This F3 tornado was moving slowly north-northeast allowing it to be surrounded by three VORTEX mobile mesonet probe cars. This photo by Daphne Zaras in Probe 1 was taken 0 .5 to 1 mile east of the tornado. While this F3 tornado was occurring, a new circulation was forming on the same storm and producing brief touchdowns prior to becoming the large tornado that originated near Chickasha, OK.

Unofficial Summary from Daphne Zaras,
Probe 1, VORTEX-99, May 3, 1999

Three VORTEX-99 crews left Norman, OK at 1:30 p.m. and drove southwest to near Lawton, OK in time to see the initial stages of the storm that produced a continuous, deadly tornado, which tracked from near Chickasha through Moore to Del City in the Oklahoma City metro area.

The teams observed the first two brief tornadoes produced by the storm near Elgin, OK and were in place for scientific data collection for several subsequent tornadoes that formed prior to the Oklahoma City metro tornado. The third tornado produced by the storm lasted over 15 minutes and was surrounded by mobile mesonet vehicles. It was rated F3--the first strong tornado from the storm that eventually went through Oklahoma City.

VORTEX-99 collected valuable data on this tornado as the new circulation in the storm organized and began forming weak tornadoes. The storm was doing what scientists call "cycling": the original circulation had spun off toward the back side of the storm while a new circulation formed. The new circulation became the fourth tornado produced by the storm and also was responsible for what became the Chickasha-Moore-Del City, OK tornado. As the deadly tornado developed, the teams abandoned the storm in favor of another storm that was over a good road network west of Chickasha. The second, western storm had produced two tornadoes and was clearly going to continue to produce tornadoes. VORTEX-99 intercepted ten tornadoes on the second storm, for a total count of 12 tornadoes intercepted on two storms.

Key regions sampled included the hook region (behind the tornado) where the air is wrapping around the back side of the thunderstorm. This region is also responsible for the "hook echo" seen in radar reflectivity that meteorologists often point out on television. One mobile mesonet was able to traverse the hook region of the storm for three different tornadoes.

VORTEX-99 is a small follow-on to the original VORTEX, which took place in 1994 and 1995. VORTEX-99 is a joint project between NSSL and the University of Oklahoma.