Know the signs of a 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.

Reference: http://www.spc.noaa.gov/faq/tornado/safety.html