PHOENIX (AND ARIZONA) WEATHER HISTORY
Many people perceive Phoenix, as well as Arizona, to be a place of unrelenting sunshine - nearly devoid of stormy, active or hazardous weather. With the exception of the intense summer heat, Arizona is a great place to live, or retire to. However, those who have lived here, and experienced the fury of Monsoon thunderstorms know that Arizona can hold its own with any state in the country when it comes to severe weather - especially during the summer thunderstorm season. The following cases represent some of the more impressive weather events that have occurred across Arizona, with special emphasis on the Phoenix Metropolitan area.
ARIZONA SUPERCELL - 1999
Arizona experiences few, if any, tornadoes each year, quite unlike the "tornado alley" states of the Great Plains. However, it is possible for a tornado to occur in Arizona, or in the Phoenix Metro area, when the atmospheric conditions become "just right". Tornadoes, for the most part, do not spring from garden-variety thunderstorms - they form in association with thunderstorms known as SUPERCELLS. Supercells are thunderstorms that are unique in that they contain strong rotation, or spin, within the core of the storm. A tornado is a violently rotating column of air, on a rather small scale, and this rotation is derived from the larger-scale rotation present within the supercell.
In order for a thunderstorm to develop this strong rotation, and thus become a supercell, the atmosphere must possess substantial amounts of WIND SHEAR. Wind shear is a change in the wind's direction, or speed, or both, with height. For example, if the winds at the surface were from the southeast at 10 mph, and at 10000 feet aloft they were from the southwest at 50 mph, the atmosphere would possess strong wind shear. A supercell storm is considered a severe thunderstorm, in that it can produce tornadoes, as well as damaging winds and large hail. Severe, supercellular storms need more than just wind shear to develop, they need an atmosphere that is very unstable.
Herein lies the problem: during the Monsoon, the atmosphere is very unstable virtually every day. However, on most occasions, the winds aloft are rather weak, and the wind shear in the atmosphere is not sufficient to promote the development of supercells. Thus, supercells and their associated tornadoes are very rare during the Arizona summer. It is a somewhat different story during the spring and fall months, however. It is possible to have both strong wind shear, and strong atmospheric instability during these months, and this increases the chances for a supercellular thunderstorm to form.
One such supercell did indeed develop on September 14, 1999. The storm was located near Crown King, north of the Phoenix metro area and southeast of Prescott. Although no tornadoes were reported with this storm (largely due to the fact that the storm moved over an area of low population density), the potential for tornadoes with this storm was very high! The following graphics, taken from the Phoenix WSR-88D Doppler Radar, show both the reflectivity, and velocity, structures of this supercell. A very prominent "hook echo" can be seen in the first image - this feature is associated with strong rotation within the storm and in some cases a tornado will develop in the vicinity of this echo! In fact, when a genuine hook echo is seen on radar, the NWS will issue a tornado warning! (Click on the image to display larger, hi-res, version)
The following 2 images compare the Arizona supercell with an Oklahoma supercell that spawned tornadoes on the ground, including an F5 tornado in the Oklahoma City area. Note that the reflectivity structures of the two storms are nearly identical...including the presence of and position of the hook echo. Oklahoma is noted for its supercellular, tornadic thunderstorms - yet it is possible for such storms to occur in Arizona, even though they do so infrequently.