nws logo  SECTION 3 - Thunderstorms and Severe Weather Spotting

D. 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. NWS Doppler radars installed around the country in the early 1990s have greatly increased the National Weather Service's ability to pinpoint severe thunderstorms and possible tornadoes and warn the public as to where these severe storms are moving. Spotter reports give forecasters the ground truth that verifies what the radar is showing and adds details such as the size of hail, the amount of rain, the depth of flood waters over a road, if wind damage is occurring, or if a tornado is sighted.

1)  Doppler Weather Radar:
image of doppler radarThe WSR-88D (Weather Surveillance Radar - 1988 Doppler) is the radar system used by NWS and the Department of Defense (DOD). It is a very powerful radar designed specifically for the detection of weather phenomena and in particular, voilent tornadoes. The computers that compile the radar data can produce as much as 100 different radar products every 5 minutes for forecaster to look at.

Typically radar has been used to tell meteorologists where precipitation is occurring, how intense it is, and where it is moving.The ability of Doppler 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 the tornado vortex.
forecaster at radar displayradar image
Mesocyclones displaying strong radar signaturessuch as storm rotation (bright red nest to bright green in the picture) can sometimes mean 10 to15 minutes lead-time on warning for a tornado before it touches down. Computer and mapping skills with this radar combined with automated rainfall gauges help meteorologists determine maximum 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 within the valleys on the other side will not always 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.

2) Satellites :
satellite in orbitGeostationary 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 there is no radar and few surface observations. 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 as northward from the Gulf of Mexico). 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. Meteorologists can watch the formation of cumulus clouds along boundaries and their growth into thunderstorms.  They can watch the change in cloud top temperatures to help determine if thunderstorm complexes are growing or weakening.

While satellites provide meteorologists with much information, they too have their limitations. A satellite is viewing a cloud from above. Only in very rare cases can you tell that a tornado has formed below. You can not know from a satellite that winds are blowing down trees or that softball size hail is falling or water is flooding a bridge.

3) 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.

4) 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 a mile wide), it would be impossible to have weather observers everywhere. Most surface observations are automated and these automated sensors give you temperature, humidity, pressure and wind information, but do not tell you hail size, do not see tornadoes nearby, or thunderstorm clouds.

5) Skywarn Spotters:
skywarn spotter on radio Skywarn is a volunteer program of trained severe weather spotters. Some are also amateur radio operators who use their added skills to pass information on to the NWS. They provide critical details to forecasters about what the storms are doing.  Skywarn training is conducted by the National Weather Service at no cost and all are welcome to take the basic spotter training class. For more information about Skywarn, check out our webpage or contact your local NWS office.

forecaster working at AWIPS console It is the combination of surface reports from automated sensors and spotters and remote sensing tools such as radar, satellite and lightning detection that bring the entire picture together for forecasters and increases their ability to issue effective, informative, and timely warnings. While new technology has enhanced the meteorologist's ability to issue a timely warning, it will be of little use if the people do not receive the warning or receive the warning but do not know what safety actions to take.

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