Project Vortex-99. Occluded mesocyclone tornado. Occluded means old circulation on a storm; this tornado was forming while the new circulation was beginning to form the tornadoes which preceeded the F5 Oklahoma City tornado. Photo #3 of sequence. Oklahoma 7 miles south of Anadarko. May 3, 1999.
(Credit: OAR/ERL/National Severe Storms Laboratory (NSSL); by NOAA Photo Library)