Tropical Cyclone Forecast/Advisory  (Source: NHC web site)

World Meteorological Organization (WMO) and AWIPS (in parenthesis) headers:
WTNT21-25 KNHC (MIATCMAT1-5) - Atlantic
WTPZ21-25 KNHC (MIATCMEP1-5) - E. Pacific

The Tropical Cyclone Forecast/Advisory contains a list of all current watches
and warnings on a tropical or subtropical cyclone, as well as the current
latitude and longitude coordinates, intensity, and system motion. The advisory
contains forecasts of the cyclone positions, intensities, and wind fields for
12, 24, 36, 48, and 72 hours from the current synoptic time. The advisory may
also include information on any pertinent storm tides associated with the
cyclone. All wind speeds in the forecast advisory in given in knots (nautical
miles per hour).

Forecast/Advisories are issued on all Atlantic and eastern Pacific tropical
and subtropical cyclones every six hours at 0300, 0900, 1500, and 2100 UTC
(learn about UTC time). Special Forecast/Advisories may be issued at any time
due to significant changes in warnings or in the cyclone.

The numbers 1-5 in the headers are assigned by cyclone number, i.e.,
advisories on the first, sixth, or eleventh cyclones of the Atlantic season
would be sent under the WMO header WTNT21 KNHC; advisories on the second,
seventh, or twelfth cyclones of the Atlantic season would be sent under the
WMO header WTNT22 KNHC, and so on.