Analysis of the Western Arizona/Eastern California Hail and Tornado Event of August 7th, 1997 Billy Warner AOS 453 5/10/06 Abstract Severe storms are not uncommon over the Desert Southwest in the summer months as a result of the North American Monsoon. On 7 August 1997, a line of storms formed in central and western Arizona that produced reports of hail and flash flooding over western Arizona, and later merged with a tornadic cell over eastern California. In this paper, I will look into the conditions that sparked these storms and caused them to develop in to severe thunderstorms and supercells. Introduction: On the afternoon of 7 August 1997, a line of strong to severe thunderstorms developed over the region of the Arizona-California border. This line of convection stretched across nearly the entire state of Arizona and was responsible for several severe weather reports throughout both states. These cells that formed in the late afternoon and early evening produced a pair of hail reports in western Arizona as well as a funnel cloud and F0 tornado in southeastern California. In this paper, I will analyze the conditions that led up to the severe weather over the AZ-CA border region. I will present evidence how the storms were able to develop under synoptic conditions that were not as conducive for supercell development compared to a case of a Great Plains supercell. However, there were sufficient meso-beta and meso-alpha forcings to support the storms. In lieu of traditional synoptic forcings, I will show that these storms began as “popcorn convection” and over the course of the evening became organized into a line of cells by convergent winds along the Mogollon Rim of the Colorado Plateau. I will show that once these storms began to develop, there were moderately good conditions for supercell development that produced the medium to large hail and the brief tornado. Data: In my analysis of this severe weather event, I used various data sources. To look at the synoptic-scale features, I used the GOES-8 (visible) satellite as well as archived upper-air plots of the US from Unisys Weather. In my mesoscale analysis, I used WSR-88D radar output from the ESX and IWA radar stations. In addition, I used analyses and short-term forecast data from the MesoETA model. Synoptic Overview: On the 7th at 12Z, the synoptic scale features (figure 1a) were quite “fair weather.” The surface analysis shows a low pressure center over theFigure 1a: Surface and upper-air plots from the Eta model analysis at 12Z 7 Aug 1997 Figure 1b: Surface and upper-air plots from the Eta model analysis at 00Z 8 Aug 1997southern AZ-CA border. This low is a classic “heat low” and is a prevalent feature over the Desert Southwest during the summer months. The warm temperatures over the region are quite clear in the 850mb analysis, with temperatures in the 28-32 degC range. The 700mb and 500mb analyses show that there is a weak high pressure center over the same region, with a very weak anticyclonic flow in association with this high. The 300mb analysis shows relatively stagnant flow in the upper levels, and there is no defined jet streak within at least 1000km of the southwestern US, therefore there is no chance for any form of dynamic lifting due to the ageostrophic winds of jet streaks. With these upper level conditions, one would not expect to see much convection. However other features such as the relative humidity and lifted index values are much more conducive. In the 850-500mb RH/LI analysis, there are moderate relative humidity values over the Arizona region in the range of 30-40% and the lifted indices are in the range of -2 to -3, which shows that the mid-levels of the atmosphere have moderate instability in the early morning hours before any sort of daytime heating has begun. The 00Z analysis (figure 1b) on the 8th shows that the heat low at the surface has remained over the region throughout the day, and the 850mb temperatures increased to 34 degC in the region where the storms were occurring. The 700mb analysis shows little change in the flow pattern at that level but there exists a region of predicted upward velocity of -4 to -6 ub/s over western Arizona, which implies that rising motion and moderate convection were likely in this region. The 700mb map also shows that the high pressure located over California and Nevada has remained throughout the day. This high shows up on the 500mb analysis as well and the general flow at both 700 and 500mb is from the north-northwest. The 500mb map shows very little in the form of mid and upper level vorticity, which means that any vorticity generated by the heat low must be confined to the surface and low levels. Like the 12Z analysis, there are light winds at the 300mb level and no distinct jet streak in the region. The RH/LI analysis shows the mid-level air over the region is still fairly dry, with relative humidities in the 30% range and lifted indices increased to -6 and -8 degC over some areas of Arizona. This set-up is similar to that of the “loaded gun” sounding, although in this case there are lower humidities in the boundary layer than with a classic “loaded gun.” These conditions reflect that low pressure at the surface could potentially initiate vertical motion and that there would be moderately favorable conditions aloft for buoyant parcels to continue rising if they are lifted high enough. Mesoscale Analysis: August 7th was marked by mostly fair weather throughout the morning and early afternoon, with clear skies over much of western Arizona. However, as I have shown in the previous section, lifted indices were conducive for buoyant and convective motions over the region. The clear skies throughout the first part of the day allowed for maximum surface heating. This resulted in theta-e values reaching into the low to mid 340’s in degK in the 15Z MesoETA analysis (figure 2a).Figure 2a: 950mb theta-e from the Meso-Eta model analysis at 15Z 7 Aug 1997 The MesoETA