, along with scientists and students from more than a dozen research facilities and universities, is investigating large thunderstorm systems that often strike at night in the central United States and can cause severe flooding. To improve forecasting capabilities, it's critical that scientists gain a greater understanding of storm characteristics, such as formation, speed, and rainfall amounts.
SCHUMACHER: These are the kinds of advances we're trying to make to better understand them and then hopefully lead to better forecasts in the future.
SHEPHERD: Using aircraft, radar, and other instruments, the team is gathering valuable information on these hazardous nighttime storms called Mesoscale Convective Systems, or MCSs. In a single thunderstorm, cool downdrafts due to heavy rainfall will sometimes spread out along the ground, bumping against the surrounding warmer moist air. The cool air pushes the warmer air upward, initiating new thunderstorms. This process can happen again and again, creating storm clusters.
SCHUMACHER: That's what we call a Mesoscale Convective System. And when that process happens, it can lead to a lot of very heavy rainfall, of course, but other hazards, such as severe winds and severe hail
SHEPHERD: Sometimes these large storm systems move slowly, even stall, and deluge an area with rain for hours and hours. Other systems move rapidly, but release extreme amounts of rain on a region.
SCHUMACHER: That huge amount of rain over a very small area is when you start getting that flash flood threat. And then it can be a very dangerous situation.
SHEPHERD: Heavy precipitation isn't the only factor in flash flooding. Geography and soil conditions also play a role in when and where flash flooding could occur. Although flash flooding can happen anywhere, some geographical areas are more prone to flooding, such as Texas and Oklahoma. Narrow water channels, multiple streams, and the slope of the land or riverbanks can increase the flood risk. When the ground is saturated, and can't absorb more water, the rain runs off into creeks and streams. River levels rise as the water continues downstream, swelling over the banks and flooding low-lying areas. Water can build up to flood levels and become life-threatening in a very short time. Loss of property and life can be prevented with flood management systems, such as levees, diversion spillways and wetland restoration. Knowing where and when to use them could allow flood waters to be stored or prevented from rushing downstream and flooding low areas. Early warning systems can also help protect against the damage and danger of flash flooding.
SCHUMACHER: Those are the kinds of advances we hope to make. As our understanding and as technology improve, we're going to get to that point where those forecasts can really be used reliably by people.
SHEPHERD: Schumacher and his colleagues are working to increase scientific understanding of severe weather-- valuable research efforts that will lead to more accurate flash flood forecasting that could save lives.
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