Arizona might be best known for its summer heat. But that sweltering heat is just a precursor to a seasonal weather pattern that can be even more dangerous. As the wind shifts, the dewpoints begin to rise. Dry lightning sparks wildfires with no hope of rain to quench the flames. Microbursts destroy homes and duststorms bring the evening commute to a halt. Weeks later, the rain arrives, and the normally-dry washes quickly fill up. Drivers foolishly attempt to cross flooded roadways, only to be rescued by police, as television news helicopters look on. The Southwestern monsoon is in full swing.
An average July day in Phoenix brings the mercury up to about 107 degrees, sometimes as high as 115 or 120 degrees. Day after day of intense surface heating produces a thermal low over the Southwest. They say "but it's a dry heat," but that's only until about mid-July. The thermal low, along with an area of high pressure that sets up northeast of Arizona, helps shift our circulation and bring in a somewhat subtropical air mass. Now it's hot and humid, announcing the monsoon's start.
Monsoon, taken from the Arab word for season, "mausim," means a seasonal shift in the wind flow. For Arizona, the usual westerly flow disappears as air moves into the state from the south and southeast. In the upper levels, we begin to see moisture from the Gulf of Mexico. In the lower levels, moisture moves in from the Gulf of California.
Our summer thunderstorms are different from those that occur any other time of the year because they are pulse-type storms as opposed to those generated by frontal boundaries. This alone makes them difficult to forecast. But the monsoon also has two distinct "seasons" within the season, and each carries its own threats. During the first half of the monsoon, moisture tends to be somewhat limited. Dust storms, microbursts and dry lightning are the main dangers.
After a couple of typically hot and dry Spring months, strong winds associated with thunderstorm outflows don't take long to cross the open desert, pick up dust and shroud the Valley. A wall of thick brown dust is often spotted moving into town in the early evening hours. The dry afternoon thunderstorms do nothing to wet the sand/dust surface and eventually, the outflows reduce visibility to zero during rush-hour traffic. What makes predicting duststorms a challenge is that you need to keep a close eye on thunderstorms happening outside your forecast area, and judge whether the outflow will remain in tact as it crosses the desert, sufficient enough to bring dust into the Valley. If storms pop-up over the Valley, dust doesn't get entrained in the storms and usually these storms do little damage. If storms move in from the Northeast off of the forested area of the Mogollon Rim, which is pretty common, they also don't carry the same threat for dust. But when storms move in from the south or southeast, moving over miles of uninhabited desert, they pick up dust and sand. You can often spot the leading edge of the dust storm at lower elevations of base reflective radar. The main challenges here are knowing where the storms are forming and will form, the direction of their outflows and the state of the atmosphere as the outflows move toward the Valley.
Another threat of the typically dry weeks of the monsoon is the microburst. Microbursts are so misunderstood by the public that when one occurs, newsrooms are flooded with viewers reporting "tornado damage," yet nobody saw a tornado. Severe wind events like this can be the most difficult to predict because not a lot is understood about microbursts. Observing and understanding cloud microphysics has a long way to go before meteorologists can warn for microbursts like they do for duststorms or thunderstorms. In the meantime, we can look at the sounding data, precipitable water and wind fields to gauge the threat. High-based thunderstorms are fairly common in the dry desert Southwest, and that means thunderstorms without the rain. Microbursts often take out power poles and mobile homes with their 60 mile-per-hour or more winds.
Dry lightning is another threat during these moisture-starved storms. With a dry atmosphere below the high-based storms, lightning-sparked wildfires and brushfires are fairly common during the early weeks of the monsoon. Thunderstorm outflows often fan the flames.
During the wet months of the monsoon, typically August and early September, flash flooding becomes the main concern. Summer thunderstorms in Phoenix are most-often of the pulse variety. This air mass is almost tropical in nature, with a weak flow aloft. This can often produce very slow-moving storms that drop significant amounts of rain. Forecasters begin to consider flash flooding potential when dewpoints are in the 60s. But moisture alone is not enough. On July 25, 2006 dewpoints were high across the region but there was no lifting mechanism. The result was a hot, humid and calm day in Phoenix. Storms developed early that evening 120 miles to the South in Tucson, and about 100 miles to the Northeast, over the Mogollon Rim. By 9pm, the outflows from the storms to the north and south collided over Phoenix and the result was 3-4 inches of rain. That's more precipitation than Phoenix usually receives from the entire monsoon.
Forecasters can better predict flooding by spotting weak flow aloft that can produce slow-moving storms, but also strong winds aloft and well-defined boundaries that can sometimes produce training thunderstorms. In other areas of the country, forecasters can look at frontal zones, but monsoon storm forecasters also have to look at much more mesoscale features like moisture differences and converging outflows.
There are several ways these monsoon forecasting challenges can be reduced. By 2012, the National Weather Service in Phoenix hopes to have access to dual polarization radar data. While this is not new technology, it would be new to the NWS forecast office. 3D images of the storms will not only help forecasters differentiate between hail and rain, but also better estimate precipitation amounts.
Looking at moisture (or lack of) is key to forecasting many of the monsoon dangers. But tracking the movement of moisture from the south is difficult because of the lack of data coming out of Mexico. Forecasters at the NWS office in Phoenix say that it is rare to get both a morning and evening sounding from many locations in Mexico. In addition, while the US has NEXRAD across the country, Mexico only offers data from major cities . This combined lack of upper air and surface data means the moisture surges sometimes aren't spotted until they get to Yuma, close to the US-Mexico border.
As mentioned earlier, there is much that is still unknown about the monsoon. Data collected in 2004 for the North American Monsoon Experiment is still being analyzed. In 2006, the CuPIDO field program took timelapse photos in hopes of helping forecasters better understand how storm clouds develop, decay and redevelop orographically over Southern Arizona's Catalina Mountains during the monsoon. Further research on cloud physics can only help forecasters' ability to understand what is going on above us during the monsoon.
Finally, the dangers of the Southwestern monsoon can be made mitigated by better educating the public. This year, the National Weather Service in Phoenix is taking a big step in that direction. Since the 1960s, the start of the monsoon was determined by looking at the dewpoints. The monsoon began for Phoenix after three consecutive days of average dewpoints 55 degrees or higher. For Tucson, the threshold was 54 degrees or higher. Technically, the monsoon could be underway in Tucson, but not in Phoenix, just 120 miles up the highway, yet both cities could be having thunderstorms. Television meteorologists would update viewers on the dewpoint status every night in anticipation of the monsoon's start. In addition, the NWS says forecasters there would field calls from people upset that there was a severe thunderstorm "before" the monsoon. The official end to the monsoon was previously subjective. So to eliminate confusion, the NWS offices in Phoenix and Tucson will declare the "Arizona Monsoon Season" to begin June 15th and to end September 30th. The idea is to take the focus off of the dewpoints and put it on the dangers of the monsoon. The new "season" will begin in the summer of 2008.
In conclusion, meteorologists are forward in forecasting the many dangers unique to the North American Monsoon. With new technology, more data, more research and better community education, taking the mystery out of the monsoon will lead to more accurate forecasts and a safer public.