Tropical Cyclone ICAO (Aviation) Advisory  (Source: NHC web site)

World Meteorological Organization (WMO) and AWIPS (in parenthesis) headers:
FKNT21-25 KNHC (MIATCANT1-5) - Atlantic
FKPZ21-25 KNHC (MIATCAPZ1-5) - E. Pacific

The Aviation Tropical Cyclone Advisory is issued to provide short term
tropical cyclone forecast guidance for international aviation safety and
routing purposes. The TCA lists the current TC position, motion, and
intensity, and includes 6, 12, 18, and 24 hour forecast positions and
intensities. Position and intensity information for forecast hours (+06,
+12, +18 and +24) is derived from interpolated forecast information -
intensity is rounded to the nearest 5 knots.

Issuance of the Aviation Tropical Cyclone Advisory occurs every six hours
at the regular advisory issuance time of 0300, 0900, 1500, and 2100 UTC
(learn about UTC time). For 2008, the bulletin's information is valid at
the routine advisory times (0300, 0900, 1500 and 2100 UTC) and is not
anchored to the synoptic times. TCAs for special advisories will be issued
for the same circumstances that apply for a standard advisory.