Whirling dust and debris at the ground level or visible beneath clouds indicates approaching tornado activity. The funnel from the cloud may not be visible to the eye, as it may be obstructed by rain. Debris may be visible at a distance.
Both the National Oceanic and Atmospheric Administration and the University of Wisconsin Stevens Point caution observers to watch the condition of the sky carefully. Prior to a tornado, the sky may appear green, as light from the sun is obstructed by the large thunderhead, allowing only green wavelengths to shine through. A tornado may form within minutes or up to 45 minutes later.
Large thunderheads that create a low-hanging wall or clouds with strong rotation may be visible. Funnels are typical but are not always visible. Large thunderheads may have bulbous clouds, called mammatus clouds, hanging from the base of the thunder clouds.
Storms may be so severe with lightning flashing so frequently that they appear to be continuous. The tornado may form as the lightning increases or be delayed for up to 30 minutes.
The characteristic sound of a tornado is often described as sounding like a train or jet plane moving at a tremendous speed. Some tornadoes contain multiple funnels that spin wildly. These funnels may produce a sound that resembles the buzz of bees.
Large hail may precede the tornado by up to 20 minutes but may also occur during the tornado. The more severe the storm, the larger the hail stones appear.
A sudden calm may indicate you are directly in the path of a funnel that is about to touch down. The funnel may touch down within seconds. Dive for shelter and tuck your head into your body. Cover your face and back of your neck with your arms to prevent injury from flying debris.