Major Severe Weather Event Across Phoenix
Updated: 12:00 PM 29 August 2008
During the day of the 28th, the sky was mostly clear across central Arizona. Early indications were that strong to severe thunderstorms were possible that evening, though by mid-afternoon little thunderstorm activity had developed. The weather balloon sounding data from Phoenix Thursday evening indicated that the atmosphere remained very unstable though a cap remained - analogous to a lid on a pot of boiling water (see graphic to right). In addition winds in the mid levels of the atmosphere were over 45 mph from the northeast, strongly unusual and indicative that thunderstorms that develop northeast of Phoenix will quickly move into the lower deserts and be capable of producing damaging winds.
Shortly after 8 PM, thunderstorms began to develop east of Phoenix and moved into the potentially very unstable airmass. The thunderstorms themselves provided enough lift to release the stored energy - they took the lid off the pot. In the ensuing four hours severe thunderstorms spread across much of the south central Arizona, including much of the Phoenix Metropolitan Area. These were not your typical monsoon thunderstorms. NWS meteorologists noted the exceptionally strong structure of the thunderstorms, with the storms themselves extending up to 60,000 feet in the atmosphere. At this height the temperature of the tops of the thunderstorms was below -110 deg F (see graphic to right). For a satellite image loop, click here (0.8 MB).
As the evening unfolded several waves of thunderstorms, about four in total, developed generally east or northeast of Phoenix and moved to the west or southwest at speeds ranging from 20 to 40 mph. These types of thunderstorms are referred to as a line echo wave pattern (LEWP); line for a line of storms, echo for radar echoes, wave for the multiple waves of storms. LEWPs are common across the Mid-West United States, especially during the late evening and overnight hours. LEWPs are rather rare across central Arizona. What was even more rare was that the waves were traveling northeast to southwest; in the Mid-West the waves often travel in a general west to east direction. For a radar loop of the full event click here (large file - 17 MB).
80 to 100 mph winds measured by radar.
Early on it became evident that these thunderstorms would be capable of producing severe and damaging winds. This is what in fact happened as significant bow echoes developed within each wave. The highest measured wind speed reported to the NWS was 85 mph, which occurred near central Phoenix. Phoenix Sky Harbor International Airport reached 75 mph. Even the NWS office in Tempe was hit by the damaging winds. NWS meteorologists estimating winds to be at or above 80 MPH at the NWS office. Winds may have actually been higher than reported by any ground-based weather station as the National Weather Service radar recorded winds in excess of 100 MPH just 1,500 feet above the ground (see to right). For a loop of velocity radar data click here (large file - 14 MB).
The National Weather Service in Phoenix issued twenty Severe Thunderstorm Warnings and received thirty reports of severe weather during the event. The most significant wind measured was a wind gust of 85 MPH 4 miles SSE of downtown Phoenix by a trained weather spotter. The very strong winds resulted in widespread damage, with central Phoenix and north Tempe taking the brunt of the storm. Media coverage indicated that downed trees occurred over a large area with perhaps thousands of trees lost during the storm. Structural damage was also observed. The images below were taken by NWS Employees. Click on each thumbnail for a full version image.
