The National Weather Service is the government agency in the U.S charged with issuing weather forecasts, advisories, watches, and warnings on a daily basis. It is operated by The National Oceanic and Atmospheric Administration and the Department of Commerce. The Weather Service operates NEXRAD, a nationwide network of doppler radars which can detect precipitation and athmospheric movement.
Volunteers called skywarn spotters relay severe weather to their local weather service office.
NOAA Weather Radio
The National Weather Service also operates NOAA Weather Radio, a network of radio transmitters that broadcasts weather forecasts, severe weather statements, watches and warnings 24 hours a day. Specially equpped radios will sound an alarm when the local weather service office issues a special weather statement, watch or warning.