SECTION 3 - THUNDERSTORMS, SEVERE WEATHER SPOTTING & DETECTION

A. Basic facts about Thunderstorms

Thunderstorms occur in all 50 states. They can occur, at any time, day or night, throughout the entire year. Thunderstorms are most common in the late afternoon and evening during the warm months. It has been estimated that 1800 thunderstorms are in progress at any given moment around the world and that lightning strikes the earth 100 times every second. Thunderstorms are basically beneficial providing necessary rainfall. In the United States, only about five percent of thunderstorms become severe and only about one percent produce tornadoes.

A list of weather brochures and learning aids are available at http://www.nws.noaa.gov/om/brochures.shtml

B. Storm Spotting Techniques

Your local National Weather Service Office provides severe weather spotter training under a program called SKYWARN^TM. It is provided free-of-charge with the request that when you do encounter severe weather, you report it to the National Weather Service. The spotter training class includes computer and video presentations that help you learn how to pick out visual clues from clouds to help determine the severity of a storm and other tips on reporting hazardous weather. It is highly recommended that the assigned district "Severe Weather Administrator and each school assigned "Severe Weather Coordinator" take the training. Although, any number of teachers and administrators are welcome to attend and receive the training. The enclosed books provide information about storms and spotting, however, they are not a substitute for official training.

C. NWS Methods of Detecting and Tracking Severe Weather

The National Weather Service uses a combination of radar, satellite, lightning detection, and surface observations, including volunteer spotter reports for detecting and tracking severe weather. The new sophisticated Doppler radars (WSR-88D) around the country have greatly increased the National Weather Service's ability to pinpoint severe thunderstorms and possible tornadoes and warn the public as to where the storm is moving and what actions to take. Spotter reports tell forecasters the size of hail, the depth of snow or flood waters, if wind damage is occurring, or if a tornado is sighted. These reports provide ground truth to the images seen on the Doppler radar.

Doppler Radar

The WSR-88D (Weather Surveillance Radar - 1988 Doppler) is the new radar system for NWS, the Federal Aviation Administration, and the Department of Defense (DOD). It is a very sensitive radar designed specifically for the detection of weather phenomena. The computers that compile the radar data can produce as many as 100 different radar products every 5 minutes for forecaster interpretation.

Typically radar has been used to tell meteorologists where precipitation is occurring, how intense it is, and where it is moving. The ability of the new radars to detect radial velocity (movement of radar targets, such as rain, toward or away from the radar derived from the "Doppler Effect") allows meteorologists to see rotation of thunderstorm updrafts and sometimes the development of tornadic circulation. Supercell thunderstorms displaying strong radar signatures (storm rotation) may allow forecasters to provide up to 20 minutes lead-time on warning for a tornado before it touches down. Computer and mapping skills with the new radar system also help meteorologists determine rainfall amounts and pinpoint areas with potential flash flood problems.

Like all technology, radars have their limitations. Radar beams can not see through mountains. This means that weather at the valley floor between mountain ranges will not be detected. Because of the curvature of the earth, as the radar beam moves away from its source, it gets higher and higher in the atmosphere and is no longer sampling the lower portion of the storm clouds. The NWS compensates by using trained severe weather spotters which help forecasters to fill in the gaps and provide ground truth information.

Satellites

Geostationary satellites (stationary above a point over the equator) and polar orbiting satellites allow meteorologists to watch the development of clouds and weather systems. Satellites are extremely useful for tracking weather systems over the vast ocean areas where no radar exist and only a few surface observations are taken. For example, satellites greatly improved meteorologists' ability to detect the formation and movement of hurricanes over the tropical waters. Satellites also help meteorologists to track movement of air masses that are either very dry (such as off the mountains) or very moist (such streams from the Eastern Pacific Ocean). This can greatly influence a storm's development. Cloud patterns also tell forecasters about the strength and movement of the jet stream which plays a large role in storm development.

While satellites provide meteorologists with much information, they also have limitations. The satellite views a cloud from above, but does not tell what has formed below it (such as a tornado). Severe thunderstorms and tornadoes can develop and dissipate fairly rapidly. Even if a satellite picture indicated a severe storm, the time delay for receiving the satellite picture may be 15 to 30 minutes which can be too long to properly warn for an event such as this.

Lightning Detection

Lightning detection systems map where cloud-to-ground lightning strokes are occurring. While this allows meteorologists to know that thunderstorms are indeed in progress and the frequency of lightning strokes, it tells little about the severity of the thunderstorm. People often report "vivid lightning" or "severe lightning," but lightning is not a criterion for whether a storm is severe, according to the NWS definition of "severe." Lightning assists forecasts in preparation for Fire Weather Forecasts and Products used by land management agencies and local fire fighters.

Surface Observations

Surface observation are the "ground truth" for tools such as radar and satellite. Because thunderstorms are very localized (the severe weather potion of the storm may only affect an area one mile wide), it would be impossible to have weather observers everywhere.

It is the combination of surface reports and remote sensing tools that brings the entire picture together for forecasters and increases their ability to issue effective, informative, and timely warnings. While the new technology has enhanced the meteorologist's ability to issue a timely warning, it will be of little use if the people who receive the warning do not know what safety actions to take.