Morning Edition’s Cathy Wurzer interviews meteorologist Bill Togstad about the science behind “The Storm of the Century” in Twin Cities on July 23, 1987. Togstad details how an unusual atmospheric condition brought massive amounts of moisture that lingered over the metro. He states there is nothing before or after that has matched this weather event for the Twin Cities.
Considered the Twin Cities “Superstorm,” the July 23-24, 1987, event dropped 9.15 inches at the Minneapolis–Saint Paul International Airport, and over 10 inches in suburbs west and southwest of Minneapolis. This rainfall cataclysm produced the worst flash-flooding on record in the Twin Cities. An F3 tornado touched down in Maple Grove.
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That was Terry Connors. We also heard from Sean Baxter and [? Jo ?] Ann Crago. Thanks to all of you who called in with your stories. Well, Bill Togstad is a forecaster at the National Weather Service office in Chanhassen. He co-authored a paper on the science of what happened in Minnesota on July 23, 1987.
And Bill joins us this morning. Good morning.
BILL TOGSTAD: Good morning.
SPEAKER: What happened, meteorologically speaking, that day?
BILL TOGSTAD: Basically, you had a combination of a number of things that are very, very unusual, and the end result was that it allowed thunderstorms to form in a corridor and repeatedly move over that same corridor and throw out pulsating bouts of heavy rain over a six-hour period.
SPEAKER: So almost like railroad tracks in a sense, when the storms kind of kept coming down the track?
BILL TOGSTAD: Yes. What normally happens is the atmosphere is set up. So on a thunderstorm day, there's a lot of moisture and heat in the low levels, and there's a reasonable amount of dry air in the middle levels of the atmosphere. What that does is, as the thunderstorms form, some of that dry air mixes in.
And as rain mixes with the dry air, it makes for a cold rain, cold layer near the surface that pushes out and makes new thunderstorms form away from where the original storms formed. In this instance, the atmosphere was structured in a way that we had monsoon moisture coming all the way up from really across Arizona, from the Gulf of Mexico, and then looping back in from the Pacific.
So the atmosphere had what we call a large amount of precipitable water, 1 and 1/2 to 2 inches. And as a result, you don't get that rain-cooled layer in the low levels when that happens, and it allows storms to repeatedly form over the same area and not move away from that point. It's fairly unusual.
SPEAKER: And how many inches of rain was dumped ultimately in the Twin Cities because of those storms?
BILL TOGSTAD: Well, what's interesting about it is it was a double-barreled event. And when I say "double-barreled," I meant there was a huge amount of rain between July 20 and 21. There was really a 6- to 9-inch core that went across southwest Hennepin County into northern Dakota County.
Then on top of that, a few days later, the 23rd through the 24th, there was a 10-inch event, and that pretty much started out by Deephaven and went down through Mendota Heights. And the net result of it was you had in some places 15, almost 16 inches of rain down around Bloomington.
SPEAKER: Yeah, I remember that night, too. And, actually, I was on duty that night at another station, and we were chasing tornadoes that night.
BILL TOGSTAD: There was an F3 tornado. I hunted that up in the records. And apparently it was ranked an F3, and it caused $5 million damage. It was up by Maple Plain and had a 5-mile path length.
And after that, there was $21 million worth of damage from the flooding event. So a total in excess of $25 million that night.
SPEAKER: How unusual was this event when you look at the record books?
BILL TOGSTAD: It is the event for the Twin Cities. Nothing before and nothing after has matched it.
SPEAKER: Well, Bill, I appreciate your time this morning. I suppose there are still some folks in the office who remember 1987. You might be talking about that today. I know you have to go back to work. Thank you so much.
BILL TOGSTAD: You're welcome.