MPR’s Mark Heistad interviews meteorologist Bruce Watson about what created the conditions for the massive amount of rain that fell on the Twin Cities on July 23, 1987. Watson describes the intensity of rain in six-hour period as something that happens about every 5-10,000 years.
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. A tornado touched down in Maple Grove.
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MARK HEISTAD: Well, let's see if we can take a little bit broader look now at the storm that went through the Twin Cities area. And here to help us with that is consulting meteorologist Bruce Watson, who is with us by phone this morning.
Good morning, Bruce.
BRUCE WATSON: Good morning, Mark.
MARK HEISTAD: You weathered the storm well enough last night?
BRUCE WATSON: I guess so.
MARK HEISTAD: Well, it's good to have you with us this morning. I have to wonder about this storm. We were talking with Jim Campbell from the National Weather Service here in the last hour. And he was suggesting that this is one of those once in a century storms. It was so unusual. It was so big and so powerful.
And apparently, this storm system parked itself over the Twin Cities, which is why we got such extensive rainfall in the Twin Cities area. And I have to wonder, first off, what causes a storm like this one to park itself in one spot or another, let alone over us in the Twin Cities?
BRUCE WATSON: Yeah. Well, what happened is that the storm actually formed right in the metropolitan area. Your first large rain cells were up in Wright County. And the cells actually were moving from west to east. And the line that they were associated with was [? line ?] [INAUDIBLE].
So what happens is that the cells are moving right along the line. So what happened? You got one thunderstorm after another following the same track. And it just kept going for six hours.
MARK HEISTAD: Yeah. One suggestion I've heard this morning, somebody on the staff, I think, brought it up. Wondered about it was so hot yesterday. And here in the Twin Cities, of course, there's a heat mass associated with just the number of buildings and the number of people that are here.
Does that concentration of heat contribute to the situation that we had last night?
BRUCE WATSON: Possibly. Certainly, any extra energy that you can put into the air gives you more uplift and possibly helps give more rain.
MARK HEISTAD: Yeah. Now, we've seen rain reports last night, 12 inches in the Lake Minnetonka area, 11 inches in Edina, nearly eight inches in the Saint Paul midway area. Those really are extraordinary rainfall numbers for, what was it, about six hours of rain.
BRUCE WATSON: Yeah. That's correct. Actually, when you're talking about anything greater than eight inches of rain in six hours, you're talking about something you can expect only once about every 5,000 to 10,000 years. So it was a very rare event.
MARK HEISTAD: 5,000 to 10,000 years?
BRUCE WATSON: Right.
MARK HEISTAD: So that really does put in perspective, the storm last night. I do have a memory, though, that we had a somewhat similar situation, what, in about August of 1977?
BRUCE WATSON: Right. August 30, 1977, the report received 7.27 inches in 4 and 1/2 hours. And that was about in the vicinity of a once in 2,000-year reign.
MARK HEISTAD: So we've had two extraordinary storms here in a little more than a decade.
BRUCE WATSON: Yeah. That's right. A lot of people, they wince when they hear about these, the rarity of these things. But you got to consider that there's a lot of places that go for, maybe one or two centuries without ever even having a 100-year rain. So it balances out.
MARK HEISTAD: Well, I guess we can hope that we've had ours for the next century. And we won't see a repeat of last night's storm anytime soon.
Bruce Watson, consulting meteorologist. Thanks so much for being with us this morning. We appreciate it.