Monday’s storms part of derecho that caused damage across Midwest

Weather News
August 10 derecho

Courtesy of NWS Chicago

SIOUX CITY, Iowa (KCAU) – A massive storm system moved across Iowa and into Illinois Monday morning and afternoon leaving behind hundreds of damage reports.

The system, which is considered a bow echo, caused so much widespread damage across Iowa and Illinois that it was classified as a derecho before reaching the Iowa, Illinois border.

In order for a bow echo to be classified as a derecho, the system has to meet one of two “requirements” according to the National Weather Service. Those requirements include:

  • Wind damage extending more than 240 miles.
  • Wind gusts of at least 58 mph reported along most of the length of the storm’s path.

A derecho is a widespread, long-lived wind storm, and typically has bands of rapidly moving showers and thunderstorms typically known as bow echos, squall lines, or quasi-linear convective systems (QLCS).

The storms that moved through Siouxland were not considered a derecho. That was just the start of the system before it grew into derecho classification. When they moved over Siouxland, it was just considered a bow echo or squall line.

By the time the storms were reaching eastern Siouxland, they had regained severe status. The reason for the storm restrengthening is due to the cornfields across central and eastern Siouxland, the warmer temperatures in those areas, and the daytime heating (the sun heating up the surface of the Earth). For storms to be classified as severe they need to have at least one of the following reported:

  • Hail at least one inch in diameter or larger.
  • Wind gusts of 58 mph or greater.
  • A tornado.

The system that moved through Siouxland started off with severe hail and wind gusts west of the metro area. By the time it reached eastern Siouxland, it was able to regain strength and produce severe hail and wind gusts.

Part of the reason the system regained its strength is due to the amount of corn growing in the rural areas of eastern Siouxland and across Iowa in general. The corn releases water vapor into the atmosphere through a process of evapotranspiration. The added moisture mixed with the warmer temperatures in central and eastern Iowa and sunshine prior to the storms moving into those areas added more ingredients needed for severe weather. There are four main ingredients for thunderstorm and severe thunderstorm production:

  • Shear: the difference in wind speed or direction over a relatively short distance in the atmosphere. In severe weather, the winds should be turning in a clockwise manner with height.
  • Lift: the source to help parcels of air to rise. In Monday’s storm system, the source of the lift was the cold front that moved through.
  • Instability: a condition where the atmosphere is generally considered unstable. Storms help the atmosphere return to its stable condition.
  • Moisture: water vapor hanging out in the atmosphere.

As the bow echo moved east, it was moving over a very unstable environment full of heat and moisture, which is part of the reason why it continued to intensify, eventually gaining the classification of derecho. The system already had the shear and lift with the cold front associated with the derecho.

As the now classified derecho moved through central and eastern Iowa, storm reports of severe wind gusts were pouring into the National Weather Service.

Over the course of the storm’s life on Monday, starting in western Nebraska and ending in Indiana, there were:

  • Over 500 severe wind reports.
  • 24 hail reports, four of which were considered large hail reports (greater than one inch in diameter).
  • Three tornado reports.
Courtesy of the Storm Prediction Center

According to the National Weather Service, the winds were so strong that semi-trucks were getting overturned near LaSalle-Peru, Illinois. The wind gusts at this location were reported around 90 mph.

Courtesy of NWS Chicago

The derecho moved across Iowa, Illinois, and into Indiana before it lost its intensity and was no longer showing signs of severe wind gusts, hail, and potential tornado production.

As you can see in the image below, the bow echo that started in Siouxland doubled in size by the time it got to central Iowa and continued to grow in size as it approached Illinois.

Courtesy of NWS Chicago

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