Monsoon

July 12, 2010

Transcript:

Ted Simons: The weekend's monsoon storms missed much of the valley, but as temperatures rise, so do our chances for more storms. Here to talk about the monsoon is Randy Cerveny, a professor of geographical sciences who directs the meteorology program at ASU. Good to see you again. Thank you for joining us. We do this every time, it seems like, this time of year. Monsoon began June 15. Who knew?

Randy Cerveny: That's right. The National Weather Service wanted to change that a few years ago so that the emphasis wasn't on when it started but on getting the safety information out. So by having a set date, they could make sure that the media was given all the appropriate safety instructions to start getting out to the public so that, in the past, the old definition of three days of having a dew point at 55 got people watching too much the dew point and not really what they should be doing, make themselves safe for the monsoon.

Ted Simons: When does the monsoon usually start?

Randy Cerveny: Actually, right around this time. This is the classic start, is the first or second week of July here in Phoenix. It starts a little earlier in Tucson, but we get enough moisture coming up into the area to trigger the thunderstorms that provide the classic monsoon.

Ted Simons: What are you looking at as far as this go around for the monsoon? What are you expecting? Do we know? Can you forecast such a thing?

Randy Cerveny: It is a really tough thing because the problem with a monsoon is it is made up of individual little thunderstorms, and so Cave Creek can get one thunderstorm and Chandler can miss it, and Cave Creek will say it is a great monsoon this year and Chandler will say, no, we didn't get anything. So it is a very localized type of thing.

Ted Simons: It is kind of like what we have been seeing the past couple of days, Apache Hills and Grand Junction got hit, but much further west of there, nothing.

Randy Cerveny: Right, That is usually how the monsoon gets it start, it gradually builds up in the White Mountains and to the north and to the south and encroaches each night a little bit further into the Phoenix, metropolitan area.

Ted Simons: How different are storms today from what the valley -- let's keep it to the valley here, what we saw 25, 45, 75 years ago? Any difference in these storms?

Randy Cerveny: Yeah, the big thing is the lack of the dust storms. The big plumes of dust that we have will occasionally happen, we get one or two maybe a season, but in the past you would get several of those during the course of the monsoon season. In today's world, because we're paving over all that desert dust, we don't get as many dust storms.

Ted Simons: I've often wondered about this, this may be a dumb question but I will ask it anyway. Say we had a monster monsoon, covered most of the state, lots of rain in the desert. Lots of it. The next day can we get a dust storm again if so much rain has fallen over desert area?

Randy Cerveny: The big problem is you generally don't get a storm the next day after a thunderstorm. One night you can get all the mechanics, all the energy available to blow up into a big thunderstorm, but the thunderstorm tends to stabilize out the atmosphere so that the next night is usually a much more calmer situation. However, again, given the localized nature of these storms, yeah, you can have a dust storm appear the night after another dust storm if it doesn't take exactly the same track as the previous night. We've had that happen with storms that have come down off of the rim and then the next night they might come up from the Eloy area and you can have a dust storm coming up.

Ted Simons: But If a desert got swamped in one spot or a general region, it is unlikely the dust would go through there or start there, those sorts of things?

Randy Cerveny: Right, because the dust itself has to be picked up by these winds, and if they've been adhered to by the moisture locking it to the soil, then it is harder for it to get up in the air.

Ted Simons: We've talked about this, you have to have high temperatures, 13, 14, 15, this sort of thing, to draw up that moisture. Can you also have an extended period of time of 108, 109, 110? Does that work the same way or do you have to get the 113 and 14 before things start happening?

Randy Cerveny: Well, it helps to have the really hot air because hot air rises and that tends to be able to act like kind of a suction pump to pull up the moisture from the Gulf of California up through Arizona, so the really hot temperatures are the best. But if you have even like 108, 110 for a prolonged period of time, that's enough energy to draw up the moisture, as well.

Ted Simons: So we don't necessarily have to top out at 115 or 16. If it is long enough, we will get there eventually.

Randy Cerveny: Unfortunately, with that said, the likelihood is on Thursday and Friday of this week we will be up to that range anyway.

Ted Simons: Can you look at winter temperatures, winter weather, spring temperatures, spring weather and say we should have at least a general monsoon of this particular nature?

Randy Cerveny: We have tried. It has not really worked that well. We thought that there was a pretty decent relationship between the amount of snow cover that the United States got in the wintertime and how strong our monsoon has been, but it's not a perfect relationship. Some years it seems to work really well, other years it doesn't. All the indicators that we have looked at seem to suggest that this is going to be a fairly normal to maybe a slightly above normal monsoon season. But we really are not good at predicting the intensity of this thing well in advance yet.

Ted Simons: And it goes along September, those lines?

Randy Cerveny: The technical definition now is September 15 we call it closed. What happens is the monsoon flow starts to merge with dying tropical storms to come up from the Gulf of California and from the Pacific Ocean, and that moisture will sometimes obscure what the true monsoonal moisture is and it gets harder to identify a true end.

Ted Simons: I think I've asked you this before, but the reason that Apache Junction, Cave Creek, Fountain Hills always seem to get the monsoon and Buckeye and Gila Bend are saying I know where it is happening, it's not happening here, why is that?

Randy Cerveny: It's elevation. What happens is that moisture flows uphill and rising air is more likely to create a thunderstorm, so you first see the true indications of a monsoon in places like Apache Junction and Cave Creek because the airflow is coming up from the deserts and flowing up that up-slope area. Central Phoenix and places like Gila Bend and such, they don't get it quite as soon.

Ted Simons: Thank you, Randy, always a pleasure.

Randy Cerveny: My pleasure.