Case of rapid tornado development from a cell with maximum reflectivity of 40dBz

On 17 June 2007, a tornado developed over open country near Pritchett, Colorado. This tornado did minor roof damage to an abandoned shelter and was rated EF0. This storm presented a challenge for the warning meteorologist. When the first report of a tornado was received, the highest reflectivity in the storm was around 40 dBz. Visually, the storm appeared as a tall and narrow column, and the visible satellite imagery showed a similar structure. Once the tornado developed, the storm rapidly developed an elevated core greater than 50 dBz, and the storm produced golf ball size hail.

The storm developed in an environment favorable for severe weather. It formed along a north to south boundary which was evident in satellite imagery. According to the LAPS model run on AWIPS, a shallow moist layer of 75 to 100mb deep was just east of the boundary. LAPS soundings suggested around 3000 to 4000 J/kg of CAPE for parcels emanating from the shallow surface layer, but with CIN around 50 J/kg.

The challenge for the warning forecaster was whether or not to issue any warning before the first report of the tornado. The warning forecaster knew the storm was in a favorable environment with the potential for rapid development; however, the radar showed a column of very modest reflectivities (greatest reflectivity of 40 dBz) with some rotation in the upper levels of the cloud. In addition, spotters and public were not even reporting any hail with this storm.

Besides highlighting the warning challenges, this paper will also present a case study of this event. The rapid tornadogenesis likely occurred when the very unstable surface air became ingested into the updraft resulting in explosive strengthening of the elevated core. Vorticity associated with the boundary likely became concentrated in the updraft to produce the tornado.