A classic hook echo. The tornado associated with this echo was part of the 1999 Oklahoma Tornado Outbreak. It reached F5 strength on the Fujita scale.

The hook echo is one of the classical hallmarks of tornado-producing supercell thunderstorms as seen on weather radar. The echo is produced by rain, hail, or even debris being wrapped around the supercell. The National Weather Service considers the presence of a hook echo as sufficient to justify issuing a tornado warning. Radar hook echo of a tornado Upload from U.S. government NOAA website File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Radar hook echo of a tornado Upload from U.S. government NOAA website File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... The Oklahoma Tornado Outbreak is the May 3, 1999 tornado event that killed 46 people. ... The Fujita scale rates a tornados intensity by the damage it inflicts on human-built structures. ... A tornado over land. ... Shelf-cloud of a supercell Supercell thunderstorms are the largest, most severe class of single-cell thunderstorm. ... A rolling thundercloud over Enschede, The Netherlands. ... Doppler radar uses the Doppler effect to return additional information from a radar system. ... Rain falling For other uses see Rain (disambiguation). ... A large hailstone Hail is a type of graupel (a form of precipitation) composed of spears or irregular lumps of ice. ... The National Weather Service (NWS) is one of the six scientific agencies comprising the National Oceanic and Atmospheric Administration (NOAA). ... A tornado warning is issued when: a tornado is reported on the ground, a funnel cloud is reported in the sky, a waterspout is headed toward landfall, or a thunderstorm with strong rotation is indicated by doppler radar. ...

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. April 9 is the 99th day of the year in the Gregorian calendar (100th in leap years). ... 1953 is a common year starting on Thursday. ...

Hook echoes are not always obvious. In the U.S. Southern states, thunderstorms tend to produce heavier rainfall which obscures the hook shape. The U.S. Southern states or the South, also known colloquially as Dixie, constitute a distinctive region covering a large portion of the United States, with its own unique heritage, historical perspective, customs, musical styles, and cuisine. ...

The use of doppler radar systems such as NEXRAD 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. NEXRAD Radar at NSSL NEXRAD or Nexrad (the next-generation radar) is a network of Doppler radars operated by the National Weather Service, an agency of NOAA, the National Oceanic and Atmospheric Administration, in the United States. ...

References

• The First Tornadic Hook Echo Weather Radar Observations
• National Weather Service presentation on Radar Interpretation - slide 26, detailing the difficulties of spotting hook echoes in the south
• July 10th, 2000 Severe Weather (NWS) - includes discussion of tornadic radar signatures
• NWS - Tornado Warning Guideance: Spring 2002 (72kb PDF)