Transition Severe Convective Events over South-Central Arizona

The vast majority of severe summertime thunderstorms over south-central Arizona occur during July and August. Many of these storms are observed in conjunction with weak easterly, northeasterly or southeasterly winds aloft as the mid-tropospheric reflection of the subtropical ridge drifts north of the area. Since these storms occur when south-central Arizona is on the northern fringe of the Mexican Monsoon, they are considered "monsoon thunderstorms". These storms typically develop over higher terrain during the afternoon, then propagate over the lower desert of south-central Arizona during the evening and nighttime hours.

However, a smaller but significant number of intense thunderstorm outbreaks over south-central Arizona occur under mid-to-upper tropospheric westerly flow, which occurs when the mid-tropospheric subtropical ridge shifts to the south of the area. This occurs as brief "breaks" in the monsoonal flow, typically observed several times during July and August, or during the seasonal shift from easterly to westerly flow aloft, typically observed during September. These so-called "transition" events have produced some of the most memorable severe weather occurrences over or near the metropolitan Phoenix area. In "transition" situations, low-level moisture availability remains plentiful while mid and upper level drying occurs, and wind fields aloft are much stronger than what typically occurs in conjunction with other summer thunderstorms. As is observed during classic "dry line" events in the Great Plains states, sharp west-to-east low-level moisture gradients can exist over southern Arizona. Vertical shear is frequently great enough to support long-lived multicell storms; occasionally, supercellular storm structures are observed, especially during the September "transition" events. These storms typically form over the lower desert and may occur at any time, though daytime events are more common. An understanding of these events is on the increase, both at the local and national level; the increased frequency of severe thunderstorm watch issuances for central and southern Arizona (especially during September, 1999, when watches were issued on three consecutive "transition" days) is testament to that fact.

This paper's primary purpose is to document the existence of severe "transition" convective events over south-central Arizona. Several of the more severe "transition" events during the period 1993-1999, including those which occurred on 12 September 1993, 4 and 13 September 1994, 27-28 September 1995, 7 October 1997, 10 August 1999, and 14 and 19 September 1999, are briefly reviewed.