MICHEL MARTIN, HOST:
Somebody who understands the factors that turn torrents of rain into a catastrophic flood is Greg Waller. He is a hydrologist at the West Gulf River Forecast Center of the National Weather Service, which means he studies the science of what happens to water when it hits the ground. And he's with us now. Greg Waller, thanks so much for joining us.
GREG WALLER: Good morning, Michel.
MARTIN: So why is part of Texas known as flash flood alley?
WALLER: It's one of those cases where every time we bring in a new forecaster, we tell them, this area you need to watch. It is close to a moisture source, plenty of moisture from the Gulf. It has a natural rise up slope in the Balcones Escarpment, which can provide a source of lift. The soils are very rocky, which means any rain that falls, very efficient at generating runoff. And then it's sloped with the hills, by the name Hill Country, so that means any runoff occurs fast. So you have fast-moving, very good runoff into the small creeks and rivers that can go from a couple of inches to a trickle up to 20-plus feet in a matter of hours, depending on the rain event.
MARTIN: Has this been getting worse over time?
WALLER: I'm not going to say it's going to get worse over time for the weather and the hydrology. But what I will say is, you know, it's a very popular area. So you have to add the social vulnerability of more people moving in, creating a larger vulnerable footprint. That is some of the most beautiful and scenic areas of the state of Texas, and it's popular for vacationers. So you mix in a holiday, you mix in a lot of visitors, you mix in a extreme rain event into a very sensitive area, and unfortunately, we see the results.
MARTIN: So we understand that in the area we're talking about where the worst of the flooding took place, within 45 minutes, 10 to 12 inches of rain fell. And the floodwater rose by 26 feet. Can you explain how that's possible?
WALLER: It's volumes of water. Think of the column of atmosphere and all the rain that comes down all at once. We had ties into a couple of tropical systems. There was a Pacific system with leftover high-level moisture, Barry from the low-level system. So we had plenty of moisture in place. We had lift. And once the thunderstorms developed, they tapped into all that available moisture - and very slow-moving. That's the key. These thunderstorms were very slow-moving and produced a lot of rain. Rainfall rates help generate runoff as much as rainfall amount.
MARTIN: So let me emphasize that you are not a spokesperson for the National Weather Service. But I just want to ask, is the forecasting able to predict such a thing, this kind of event?
WALLER: I am a forecaster for the National Weather Service. And I will say, in my career, the science is getting better. It is not perfect. We are doing our best to try to push the limits with ensemble modeling, better high-resolution modeling. But this is Mother Nature. And the difference in 15 to 20 miles in a placement of a thunderstorm, which is a good weather forecast, that could be a bad river forecast, hydrology forecast because that could be a completely different river basin. Our science and skill is getting better, and we need to continue to strive to make it even more so.
MARTIN: So even with that, I think what you're telling me is that these events still are unpredictable to a certain degree.
WALLER: I'm not going to say unpredictable, to the point of we were familiar that there was the possibility of a heavy rain event. It's the precision that we need to work on. We knew that there would be thunderstorms. We knew there'd be locally heavy rain. It's the precision of getting that exact rainfall amount and the exact location to assist. The weather forecast offices, you know, capture that with their flash flood warnings and flood watches. But there's always room for improvement in this science.
MARTIN: And so then the question becomes, are our systems for response able to keep up?
WALLER: That's not in my field, the response. We try to provide the best forecast possible, the best information possible so that those decision-makers can make the best decision. We will be involved in the process. But the response, education, awareness, and you've got to be able to respect what Mother Nature can bring.
MARTIN: That's Greg Waller. He's with the West Gulf River Forecast Center of the National Weather Service. Mr. Waller, thanks so much for sharing this expertise with us.
WALLER: Thank you very much.
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