A Blizzard Warning (SAME code: BZW) is an advisory issued by the National Weather Service of the United States which means sustained winds or frequent gusts of 35 mph / 15 m/s or greater with heavy snow is forecast for a period of 3 hours or more. A blizzard tends to reduce visibilities to 1/4 of a mile (400 m) or less. A Severe Blizzard Warning is a variation issued in some cases of winds above 45 mph / 20 m/s and temperatures below 10 °F/-12 °C. Most local weather offices will activate and broadcast the SAME alarm tone on relevant NOAA Weather Radio stations for both varieties of warning, and many offices will also request activation of the Emergency Alert System.