After a thunderstorm rolled through the Twin Cities that created severe flash floods, MPR’s Tom Crann asks meteorologist Paul Huttner about the moisture conditions and urban landscape environment that made for the ingredients for flash flooding.
On July, 6, 2016, up to 3 inches of rain fell in some parts of the metropolitan area, along with strong winds. About 250,000 Xcel Energy customers lost power, cars were submerged, and structural damage littered streets.
SPEAKER: As you recall last night, severe thunderstorms dumped torrents of rain across the state, including the Twin Cities. And as last night's rain overloaded storm drainage systems across the metro, flash flood warnings were popping up, along with tweeted images of cars afloat on streets that looked more like rivers.
Our chief meteorologist, Paul Huttner, was on the air with me last night into the night with the latest weather warnings, and he's back now this afternoon with more on the flash flooding that we went through. Hi, Paul.
PAUL HUTTNER: Hey, Tom. Good to talk to you.
SPEAKER: So what conditions came together last night to create this flash flooding, especially in the metro?
PAUL HUTTNER: It was the perfect storm in terms of heavy rainfall totals. We had plenty of moisture in the atmosphere. Dew points in the 70s. You felt how sticky it was yesterday afternoon. Anytime we get that kind of tropical moisture around, the atmosphere is ripe for heavy rainfall events.
So once you have the moisture, then you get the storm front moving in. And if the storms move slowly enough or stay across any given area long enough, rainfall rates in that type of situation, Tom, can be 1, 2, even 3 inches an hour.
So if you get under an hour or two of that really heavy tropical torrential type rainfall, you can easily pile up 2 to 3 inches of rain in a hurry. And most of the landscape, most of the urban environment is just not able to handle that kind of rainfall rate, Tom.
SPEAKER: Is it the concrete or is this a matter of the inability of storm drains to keep up? Or is it that too much rain all at once or a combination of all of it?
PAUL HUTTNER: Yeah, it's all of the above, I think. I mean, you will get flash flooding in rural areas, to be sure, and that's really dependent on how saturated the soils are. If you've got some head room, as we call it, and the soils are kind of dry, they can absorb maybe that first half inch or inch of rainfall before you really start to get into a flash flooding situation.
But that's not true in an urban environment. So as that rain comes down on asphalt, parking lot, concrete, our roads and funnels its way into the storm drain systems, first of all, it's all runoff. Nothing is absorbed when it hits these paved urban areas.
And then second of all, the storm sewer system is designed to handle a certain amount of rainfall. And anything above that isn't going to be able to drain off. So what happens is you get these urban flash flood situations that happen pretty quickly.
And we saw that last night where some of the bridge underpasses fill up. If you get a clogged drain in there, well, then you're pretty much toast and the water begins to rise very quickly. And that's why we tell folks, especially at night in flash flood situations, really just back off from those areas that look like they have too much water. Thankfully, everybody got out of their cars safe last night, but it was a dicey situation for a few hours.
SPEAKER: And then from a meteorological standpoint, the differences between flash flooding and longer term flooding, we've seen because both are pretty dramatic.
PAUL HUTTNER: Yeah, flash flooding is just that. It happens in a flash. And it can disappear in a flash. So it's an overabundance of heavy rainfall causing rapid rises in water, either creeks, small streams, especially the urban environment as we talked about. So that's flash flooding.
Flooding or flood warnings are longer term warnings for river systems. So in the spring, we get a lot of flood warnings from snowmelt and spring rainfall on bigger rivers. Flash flooding really is a shorter duration event, but it can be-- every bit as dangerous as a flood.
SPEAKER: Now, because of climate change, are we seeing more flash flooding incidents? And I suppose the bigger question is, are we seeing more heavy rainfall causing flash flood incidents?
PAUL HUTTNER: We are. And that's one of the well documented areas of climate change in the United States. And Midwest and the Northeast, for some reason, seems to be having a much higher increase of these heavy rainfall events, Tom.
When you look at the 3 inch plus rainfall events, the occurrence of those heaviest rainfall events, we're seeing about a 40, 50% increase in those across the Midwest, more like 70% in the Northeast.
And that is part of climate. You create a warmer climate, you put more water vapor into the atmosphere, and some indication we may get slower moving storms. So you combine all that, you're generating these tremendously heavy rainfall totals, Tom. We're seeing more of that in Minnesota. We've had four 1,000-year floods in Minnesota in just the last 9 or 10 years.
SPEAKER: All right. Paul Huttner, thanks so much for the insight.
PAUL HUTTNER: Thanks, Tom.
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