A typical tornado

Weather forecasting science is not perfect and some tornadoes do occur without a tornado warning. There is no substitute for staying alert to the sky. Besides an obviously visible tornado, here are some things to look and listen for: 1. Strong, persistent rotation in the cloud base. 2. Whirling dust or debris on the ground under a cloud base -- tornadoes sometimes have no funnel! 3. Hail or heavy rain followed by either dead calm or a fast, intense wind shift. Many tornadoes are wrapped in heavy precipitation and can't be seen. 4. (Day or night) - Loud, continuous roar or rumble, which doesn't fade in a few seconds like thunder. 5. (Night) - Small, bright, blue-green to white flashes at ground level near a thunderstorm (as opposed to silvery lightning up in the clouds). These mean power lines are being snapped by very strong wind, maybe a tornado. 6. (Night) - Persistent lowering from the cloud base, illuminated or silhouetted by lightning -- especially if it is on the ground or there is a blue-green-white power flash underneath. Image: Alfalfa, OK, tornado of 22 May 1981.

Credit: Storm Prediction Center

For more info: http://www.spc.noaa.gov/faq/tornado/