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A storm that dumped over three inches of rain near South Mountain in Phoenix is being called a 1,000-year event. State climatologist Nancy Selover, who lives in the affected area, will tell us more.

Ted Simons: Good evening and welcome to "Arizona Horizon." I'm Ted Simons. Heavy rains hit the valley last night resulting in flooding, downed trees, and over a dozen water rescues of stranded motorists. Joining us now is Nancy Selover, the state climatologist, and a resident of an area near South Mountain that was hit especially hard. Good to have you here. Thanks for joining us.

Nancy Selover: Thank you.

Ted Simons: What did you see -- you're by South Mountain, correct?

Nancy Selover: Yes.

Ted Simons: What did you see last night?

Nancy Selover: We had an extreme rainfall event that was tropical moisture. We had some dynamics, very unstable atmosphere and everything came together to squeeze that water out. We had 3.37 inches at my house in 100 minutes.

Ted Simons: What is 3.37, or three and a third -- in a little over an hour and a half?

Nancy Selover: Yes.

Ted Simons: I was going to ask if this is typical. That doesn't sound typical.

Nancy Selover: No, that is not typical.

Ted Simons: What happened here?

Nancy Selover: We just had an incredible amount of moisture that had come up from Mexico, tropical moisture. We saw low cloud levels, and it didn't take much to raise that up and get that -- get those rain events to start happening. And it just sat over South Mountain for an extended period of time. There was graphic lifting, surface heating, some dynamics, what we call an inverted trough, low pressure system and additional positive - spinning that cause uplift.

Ted Simons: It seemed like it parked over South Mountain. It came from Mexico. Could you see it building? Was it waxing and waning?

Nancy Selover: The weather service knew the moisture was up there. The storms themselves, a single storm doesn't form down in Mexico and typically continue just as one storm. It is just a continual reformation of a storm cell. One dies out and the next one forms.

Ted Simons: It forms in Mexico, moves up here in various forms. This one parked over South Mountain and went nuts on your house in that area out there where they stranded motorists and everything. Where does a system like this go? How far north does it go?

Nancy Selover: This one didn't go too terribly far past South Mountain and kind of fell apart. There was a little redevelopment, later in the day,a little formation over the west valley as it went north but it dumped its load on South Mountain. There wasn't enough moisture left after that to really provide too much rainfall.

Ted Simons: Monsoon, storms in general, it seems like they bounce off the rim and come back. Or go straight up. They don't go to Canada. How far north do they go?

Nancy Selover: Monsoon moisture typically goes into Northern Arizona. If we get a storm or a system that moves up the Colorado river, lower Colorado river valley, it will go up through into Las Vegas and it will dump a bunch of precipitation on them up there and it doesn't usually -- moisture doesn't usually go much further north than that.

Ted Simons: So, obviously, this was -- you said -- what happened on South Mountain could legitimately be described as a once in a thousand year event.

Nancy Selover: Yes, based on the annual return interval from the NOAA Atlas precipitation frequency tables, yes.

Ted Simons: The fact that it parked on the mountain, not necessarily the amount of moisture in the storm but that it stopped there and dumped.

Nancy Selover: Tables are calculated based on how long a time it rains and how much rain falls in that time. If you had two inches in 10 minutes, that would be a very, very significant event. Something that you would not anticipate happening frequently. In this case, we had a 200-year event, if I looked at 10 minute precipitation, highest 10 minute value. But looking at the highest 50 minute value, and the highest 90 minute or two hour value, we were at a thousand year event.

Ted Simons: Looking at washes, I think these were around your house and the washes, something like this, will change the nature of the landscape, won't it?

Nancy Selover: It absolutely will. When you get that - it's very steep terrain up there. When you get that water that falls that quickly, it doesn't infiltrate. It just runs off the surface and as it moves, it gains power and steam and more and more water comes down and it picks up and carries larger and larger particles, and pretty soon gravel and rocks and then it is ripping trees out and carrying limbs and bushes down. When it gets to the end of the wash, for example, in our neighborhood, we have culverts that go under the street to drain the wash. Culverts about / foot in diameter with a grading on them. The bottom part of the culvert got filled with mud and sand. But then the top part was blocked when all of the bushes and branches and things came down. The water came up over the wash, out of the wash, over the street, down on the other side of the street and started to scour out the dirt over there.

Ted Simons: So, the concept of cleaning up after the storm, yeah, you have to clean up. You also have to look at what has changed and start preparing for the next storm.

Nancy Selover: Sure, sure. We have to clean out the washes and get those things unblocked so that the water stays where it is supposed to stay.

Ted Simons: The monsoon so far, other than this obviously, other than that, Mrs. Lincoln, how did you like the play? Other than that, pretty typical.

Nancy Selover: So far normal -- Sky Harbor, this year, donut hole -- Tucson airport last year -- a lot of rain in Tucson but not at the airport. This year, the airport here has not yet had its rainfall. I think it still will.

Ted Simons: You think the monsoon has some punch left in it.

Nancy Selover: Another four weeks.

Ted Simons: And that would be not so unusual.

Nancy Selover: No.

Ted Simons: Does it seem like in the old days, East side, East Valley, eastern parts would always get the rain and you would feel sorry for the folks, almost like in the West Valley, they wouldn't get the storms. They wouldn't get the rain. It seems like they're getting a lot of storms these days. Is that just me? What's going on here?

Nancy Selover: Partly depends on where that moisture and where the core of the monsoon sets up in Mexico. If sometimes it is over to the east side, so the Southeastern part of Arizona gets that or the storms go up through western Mexico, and some years the central path is up the lower Colorado River. And then sometimes just up through Central Arizona. So, where in Phoenix that splits, East Valley, West Valley, it's always hard to say. It is not a pattern. It is not just creeping in one direction.

Ted Simons: We are not seeing necessarily --

Nancy Selover: A trend --

Ted Simons: More people out there, and more people saying it is raining out there.

Nancy Selover: They're noticing it.

Ted Simons: EL Nino -- what is going on this winter?

Nancy Selover: Coming and going and on again and off again. At the moment, it looks like it will have something. Looking like it might be moderate to weak, not necessarily strong, although that keeps changing. It is hard to say. If it is a moderate EL Nino, then we typically would get a little more precipitation in the winter than we usually get. And the past three winters have been consecutively dry. We're happy for anything.

Ted Simons: Moderate EL Nino is a good thing. What does that mean for the mountains in Colorado?

Nancy Selover: It doesn't make so much difference. Colorado -- EL Nino, southern tier states that provide us with more moisture than normal. Colorado, Northern California, they don't usually get a huge impact.

Ted Simons: The Colorado River water, the mountains, hard to tell though.

Nancy Selover: The good news for us, last year it was our third consecutive dry year in the state, but Colorado, Rockies got a lot of precipitation, they had a lot of snow pack and that helped bring -- up a little bit. Not huge.

Ted Simons: That is still a major concern. Good to have you here.

Nancy Selover: Thank you.

Nancy Selover:State Climatologist, Arizona;

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