Air Date: Week of May 6, 2011

In April the Mississippi River experienced its worst flooding since 1927 and the Southeast had a record number of severe tornadoes. Host Bruce Gellerman talks with meteorologist Jeff Masters about how a changing climate factors into the recent rash of extreme weather events.

Transcript

GELLERMAN: Relentless rainfall - a deluge that dumped five months worth of rain in just 14 days - and a massive spring snowmelt have had a devastating effect along the Mississippi and Ohio Rivers, forcing the Army Corps of Engineers to take extreme measures. Demolition experts blew up levees along the Mississippi in a daring effort to protect cities and towns from record-breaking floodwaters. Meanwhile, states in the South and Southeast have been experiencing an unprecedented number of intense tornadoes.

In April, nearly 900 tornadoes tore through the region - four times as many as usual. Well when the weather gets weird and wild, we turn to meteorologist Jeff Masters. He’s co-founder of the website Weather Underground. And Jeff, welcome back!

MASTERS: Thank you Bruce.

GELLERMAN: So where’s all this water coming from that’s flowing into the Ohio and the Mississippi?
MASTERS: Well it’s springtime, and it’s very common to get some intense low-pressure systems moving across the country during the spring. And what powers those low-pressure systems is you have a supply of very warm, moist air from the Gulf of Mexico, which is, right now, near record temperatures at the surface of the ocean there. So all that warm water evaporates off the ocean, gets pulled northwards into the center of the country, where it collides with some very cold air still coming down from Canada.

Army Corp of Engineers Blow up the Birds Point Levee in Missouri

And we had a very, very strong jet stream over the central U.S., and that really made some very intense thunderstorms that helped power these intense rainstorms that we saw. We had some exceptionally heavy rains during April, in particular during that huge outbreak of tornadoes that went across the southern U.S. That was accompanied by some really, really heavy rainfall - as much as 19 inches fell on Arkansas during that week.

GELLERMAN: The upper Midwest also got whacked this winter with snow!

MASTERS: Yeah, we had near-record heavy snowpack over the upper Mississippi watershed. North Dakota, Minnesota, South Dakota - we had a couple feet of snow up there on the ground up until early April, and of course when all that melted it sent this huge floodwater pulse down the Mississippi.

GELLERMAN: So we had a lot of snow and a lot of thunderstorms and rain - why? What’s driving that?
MASTERS: Well if you look at the history of precipitation over the Mississippi Valley region over the past century, it has increased ten to twenty percent. That is something we expect to see in a warmer climate - warmer air holds more water vapor, and climate models predict that by the end of this century, we’ll see another 20 percent increase in the rainfall over the Mississippi Valley. So this is all quite expected and due in part to a warmer atmosphere and climate change.

GELLERMAN: So hold on - warm atmosphere holds more water vapor, right?

MASTERS: Yeah, you can evaporate more moisture into the air when it’s warmer.

GELLERMAN: Uh huh. So then we get these heavier downpours.

MASTERS: Yeah, and it turns out - okay well you’ve got more vapor in the atmosphere, it’s got to come down once it goes up - so how does that rain occur? Well as it turns out, the kind of average, steady rains that we get don’t change much. What does change is those heaviest downpours, those upper one percent and those upper point-one percent downpours - the really, really heavy rains that can most likely cause flooding. Those are the type of downpours that are increasing due to all this atmospheric moisture that’s increased.

GELLERMAN: So if you were to take these climate models and project them out, how bad would it get, say, in 90 years - 2100?

MASTERS: Yeah, well we’re thinking another 20 percent increase in the rainfall over the Mississippi Valley. And you’d think that would maybe cause an extra 20 percent in flooding - no. The thought is it would increase runoff by more like 50 percent. Because what happens when you start getting heavier rains is now you’ve got a saturated soil that can’t absorb rain anymore - so you tend to get more runoff.

GELLERMAN: So this is consistent with the global warming climate models, but couldn’t it be something like La Niña or El Niño?

MASTERS: Sure. There is a La Niña event going on right now in the Pacific, and that does alter the course of the jet stream, affects the storm track for storms going through the country, and in particular, some scientists have thought that maybe La Niña makes it more likely we have strong tornadoes.

GELLERMAN: So there are 800 tornadoes, I understand, that were reported in April. That’s four times the average for the month.

MASTERS: Yeah, it’s pretty ridiculous. We had two outbreaks in April that each generated more than 150 tornadoes. Two of the top four events going back to 1950 occurred last month. So we had a very unusual atmospheric situation in April that’s unprecedented.

GELLERMAN: What’s the role that water vapor and warm air play there?

MASTERS: Well they play a big role, because in order to get the parent thunderstorm that spawns a tornado, you need instability. And in order to have an unstable atmosphere, that means you need to have low-density air near the surface, high density air aloft, so that when an updraft forms and starts rising, it will continue to rise in an unstable atmosphere if it’s less dense than its surroundings. So in order to increase your instability, the best thing you can do is to have very moist and warm air near the surface, because moist air is less dense than dry air, and you need to have very cold air aloft.

GELLERMAN: So if you were a betting man, what role does climate change, do you think, play in the tornado season that we’re experiencing?

MASTERS: I think it’s a flip of a coin. And that’s because with climate change, we expect it to increase instability, which is favorable for generating intense thunderstorms - okay, because now we’ve got hotter waters in the Gulf of Mexico, more unstable air helping to fuel thunderstorms.
But the flipside of that is that in order to get a tornado, you need more than just unstable air - you need something to get it spinning. And the way you get it spinning is if you have a very strong jet stream that changes direction with speed and height. But in the future, we expect the jet stream to weaken with climate change - that’s one of the things that these climate models tell us. So I think these two factors are going to offset each other to some degree, and it’ll be…some areas maybe will get more tornadoes, some areas less - it’s hard to say what’s going to happen.

GELLERMAN: Could we also get tornados where they don’t occur now? Could they move, let’s say, northward or northeast?

MASTERS: Yeah, I do expect the tornado belt to migrate northward. They’ll be more common in Canada. You’ll just get more, you know, hot, moist air, getting up further north during the year - because, you know, all the landmasses are going to warm. Kind of my motto for the coming decades is going to be: expect the unprecedented.

GELLERMAN: Well, Jeff Masters, thanks so much, I really appreciate it.

MASTERS: Alright, my pleasure.

GELLERMAN: Jeff Masters is a meteorologist and co-founder of Weather Underground.

 

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