JULY 28-29 1986 DERECHO

"The Supercell Transition Derecho"

Fig. 1. Severe weather outbreak on July 28-29, 1986. Tornadoes are indicated by "T" or as linear paths with intensities noted in "F" scale (F0=weak, F4=violent). Large hail is indicated by "A" with hail diameter in inches. Damaging winds are indicated by "W". The widespread severe winds of the derecho are included within the dashed line. Times are given in Universal TIme (UTC) which are converted to CDT in the description below. Note the map is skewed so that the top of the map is towards the northeast rather than north. (From Johns and Leftwich 1988)

On Monday, July 28, 1986 isolated supercells (see Figure 2) developed over portions of South Dakota and began producing large hail, damaging winds, and tornadoes. The first reports occurred at 4:30 PM CDT (2130 UTC). The supercell that moved south southeastward through southwestern Minnesota evolved into a bow echo as it moved into northwestern Iowa (see Figures 2, 3 and 4). This bow echo grew in scale and produced a derecho as it moved rapidly southeastward. Gusts greater than 80 mph occurred in the Des Moines, Iowa area about 10:15 PM CDT (0315 UTC) and 100 mph winds caused considerable damage as the storm raced through northeastern Missouri and western Illinois. The derecho finally diminished in the St. Louis, Missouri area about 3:30 AM CDT (0830 UTC) on July 29th. An example of some of the severe damage is shown in Figure 5 below.

Fig. 2. Radar reflectivity imagery at 4:52 PM CDT showing isolated supercells.

Fig. 3. At 6:42 PM CDT supercell over southwestern MN is evolving into a bow echo

Fig. 4. By 9:40 PM CDT the bow echo has become well defined and is producing a derecho.

Fig. 5. As the derecho passed the Des Moines River near Boone, IA, cars of a train crossing an 180 foot high bridge were blown off into the water below. (From Storm Data for July 1986)