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Severe Thunderstorms in Arizona

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Severe thunderstorms are fairly common during the monsoon in Arizona. Strong downburst winds, flash flooding and cloud-to-ground lightning are the most frequently observed thunderstorm phenomena but hail, blowing dust and even an occasional small tornado can also be observed.

Weather Patterns Associated with Severe Thunderstorms.

The Type I - Southern Plains/Four Corner High - pattern is the "classic" monsoon pattern. The streamlines depicted below are for the mid-levels or about  20,000 feet above sea level. In this situation the monsoon ridge sets up over the southern Plains. This broad high is positioned across the southern half of the United States with easterly flow located over portions of Texas, New Mexico and extends west to the Arizona-Utah border The Easterly flow in Mexico and a more Southeasterly flow into Arizona means lots a of thunderstorms in the mountains and Southern Eastern Arizona foothills. A secondary high usually develops near the Four-Corner region. When this happens, mid level temperatures across southern Arizona cool, low level moisture increases from the south or east, and winds between 10,000 and 20,000 feet increase out of the east. This causes thunderstorms to tilt slightly, and allows them to maintain themselves for longer periods of time while organizing into lines or clusters. If the lower levels of the atmosphere are rather dry, straight line winds and dust storms are a major concern. If the lower levels are moist, flash flooding becomes a problem as well. Strong High pressure over the southwest United States almost guarantees that the surface based thermal low is deepening (refer to the Gulf Surge discussion).

Initially, thunderstorms on Type-I days form on the mountains and spread east-to-west or southeast-to-northwest. Thunderstorms on the Mogollon Rim and in the White Mountains tend to remain where they develop, while the storms in the mountains of southeast Arizona or northern Sonora tend to move into the valleys and eventually the low deserts. As these storms move progressively farther to the north or the west, they typically encounter a more stable atmosphere and dissipate. When a Gulf Surge is underway, though, the atmosphere remains unstable as the storms move into the lower deserts. In these instances, storms may continue to travel all the way to the Colorado River Valley.

The Type II: Great Basin High - pattern is associsted with a high amplitude ridge over the western United States. The streamlines indicate that easterly to northeasterly flow is located across Arizona with this pattern. Once again all the elements are there, easterly flow over Mexico for lots of storms and intense heat over the west building the surface based thermal low. Over the eastern U.S., an unusually deep upper level trough develops which sometimes pushes a cold front south through the Plains and west toward the Arizona-New Mexico border. With this pattern the ocscasional weak cool fronts can actually bring moisture from the High Plains into Arizona.The clockwise circulation around the upper level high causes winds between 10,000 and 18,000 feet increase out of the northeast over Arizona.

As the thunderstorms develop on the mountains, the Mogollon Rim readily forces them to organize into squall lines, which are then pushed southwest by the winds aloft into the deserts. The northeast winds aloft usually bring drier air into Arizona, so if the drying is deep enough, the thunderstorms may dissipate before moving very far away from the higher terrain. However, if there is only drying aloft and low level moisture remains plentiful, the downdrafts associated with these lines of storms can become large and severe. Areas most susceptible to Type II events are those immediately downwind from the Rim or White Mountains of east central Arizona, including: the Phoenix Metro Area, the Gila River Valley, and the valleys of Yavapai County. The changing wind direction and speed with height also helps to sustain the thunderstorms for even longer periods than in Type-I patterns, which can allow them to persist well into the night as they move southwest through Tucson and the Colorado River Valley.

The Type III: Trapping High - pattern is also associated with active periods of the monsoon. The elements are similar to the previous two pattern types easterly flow over northwest Mexico with hot temperatures across Arizona by virtue of there being a high pressure system nearly directly overhead. Typically below 20,000 feet the This pattern is quite different than the other two in that the monsoon ridge is weaker and sometimes suppressed father to the south - sometimes extending along the U.S.-Mexican border. The ridge will sometimes break into two separate centers with one over south Texas and the other over northern Baja. If this occurs either in June or September, the ridge placement tends to block moisture coming north from Mexico. However during monsoon peak, moisture still finds its way into Arizona from the south and east. Meanwhile, upper level disturbances can move into the region from several different directions, and either slow down, or become trapped within the ridge and stall. The presence of a weak upper low keeps temperatures aloft relatively cool, and the entire atmosphere unstable.