Rain that caused Duluth, N.E. Minn. flooding was much heavier than NWS expected

A year ago today the National Weather Service in Duluth was predicting that an approaching storm would bring about two to four inches of rain to northeastern Minnesota. A flash flood watch was issued for the region. But the storm turned out to be much bigger than anticipated. It started raining and it kept raining. Ten inches of rain fell on parts of Duluth. The ground was already soaked from previous storms and the result was massive flooding in Duluth and other northeastern Minnesota communities.

Carol Christenson, the warning coordination meteorologist for the NWS in Duluth, spoke to Morning Edition's Cathy Wurzer. Here's an edited transcript of that conversation.

CAROL CHRISTENSON: We knew it was going to be a big rainmaker. That's why we had the flash flood watch in effect, and then later on in the evening, a flash flood warning. So we knew it was going to produce a lot of rain. Ten inches? You're right, 10 inches was not in the forecast. But earlier in the evening we started out with severe thunderstorms with hail and damaging winds and then, as happens frequently with this type of situation, it turned into heavy rainmaker.

CATHY WURZER: How did that happen?

CHRISTENSON: It was what we call "training." We had a warm front - a stationary front just to the south of us. And the storms just continually developed north of a front and then moved over the same area all night long. Just a continuous stream of storms, one right after the other. And with that situation it produced a lot of precipitation as we saw.

WURZER: Was there anything else unique about this storm system?

CHRISTENSON: What was unique was that it was a very sultry, humid, very heavy day so there was a lot of moisture in the atmosphere. And then with that stalled front just to the south of us, and we're just north of it, we're in the prime location for these training thunderstorms. And as you mentioned the soil was already saturated because we had just been recovering from previous floods from May - late May especially was extremely wet. So any rain that fell did not go into the soil whatsoever. It just ran right off.

WURZER: When did you all know that you were in for a big problem? About 1:00 a.m.?

CHRISTENSON: It was much earlier than one o'clock in the morning. We issued the flash flood warning about 7 p.m. We started getting reports of flooding to the west. And it was at that moment we issued the flash flood warning with enhanced wording that it was a high-end, life-threatening flash flood event.

WURZER: Do you think you'll ever see anything like that again?

CHRISTENSON: Probably. We've had extreme flooding in the Duluth area in the past 30 years, back in '72 there was extreme flash flooding in Jay Cooke State Park, where they had 10 hours of continuous thunderstorms and like we say in May, just this year again, major flash flooding. So everything just needs to come together: saturated ground, intense rainfall.