Although predicting tornadoes is still far from a perfect science, Doppler radar used by the NEXRAD system has improved the accuracy of prediction to about 80 percent (versus about 30 percent before NEXRAD). Smaller, quickly forming tornadoes are still nearly impossible to anticipate, but larger tornadoes, especially huge tornadoes in the EF4 or EF5 category range, are being anticipated with much better results. In fact, the National Weather Service is often able to warn nearby residents of a large tornado 20 or more minutes before it actually strikes. NEXRAD can do this by searching for tornadic mesocyclones, which are a good indicator of twisters. Any large tornado vortexes can actually be seen by Doppler radar these days. Thanks to these advances, it is much less likely that a big tornado will take nearby populations completely by surprise. Even a 10-or 20-minute warning can be enough to save dozens or even hundreds of lives.