The Monsoon Halftime Review
It seems like only yesterday when various groups were putting out their 2013 monsoon forecast for the Southwest. In fact, I gave several presentations where I danced around direct questions relating to our expected rainfall for this season. Here at the National Weather Service, we fielded numerous phone calls from overheated residents who wanted to know when the rain would start. Well, we are almost halfway through the 2013 monsoon and I can honestly say this year has provided yet another example why forecasting the monsoon is no easy task.
I do need to give credit where credit is due. Both the Climate Prediction center in its official forecast, and the University of Arizona in a non-official forecast, suggested this year would start off wet. They were right for a good portion of Arizona. Unfortunately, for many Tucson residents, this year has started off a bit spotty.
The map at left shows a 30-day radar estimated percent of normal rainfall analysis for Arizona. I do need to point out that radar rainfall estimates have their limitations so this is mainly used to give you a general idea of where rain has fallen this year. I want to focus in on two areas in Southeast Arizona. Let’s start with Tucson.
One of the interesting features of this map occurs across Yuma County in far southwest Arizona. One might think southwest Arizona is swimming under all that rainfall. However, this area of Arizona is normally very dry so it only takes a few strong storms to dramatically increase their percent of normal rainfall. In comparison, much of Pima County has experienced twice as much rain, but it appears drier on this map because normal rainfall is greater in that area.
Let us now zoom into Tucson. The map at right shows the radar observed rainfall for the Tucson area over the last 30 days. Pay attention to the color scale because it is different from the percent of normal map above. In this case, red indicates a large amount of rainfall. This map also helps demonstrate that a few miles can make a significant difference in rainfall during the monsoon.
If you live around Grant Road or further south, your monsoon has either been near normal or on the wetter side. Congratulations. However, if you live north of Grant Road and south of Oro Valley, then you have missed out on the bigger storms. In fact, you are sitting at roughly 50 percent of your expected rainfall.
I put together a quick chart showing the observed rainfall and average temperatures from the Tucson airport compared with our normal conditions through July 28th (“normal” implies a 30-year average). This is intended to give you an idea of how the monsoon has progressed so far.
Warm air sat over Tucson the first few weeks of the monsoon, but the air started to cool around July 10th. That is also around the same time a few rain events impacted the airport. From the airport’s perspective, this monsoon is running slightly wetter than normal and the last few weeks have been cooler.
Now let’s jump to Douglas, Arizona. The light blue line on this graph should jump off the page at you. In the month of July, the Douglas airport received 10.12 inches of rain—there were only four days in which it did NOT rain at the airport. This was the wettest 30-day period on record for Douglas, dating back to 1948. NOAA keeps track of rainfall return intervals for numerous locations around the United States. As it turns out, recording over 10 inches of rain over a 30-day period in Douglas equates to about a 1 in 200-year event. Another way of viewing that data is to say that in any single year, Douglas has a ½ percent chance of receiving over 10 inches of rain during any consecutive 30-day period. This is a rare event for Douglas.
Part of the explanation for the extremely wet conditions across portions of southeast Arizona is a series of low-pressure systems that brought increased support for rainfall on several July days. Of particular note was a low-pressure system that originated in the Gulf of Alaska and followed a normal path east over Canada and the northern United States. This system then slid south toward Pennsylvania and slowly began traveling west. For several days, this system tracked quietly west across the central United States, providing only minor impacts of cooling temperatures and a few sprinkles.
However, when the system arrived in Oklahoma, abundant moisture combined with the cooler upper atmospheric air to produce widespread heavy rainfall. Eventually, this system meandered into the desert Southwest and lingered for several days. Much of Arizona and western New Mexico received widespread rainfall thanks in part to this low-pressure system.
These systems have a tendency to modify the normal diurnal trends of the monsoon. Overnight showers and thunderstorms with activity lasting into the early morning hours become more common. So if you noticed more overnight rain than usual this month, then you can thank the low-pressure system that start in the Gulf of Alaska and eventually made its way into Arizona.
Finally, what does August have in store for southeast Arizona? I’m sure you can guess how the Climate Prediction Center (CPC) feels about our chances for being warmer than normal. The image at left is the official temperature outlook for August. The CPC once again shows a higher chance of being warmer than normal in August. However, you should take some solace in noting the probability of being warmer than normal is just over 33 percent. In other words, there is still a decent chance of being either near normal or even cooler than normal. Often times during the summer, Arizona is under a very high (greater than 50 percent) probability of being warmer than normal.
As for the precipitation outlook, the CPC places a 40 percent chance of being wetter than normal over far southeast Arizona (see image, right). If you recall, the CPC produced a very similar forecast for July. Please keep in mind this forecast is a probability of being wetter than normal. Just like with the temperatures, we still have some potential of either being drier than normal or near normal.