I recently got a phone call from an avid Ask Josh weather blog reader named John Donohue from Kentucky. He asked "what is a hook echo and how is it considered severe in weather?"
Great question, John and thanks for reading my blog!
Here's the answer and some radar images to illustrate what a hook echo is.
The hook echo is one of the classical hallmarks of a tornado producing thunderstorm as seen on weather radar. The echo (name given to precipitation areas on radar) is produced by rain, hail or even debris being wrapped around the thunderstorm. The National Weather Service considers the presence of a hook echo as sufficient to justify issuing a tornado warning.
Below is a Doppler radar image from the National Weather Service in Dodge City, Kansas of a thunderstorm producing a tornado. The twister is located in the hook or in the southwest part of the storm.
The hook echo has been recognized as a sign of tornado development for most of weather radar's existence. The first documented tracking of a hook echo was on April 9, 1953 by the Illinois State Water Survey, during preparations for an early test of radar's ability to measure rainfall rates.
Hook echoes are not always obvious. In the Southern U.S. thunderstorms tend to produce heavier rainfall which leads to the high precipitation (HP) thunderstorms and obscures the hook shape. These storms will instead take on a kidney bean shape.
The use of Doppler weather radar systems such as NEXRAD (next generation radar) allows for detection of tornadoes even when the hook echo is not present and for greater certainty when it is. By detecting the relative velocities of different parts of a storm, Doppler radar can detect areas of rotation.
Rarely is a signature this classical, this obvious. At the of this image (above), a tornado was crossing interstate 44 south of Oklahoma City moving towards the town of Moore, OK where an F5 tornado hit! This F5 tornado (the strongest and rarest of all twisters) had winds of up 318 mph! That wind speed, which was detected by Doppler radar was the strongest wind speed ever recorded in history.
The bright red colors at the tornado location represent not hail or rain, but the aggregate signature of car parts, pieces of houses, shredded tree branches, dirt and other debris, hoisted thousands of feet skyward by the vortex!
Keep those great questions coming! Have you ever been in a tornado? Share your severe weather experiences in the comment section below.
Thanks for reading!