Oklahoma Tornadoes and Downbursts

The most significant tornado outbreak in Oklahoma since 3 May 1999 was observed during the afternoon and evening of 10 May 2010. Associated with the tornado activity were numerous downbursts, especially over central Oklahoma. The period between 2218 and 2238 UTC 10 May was especially active with two tornado touchdowns and reports of wind gusts between 51 and 56 knots in the Norman area. The GOES Microburst Windspeed Potential Index (MWPI) and GOES imager band 3-4 brightness temperature difference (BTD) products effectively indicated the potential for severe downbursts from three hours to 10 minutes prior to the observation of high winds in Norman.


Figure 1. Surface analysis of Oklahoma Mesonet observations at 2000 UTC (top) and GOES MWPI product at 1919 UTC (bottom).

Figure 1 shows the favorable environmental set-up for severe convective storms and downbursts during the afternoon of 10 May. At 2000 UTC, the dryline extended from near Freedom to Grandfield. The dryline served as a trigger for supercell convective storms as well as primed the atmosphere for downbursts over central Oklahoma. Figure 2, a radiosonde observation (RAOB) from Norman at 2000 UTC, displayed a classic "loaded gun" profile with large CAPE, a shallow, well-developed mixed layer, and a significant dry-air layer between the 500 and 700-mb levels. The dry-air layer, in conjunction with heavy precipitation resulting from large CAPE, played a major role in forcing intense convective downdrafts in the supercell storms.


Figure 2. RAOB from Norman, OK at 2000 UTC.



Figure 3. GOES MWPI product at 2121 UTC and BTD image product at 2215 UTC with overlying radar reflectivity image from Oklahoma City TDWR.

The MWPI image in Figure 3 indicated values between 60 and 70 southeast of Norman, corresponding to wind gust potential near 55 knots. The BTD image displayed overshooting tops associated with the supercell storms moving through Cleveland County as well as a dry-air notch on the southwestern flank. The overshooting tops signified the presence of intense storm updrafts that were generating heavy precipitation. The dry air notch most likely indicated the presence of drier air that was channeled into the rear of the storm and provided the energy for intense downdrafts due to evaporation within the intense precipitation core. The Norman RAOB showed southwesterly winds greater than 50 knots in the dry-air layer.

Severe weather occurred in the Norman area with the following time line:
2218-Tornado touches down near Moore associated with supercell #1
2225-51 knot wind gust recorded by Norman OK mesonet station
2232- 56 knot wind gust recorded at National Weather Center
2233-2238- Tornado touches town in southeast Norman, associated with supercell #2, with wind gust of 90 knots recorded