Arizona Downbursts: 18 May 2009

During the afternoon of 18 May 2009, widespread convective storm activity developed over Arizona as a result of strong solar heating and the presence of an upper-level disturbance that acted as an additional lifting mechanism. The convective storm activity persisted into the nighttime hours, producing isolated strong downbursts near 0500 UTC 19 May. A downburst, with an associated wind gust of 35 knots, was recorded at Magma FRS ALERT station, 45 miles southeast of Phoenix, near 0500 UTC. A stronger downburst, with a wind gust of 52 knots, was recorded at Sedona Airport, 95 miles north of Phoenix, at 0530 UTC. The GOES imager microburst product indicated moderate risk in proximity to the location of these downbursts two hours prior at 0300 UTC. The pre-downburst environment was indicated to be favorable for dry microbursts, with significant mid-level moisture overlying a deep and dry residual mixed layer that had evolved during the previous afternoon. Interestingly, the convective downdrafts were able to penetrate the shallow stable boundary layer and produce strong wind gusts at the surface.


The images above are GOES imager microburst products at 0300 UTC with overlying radar reflectivity imagery at 0453 UTC (top) and 0529 UTC (bottom). Both images show elevated risk values in proximity to downburst producing convective storms over Magma FRS ALERT station (top) and Sedona Airport (bottom). Output brightness temperature difference (BTD) near 35K (light blue shading) was indicated immediately downwind of the convective storm over Magma FRS ALERT station. The 0300 UTC Rapid Update Cycle (RUC) sounding profile over Magma station (below, top) displayed a classic inverted V profile typically associated with dry microbursts. Especially apparent in the sounding is a nearly 500 mb deep dry adiabatic layer with a large relative humidity gradient between the 500 and 900 mb levels (just above the surface) that fostered significant downdraft instability. However, due to the time of microburst occurrence (0500 UTC), about 2 1/2 hours after sunset, a shallow stable boundary layer had developed due to radiational cooling at the surface. The depth and moisture stratification of the overlying mixed layer did promote evaporational cooling and negative buoyancy as precipitation descended in the sub-cloud layer. Downdrafts of sufficient intensity were initiated to impact the surface and produce the strong wind gust recorded at Magma station. At 0530 UTC, a stronger downburst (52 knots) was recorded at Sedona Airport. This downburst occurred in proximity to slightly higher output BTD (>40K, light green shading). The 0300 UTC RUC sounding profile over Sedona (below, bottom) displayed a shallower inverted V profile (due to the higher surface elevation) with a nearly 500 mb deep dry adiabatic layer, similar to the Magma sounding. Again, apparent in the sounding profile was a shallow stable boundary layer that was easily penetrated by the convective downdraft to produce the severe wind gust at Sedona Airport.


The parent storms of both downbursts were characterized by low reflectivity and thus could be classified as "dry" type. Although these downbursts occurred during the late evening, the deep, dry residual mixed layer that evolved during the previous afternoon promoted significant downdraft instability. The convective downdrafts penetrated the stable boundary layer to result in strong downbursts in a similar manner to previously noted nighttime downburst events.