An intense, long-lived mesoscale convective system (MCS) developed over south-central Arizona during the early morning hours of 17 August 2001. The MCS reached its peak intensity around 0930 UTC (230 am MST) 5 km (3 miles) north of Gila Bend, Arizona, where in excess of 100 electrical power poles were downed by straight line winds estimated at 80 to 100 mph. An electrical substation was severely damaged by the intense wind, leaving a number of communities in southwest and south-central Arizona without electrical power for several days. The Governor of Arizona declared a "state of emergency" for the area. This MCS was unusual in that it developed after 0800 UTC (1 am MST), then maintained its identity and produced severe weather through 1500 UTC (8 am MST) as it propagated south-southwest into northwest Mexico.

Given the degree of instability, coupled with stronger than normal north to northeast winds aloft as well as moderate vertical wind shear, NWSFO Phoenix expected isolated severe convective storms, with damaging straight line winds, to affect portions of south-central Arizona during the evening of 16 August. However, a large, long-lived MCS was not forecast for several reasons: 1) an unusually inactive convective day and early evening was observed over the higher terrain north and east of south-central Arizona; 2) no detectable upper level disturbance was expected to affect the area overnight; and 3) for the vast majority of situations with similar meteorological characteristics, only isolated severe convective storms are observed.

Investigation of MCS evolution showed that the area of organized intense convection, with unusually high cloud-to-ground lightning flash density, was triggered by deep, rain-cooled thunderstorm outflow boundaries from two widely-separated and relatively small thunderstorm clusters. This paper will briefly discuss the climatology of convective storms over southwest and south-central Arizona during its active summer convective season, review the meteorological conditions preceding this MCS development, and describe MCS evolution as observed primarily by WSR-88D radars, the GOES-W satellite, and lightning strike data. Specific damage reports and verification statistics for this event, which occurred primarily over a sparsely populated region, will also be provided.