The National Weather Service issues Center Weather Advisories (CWA) when conditions exists meeting or approaching the national in-flight advisory (AIRMET, SIGMET, or Convective SIGMET) criteria. CWAs are unscheduled weather advisories which are valid for up to 2 hours and may include forecast conditions that are expected to begin within 2 hours of the issuance. If meteorological conditions are expected to continue beyond the advisories valid period a statement to this effect will be included in the advisory.

The CWA will describe the location of the phenomenon using ARTCC relevant points of reference, such as VORs, and will include the height, extent, intensity, and movement of the phenomenon. Each CWA will have a phenomenon number.

The format of the CWA communications header is: (ARTCC designator)(phenomenon number) CWA (date/time issued in UTC)/(ARTCC designator) CWA (issuance number) VALID UNTIL (date/time in UTC)/ (FROM) (affected area)/(text).