Tropical Cyclone Discussion  (Source: NHC web site)

World Meteorological Organization (WMO) and AWIPS (in parenthesis) headers:
WTNT41-45 KNHC (MIATCDAT1-5) - Atlantic
WTPZ41-45 KNHC (MIATCDEP1-5) - E. Pacific

The Tropical Cyclone Discussion explains the reasoning for the analysis and
forecast of a tropical or subtropical cyclone. It includes a table of the
forecast track and intensity.

Tropical Cyclone Discussions are issued on all Atlantic and eastern Pacific
tropical and subtropical cyclones every six hours. Special tropical cyclone
discussions may be issued at any time due to significant changes in warnings
or in the cyclone.

Tropical Cyclone Discussions for Atlantic tropical cyclones are normally
issued every six hours at 5:00 AM EDT, 11:00 AM EDT, 5:00 PM EDT, and 11:00 PM
EDT (or 4:00 AM EST, 10:00 AM EST, 4:00 PM EST, and 10:00 PM EST). 

Tropical Cyclone Discussions for Eastern Pacific tropical cyclones are
normally issued every six hours at 2:00 AM PDT, 8:00 AM PDT, 2:00 PM PDT,
and 8:00 PM PDT (or 1:00 AM PST, 7:00 AM PST, 1:00 PM PST, and 7:00 PM PST). 

The numbers 1-5 in the headers are assigned by cyclone number, i.e.,
discussions on the first, sixth, or eleventh cyclones of the Atlantic season
would be sent under the WMO header WTNT41 KNHC; discussions on the second,
seventh, or twelfth cyclones of the Atlantic season would be sent under the
WMO header WTNT42 KNHC, and so on.