Myth-busting urban tornadoes and severe weather

NINA MOINI: This week brought another significant tornado outbreak to parts of Minnesota. The Twin Cities was spared this time. But are the Twin Cities and other downtowns safer because of urban heat islands? Maybe not so much. Kenny Blumenfeld studies urban tornadoes. He's a senior climatologist with the Minnesota State Climate Office. He did some myth busting in a conversation with MPR News chief meteorologist Paul Huttner.

PAUL HUTTNER: Let's start with this week. Do you have any thoughts on the latest outbreak from Monday?

KENNY BLUMENFIELD: Oh, well, I mean, it's a good reminder, if nothing else, that we get tornadoes all over Minnesota. And we get some inquiries to our office about, oh, it seems strange to have tornadoes in the Brainerd Lakes area. And in fact, it's not unusual at all. That area has been hit hard numerous times, including some colossal tornado outbreaks.

And really, we've seen tornadoes in every part of the state, including right up to the North Shore, even across the Boundary Waters, and pretty much everywhere you could imagine-- river valleys, up on tops of bluffs-- on top of bluffs, in cities, in rural areas, near Lake Superior, crossing lakes. We get tornadoes in Minnesota.

PAUL HUTTNER: And let's go with that, because we have had tornadoes in Minneapolis. I mean, 1981, the Edina to Minneapolis to Roseville tornado. I was working the 2011 North Minneapolis tornado. Joplin, Missouri, happened that same day. Just devastated that city. What other US cities have been hit by tornadoes?

KENNY BLUMENFIELD: Oh, yeah, I mean, almost too many to name. The Storm Prediction Center maintains a list of tornadoes that have been in or very near central business districts of major cities. And that list is dozens deep. But some of the more notable ones would include Raleigh, North Carolina, Nashville twice, including one strike on downtown. And both of these were devastating tornadoes that injured or killed people. Atlanta, Georgia. Fort Worth, Texas. Miami. Even Salt Lake City, which most of us don't think of as a tornado haven.

Closer to home, Des Moines, Iowa, right in-- they have a small downtown, like St. Paul or Rochester. And Des Moines has been hit right in downtown. Of course, Minneapolis.

And it wasn't just the 1981 tornado that you mentioned. That was a formative tornado for me. I was a young kid, and I was about six blocks away from the path. We also had, in 1984, a tornado that started in Minneapolis went into St. Anthony. That was a killer tornado. We, of course, had the Joplin day tornado. And we had a tornado that went right into downtown Minneapolis by-- after going down Portland and even 35W, heading northbound, into downtown Minneapolis, back in 2009. If you look at all the tornadoes that we've had inside Minneapolis's city limits, you're up around 10 in the last century. Yeah.

PAUL HUTTNER: Wow. Let's talk about the geography a little bit, because cities are relatively small areas in a bigger landscape. How much is the geography of the relatively small cities' role in seeing more tornadoes outside of cities? I mean, is it like the dartboard and the bull's eye, just the odds playing out that cities don't seem to get hit as often?

KENNY BLUMENFIELD: Yeah, that's exactly right. It really comes down to a matter of perception. And in fact, you could take this beyond cities. Pretty much, unless a person has been hit by a tornado or had a significant encounter with a tornado, the tendency is for them to believe that tornadoes are more common elsewhere. And in a way, they're right, because elsewhere is huge compared to right here. So the odds are much better elsewhere.

And if you think about it, even astute-- some astute weather observers and even some meteorologists are prone to thinking, after a long career, a long life observing these things, and the tornadoes do always hit somewhere else, if you're lucky. And it can lead to a perception that maybe where I live gets-- is avoided somehow. And that is most common where you have high densities of population in urban areas. So there tends to be this belief that the cities are actually going to somehow destroy or deter a tornado.

But we've got so much not just scientific, wonky evidence but also documentary evidence-- I mean, videos and photos of tornadoes moving right through the hearts of major cities, not just in the US but in other parts of the world too. We should definitely disabuse ourselves of this myth.

PAUL HUTTNER: Let's talk about the physics of cities and tornadoes, because we know that urban heat island is a real thing. Cities can be easily 10 degrees warmer than surrounding areas. But what do we about how urban heat island can affect storm development, circulation, and tornado formation?

KENNY BLUMENFIELD: Yeah, that's a great question. So the urban heat island does change the behavior of many thunderstorms and many precipitating weather systems, at least locally, by essentially changing the way that those storms would organize. So when you do build a good heat island, on those days, you tend to have thunderstorms that would develop over or maybe a bit downwind of the city. And you end up with an effect on rainfall patterns and even lightning patterns.

But one thing to keep in mind-- and this is where I think some of the confusion comes in, is that the vast majority of thunderstorms are just normal, common, backyard, garden variety storms. And those conditions that we're talking about where you have a heat island, those aren't the conditions that are likeliest to produce the kinds of thunderstorms that would lead to tornadoes.

And once you get into those potentially tornado-producing storms, the really big, really severe supercell thunderstorms, often, the regional winds are behaving differently than they do on strong heat island days. And this is where there's been some difference in the research that once you have a lot of motion in the atmosphere, storms are going to behave differently as they approach urban areas.

Basically, the proper inference to make is that if you are in an urban area that happens to be in a pretty healthy risk for tornadoes, say, an enhanced risk for severe weather or moderate risk, or you have a tornado watch in effect, that means most likely the urban heat island and those local effects probably would not save you if the storm were in a position to enter that urban area.

What I take from all of the research and all of the ways that we've looked at data over the years, and the other researchers who've studied this, is that when you're talking about supercell tornadoes especially, no landscape is immune. And that's the take-home message.

PAUL HUTTNER: Here's another one. I hear this a lot, and I'll bet you do too. People say the storms always split and miss the Twin Cities. And some of that impression might be the radar site and the way it works. When storms pass over the radar beam, it looks like they split. But any evidence to the idea that storms miss the Twin Cities?

KENNY BLUMENFIELD: There actually is some. And this is where scientific is full of nuance. And it gets difficult. There's something called a building barrier effect. And that's been studied by urban and microscale and mesoscale meteorologists, people who are looking at the interactions between moving boundaries and other landscapes.

And what they found is that when a gust front or a cold front or a line of thunderstorms comes into a really rough area, like an area with a lot of buildings and a lot build-up, it can break apart, and so especially when there's not a lot of support for those storms. So there is a real effect of some storms splitting apart or-- they call it bifurcating-- breaking around an urban area.

But again, when there's enough energy in the atmosphere, the storms will overwhelm that building barrier effect. And that's why we've had some of our high-end severe weather events that have produced widespread wind damage or widespread even tornado activity. Those have gone right through the heart of the Twin Cities area. So there is some truth to it, that urban areas could break up storms, the same way that there's truth to the urban heat island occasionally causing storms to form and move elsewhere. It's just that in the situation where you have really severe weather or the ingredients would support really severe weather, then you're probably not going to be saved by the urban heat island or the building barrier effect.

PAUL HUTTNER: Tell us a little bit about your research into urban tornadoes. What are the big takeaways people should think about when it comes to cities and tornadoes?

KENNY BLUMENFIELD: I started with the Twin Cities. And the clear takeaway there was-- and just to be clear, I actually thought that I was going to find what everyone else believes, which is that I thought I was going to find, yeah, there are more tornadoes on a per area basis outside of the heart of the Twin Cities than there are inside the heart of the Twin Cities. Actually, that was incorrect.

So locally, the takeaway is the probability of tornadoes inside the 494/694 loop or even inside Minneapolis and St. Paul, it's just as high as it is in pretty much any other part of the area. And then when you expand that to other cities around the country, some of the preliminary research seemed to indicate that the same was true with other major cities, at least in tornado-prone regions, like the Midwest and the South and the Great Plains.

All the big cities in those areas have relatively high amounts of tornado activity in the city limits, just like St. Louis got hit about a month ago. A pretty major tornado. And it's a lesson for all of us that tornadoes can and do hit cities.

NINA MOINI: That was MPR News Chief Meteorologist Paul Huttner talking to Kenny Blumenfeld, a senior climatologist with the Minnesota State Climate Office. You can hear more conversations like this every Thursday, today, on Climate Cast during All Things Considered, beginning at 3:00.