June 29, 1998 Derecho

On June 29th, 1998 a supercell thunderstorm developed over eastern Nebraska and tracked into central Iowa. As the thunderstorm reached central Iowa, a strong rear-inflow jet developed which caused the thunderstorm to take on a different characteristic, known as a derecho. This type of storm is known for its widespread high winds and long, persistent life-cycle.

Over the Davenport area of responsibility, numerous reports of wind gusts ranging from 80 to 100 mph were received. The highest measured wind gust was reported in Washington, Iowa...a whopping 123 mph. This is the highest unofficial recorded wind gust in the history of the state of Iowa.

Below are a few radar images from the KDVN WSR-88D.

This was just as the derecho was really beginning to take shape.

This was around the time that Iowa city was getting hit. An obsever near Iowa city reported wind gusts above 65 mph for more than 20 minutes. Another observer reported a brief tornado touchdown.

This is the SRM (Storm Relative Velocity Map) data at the same time as the above reflectivity data. Note the red and green couplet over Iowa city. The red color indicates winds moving away from the radar (the radar is to the west) and the green indicates winds that are moving towards the radar. When these two colors are close together it is an indication of rotation. This area of rotation persisted for about an hour an was likely a meso-low...or small intense low pressure system.

This was when the storm was nearing it's maturity. Note how the storm is really beginning to bow outward. This is due to the strong rear inflow jet that has developed on the backside of the thunderstorm. Notice also what is called the warm advection wing, oriented west-east on the northern part of the storm.

The meso low at this time was located near the Mississippi River and has just passed Muscatine. An observer in Muscatine recorded a 90 mph wind gust.

This is the derecho in full maturity. Surface winds of 50 to 60 mph were being reported in the rear inflow jet region at this time...which was well behind the strongest reflectivities. Winds of 80 to 100 mph along with brief tornado touchdowns were still occurring along the main line of thunderstorms.