United States. Particularly dangerous situation wind chill warning (NPW) – Extremely low wind chills of or lower creating an enhanced risk of frostbite, hypothermia and death are imminent or occurring. Wind chill advisory (NPW) – Dangerous wind chills making it feel very cold are imminent or occurring. The apparent temperature and wind speed criteria vary significantly over different county warning areas based on climate variability. Wind chill watch (NPW) – Extreme wind chills that are capable of causing life-threatening medical conditions associated with accelerated heat loss from exposed skin are possible within the next 12 to 48 hours; the apparent temperature and wind speed criteria vary significantly over different county warning areas. Aviation The following advisories are issued by the National Weather Service Aviation Weather Center (outside of Alaska) or Alaska Aviation Weather Unit. Atmospheric ash plume advisories/warnings are also issued by the United States Geological Survey (Aviation Color Codes). Airport weather warning (AWW) – Addresses weather phenomena (including but not limited to surface wind gusts around or above , freezing rain, heavy snow or thunderstorms producing cloud-to-ground lightning within of the airport and/or hail) capable of adversely affecting ground operations at regional and international airports. AWWs are intended for the use of airport managers, fixed-based operators, airline ground personnel, and other personnel responsible for the safety of ground operations. Center weather advisory (CWA) – Advisories issued when conditions just below severe (SIGMET) criteria. CWAs are issued for thunderstorms, turbulence, icing, and ceiling and visibility limits (IFR). SIGMET (significant meteorological information; SIG) – Advises of weather that is potentially hazardous to all aircraft and is affecting or is forecast to affect at least . Convective SIGMET (SIG) – Implies severe or greater turbulence, severe icing, and low level wind shear. Issued for severe surface weather (including surface winds greater than or equal to , hail at the surface greater than or equal to in diameter, or tornadoes); embedded thunderstorms; line of thunderstorms; thunderstorms greater than or equal to VIP level 4 affecting 40% or more of an area at least . Non-convective SIGMET (SIG) – These SIGMETs may be issued for: severe icing; severe or extreme turbulence; dust storms and/or sand storms lowering visibilities to less than ; or volcanic ash. SIGMET advisories are issued for six hours during hurricanes and four hours for other weather-related events. Volcanic ash advisories (VAA) – Advisory issued for all ash plumes detected by