NOAA and 1974 Tornado Outbreak

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RADAR TECHNOLOGY — THEN AND NOW


Radar Tower
Weather radar is vital for detecting and monitoring the movement and development of severe storms. Twenty-five years ago, when the Nation's worst tornado outbreak occurred across a dozen states on April 3-4, 1974, National Weather Service forecasters could see only green blobs on their radar scopes and relied on visual confirmation to issue tornado warnings. Thanks to a $4.5 billion modernization program and an explosive growth in technology, today's National Weather Service has significantly advanced its warning capabilities.

Prior to the modernization program, the National Weather Service (NWS) depended on 1950s and 1970s vintage radar technology to operate a combination of network and local-warning radars to monitor weather systems. The radar units in the national network were obsolete and difficult to service, and large areas were not served by any NWS radar. The radars were so old that some of the parts were no longer manufactured.

Today, NWS forecasters detect severe weather events threatening life and property using Doppler Weather Surveillance Radars (Model WSR-88D). The WSR-88D observes the presence and calculates the speed and direction of motion of severe weather elements such as tornados and thunderstorms. Doppler radars also provide quantitative area precipitation measurements, important in hydrologic forecasting of potential flooding. The severe weather and motion detection capabilities offered by radars contribute toward an increase in the accuracy and timeliness of NWS warning services.

The WSR-88D uses Doppler radar technology to:

Substantially increase tornado warning lead time. Improve the detection and measurement of damaging winds, severe turbulence, wind shear and hail associated with severe thunderstorms. Improve the forecast of the location and severity of thunderstorms. Increase the accuracy of identifying threatened areas. Substantially reduce the number of false alarms.

The WSR-88D capabilities also:

Increase the accuracy of rainfall estimates for flash flood warnings. Improve water resource management and river flood forecasts.

How Doppler Radar Sees Into the Cloud

Radar detects the presence and location of an object by bouncing an electromagnetic signal off of it and measuring the time it takes for the signal to return. This measurement is used to determine the distance and direction of the object from the radar. In the case of radar meteorology, the "objects" being measured are the particles of water, ice or dust in the atmosphere. Doppler radars take additional advantage of the fact that radar signals reflected from a moving object undergo a change in frequency related to the speed of the object traveling to or away from the radar antenna.

Doppler radar detects objects moving toward the radar and objects moving away from the radar. The return signal is different for objects moving away from the radar and objects moving toward the radar.

Doppler radar detects two motions associated with clouds. The WSR-88D calculates both the speed and direction of motion of a severe storm. It also detects internal motions of the storm, and certain unique internal motions can be a precursor for tornado formation. A developing tornado, for example, can be detected forming above the earth before it reaches the ground. This means earlier detection of the precursors to tornadoes, as well as data on the direction and speed of tornadoes once they form.

A Tri-agency Approach

In cooperative effort with the Department of Defense and the Federal Aviation Administration, the NWS has deployed more than 160 radars across the country. Through an integrated network spanning the entire United States and its island territories, from Guam to Puerto Rico, the WSR-88D dramatically enhances the NWS' ability to safeguard life, property and commerce.

For more information contact Barry Reichenbaugh at (301) 713-0622.