Record Rainfall

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Near-record rainfall has hit Arizona because of Tropical Storm Lorena. Arizona State University climatologist Randy Cerveny will discuss the rainfall and the storm.

Ted Simons: Good evening, and welcome to "Arizona Horizon." I'm Ted Simons. Recent rains in the valley and around the state have reinforced what's turned out to be a pretty good summer rainy season. Here to tell us what's going on is ASU climatologist and president's professor Randy Cerveny. Good to have you here.

Randy Cerveny: Thanks.

Ted Simons: What is going on out there? There's some crazy stuff here the past few days.

Randy Cerveny: It's the typical monsoon winds, circulation. But it's on steroids, because it was able to pick up all that moisture from the dying tropical storm that was down off the coast of Mexico. As all that moisture was pumped up the Gulf of California, it basically really, really just dumped right on to Arizona.

Ted Simons: Is that unusual for Arizona? Is that unusual for this time of year?

Randy Cerveny: Actually this is the perfect time that we have these kind of things, because the monsoon never really sharply ends. It slowly dwindles away, and as it does, there's a tendency for these dying tropical storms that are off Mexico to work their way up into near Arizona. We never get a tropical storm up here, but we get the moisture, and we can have these rains in September and October.

Ted Simons: So the storm sees a vacuum, moves up because the monsoon is easing off a little bit.

Randy Cerveny: Right. Actually, the reason it comes up into Arizona is it's kind of like a giant pump. When we have high pressure over in the Great Plains, it circulates around in a clockwise manner, so the airflow comes over Mexico, pushes the moisture up into Arizona. So when you have really hot, dry conditions like they've had in the Great Plains this year, that kind of pumps the moisture up into Arizona.

Ted Simons: Let's talk about that. It seems to me that this has been an above average monsoon. Has it been above average?

Randy Cerveny: It is slightly above average right now. In terms of the Phoenix metropolitan area, we have more rain than what we've had for several past monsoon seasons.

Ted Simons: And that's because --

Randy Cerveny: Because primarily we've had a moisture source. We've had this flow of moisture, the typical monsoon we get this moisture coming up from the Gulf of California, from the Pacific Ocean. But this year it's been very, very consistent. It's day after day of this pumping action.

Ted Simons: Is that because that high you were talking about is parked a little east of us?

Randy Cerveny: Yeah. And it basically is set over Texas and baked Texas all summer.

Ted Simons: And they're having problems there with drought, they're having problems with not enough rainfall. So when it's parked over us, they get the rainfall?

Randy Cerveny: Yep. Our monsoon moisture usually will arc up along the Colorado River and then go up into Utah. This year it hasn't.

Ted Simons: That's interesting. So if we have a good monsoon, they're hurting in the Plains. If we have a not so good monsoon, they're getting a lot of rain in the plains.

Randy Cerveny: That's right. People don't realize, but when you get one weather occurring in one part of the country, it usually means the exact opposite in another part of the country.

Ted Simons: Was there any way back in April or May to forecast we would have an average to above average monsoon? Any way at all?

Randy Cerveny: I wish I could say there was, but no. We frankly, the monsoon is such an odd phenomenon, and we've only been studying it for the last 10, 15 years, we really don't yet have a good feeling for what causes it well in advance of when it actually starts.

Ted Simons: So now we've got the monsoon, and it's slowly ending, it seems to me, it seems like in September maybe not so many storms, but the storms we get can be real lulus. Is that accurate?

Randy Cerveny: That is accurate. The reason again has to do with these tropical storms. Even though they don't come all the way up into Arizona, the moisture, when they're dying down by Mexico, does get pumped up into Arizona, and so in September and October we will not have as many storms as the monsoon time, but when they do they really dump a lot of water.

Ted Simons: It's also my impression once the monsoon ends, statistically you have it the end of September, usually mid- to late September, we go through an extended dry period, similar to what we see maybe in the spring before we get -- The temperatures aren't the same, but the dryness level seems to be the same. Is that accurate?

Randy Cerveny: We'll lose the moisture. In the atmosphere. Because our storms switch from being powered by moisture in the air, to being powered by things like cold fronts coming from California. So we'll lose that moisture and that means we'll start to have really nice cool, crisp mornings, and we'll get away from that monsoonal humidity that we're all used to in the summertime.

Ted Simons: But it's not quite as dry for quite as long as we see in the spring?

Randy Cerveny: Right.

Ted Simons: So that's when it really bakes.

Randy Cerveny: Yes. Because that's part -- What causes the monsoon in the first place is the baking in the spring.

Ted Simons: OK. So on we go to winter, we've got a La Niña, El Niño, can we figure out what's going to happen this winter?

Randy Cerveny: Right now what they are basically saying is that we're going to be tending towards dryer conditions here in the southwest. Most of the storms that we're going to have, and it's a fairly typical situation, will get shuttled up towards the Northwest part of the United States. Up into Oregon and Washington. And we'll get a few of those, but nowhere like the El Niño kind of seasons that we've seen occasionally in years past.

Ted Simons: So tipping toward La Niña.

Randy Cerveny: It's going to be tipping toward a dryer season.

Ted Simons: Everything we've talked about, from above average monsoon to the plains, to September, to whatever, global warming and climate change. Any factor whatsoever?

Randy Cerveny: Well, one of the things that we have figured out is that the big high pressure systems like we've seen setting over Texas and the southern part of the country, particularly over the last few weeks, those will become more prevalent. But as they set there, they also tend to pull moisture up and circle it around. So that interestingly, one of the things we found with global warming is that for the world as a whole, which will become a wetter place, we're still going to have droughts, but there's going to be more chances for big flooding events under a global warming world than under a non-global warming world.

Ted Simons: From an Arizona perspective, if a global warming world means that high pressure system moves over Texas and God bless them, they don't get anything going, they got their own concerns, we wind up getting good summer rains.

Randy Cerveny: Yeah. Because the critical thing to us is being able to force that air from Mexico up into Arizona. And when you have something over in Texas that can circle that air up into Mexico and us, that's what you need to have.

Ted Simons: Real quickly, we're talking about the monsoon, the importance of the monsoon pales in comparison to the winter rainy season and the snow cap.

Randy Cerveny: For things like the Colorado River, and for the watersheds in the Salt and Verde, the winter time snows are the critical thing. You want that water slowly get into the reservoirs as opposed to just rushing in and rushing out.

Ted Simons: All right. Randy, good stuff, good to have you here.

Randy Cerveny: Thanks. My pleasure.

Randy Cerveny:Climatologist, Arizona State University;

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