"Desert SW Severe Weather"
07 August 1997

On 7-8 August 1997, the desert southwest experienced severe weather, the bulk occurring in Arizona. Several reports of hail up to 1.75" in diameter were recorded, as were strong thunderstorm winds, and flash flooding. Thunderstorms are most common during July and August in Arizona, due to the monsoonal flow, which brings moisture up from the Gulf of California, the Pacific Ocean and the Gulf of Mexico. As this moisture moves into the region, a combination of orographic lift and daytime heating causes thunderstorms to develop.

On 7 August at 1200Z, the water vapor image shows a large area of moisture over northern Mexico, which would spread north to blanket all of Arizona by 0600Z 8 August. Surface moisture was also present, note the 60F+ dewpoints in southern Arizona. Skew-T's from Tucson at 0000Z 8 August and 0000Z 9 August show good moisture and instability present to aid in thunderstorm development. Further to the north, in Flagstaff, the 1200Z 8 August skew-T shows good moisture; and with the daytime heating, the necessary instability would then develop.

The mountains acted as a focusing mechanism for the initiation of these storms, as you can see clearly when comparing a nowrad image with a topography map of Arizona.

The storms of 7-8 August 1997 caused only minor damages in Arizona, an estimated $88K in damages from wind and flash flooding.

Case Study 017