Derecho - 100 mph Winds on Monday, August 10, 2020

Barometric pressure trace from Cedar Falls, Iowa for August 9th through August 15th. The sharp pressure jump at Noon on August 10th occurred as the Derecho side-swiped Cedar Falls. We experience the pressure rise but not the damaging winds. Almost immediately after the pressure rise it dropped quickly in the wake of the storm, reaching a minimum at mid-afternoon.

Radar image from the National Weather Service in Des Moines, Iowa captured as storm with classic Bow Echo formation swept across Iowa on August 10, 2020. Winds in excess of 100 mph were measured with sustained winds of 60 to 100 mph lasting for 60 to 90 minutes.

Notice the notches of weaker radar reflectivity on rear side of the storm. These are areas of strongly descending air dropping under the rain before exiting out the front of the formation. The rain cooled air accelerates downward as evaporation of the rain into the descending air carries stronger winds to the surface. The evaporation cools the air even more which increases its density causing the air to accelerate to the ground.

Approximately 80% of the buildings in Cedar Rapids were damaged. About 1/3 of Iowa’s corn crop was damaged. It remains to be seen how much of it will be available for harvest. The damage is so extensive that is it visible on weather satellite imagery.

The radar image is a screen grab from RadarScope sofware. Go to radarscope.com to purchase the software. The radar data is free from the National Weather Service.

Clear HERE to learn more about Derechos.