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How do meteorologists issue tornado watches before storms develop?

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Weather forecasting Doppler radars, such as this one operated by the U.S. National Weather Service at New Underwood, South Dakota, extract the Doppler frequency shift of the echo produced by a moving target by noting how much the frequency of the received signal differs from the frequency of the signal that was transmitted.

Meteorologists issue a tornado watch by identifying regions where atmospheric conditions are favorable for the development of severe thunderstorms capable of producing tornadoes. This process involves monitoring specific environmental factors, often one to seven hours before storms develop.12

Forecasters analyze the atmosphere for the following key ingredients:
* Instability: They look for a layer of moist, conditionally unstable air near the surface, topped by cool, dry air at middle levels of the troposphere.13
* Wind Profiles: Meteorologists monitor for a veering wind profile—where wind direction shifts clockwise with height in the Northern Hemisphere—along with strong high-altitude winds provided by the jet stream.1
* Frontal Boundaries: They track the movement and intensity of fronts that separate cold, dry air from warm, moist air, as these boundaries often trigger storm development.13

Using satellite imagery, wind profilers, and weather maps, forecasters identify these potential areas and issue a watch to alert the public that conditions are ripe for severe weather, even if the skies are currently clear.12