Tropical Cyclone Valid Time Event Code (VTEC)  (Source: NHC web site)

World Meteorological Organization (WMO) and AWIPS (in parenthesis) headers:
WTNT81-85 KNHC (MIATCVAT1-5) - Atlantic
WTPZ81-85 KNHC (MIATCVEP1-5) - E. Pacific

The Tropical Cyclone Watch/Warning text product (TCV) is based upon the
Valid Time Event Code (VTEC). It summarizes all new, continued, and canceled
tropical cyclone watches and warnings issued by the National Hurricane Center
(NHC) for the U.S. Atlantic and Gulf coasts, Puerto Rico, and U.S. Virgin
Islands. Beginning in 2008, the TCV will also include areas of southern
California from Los Angeles to the United States/Mexico border.

Once watches and/or warnings are in effect for the areas mentioned, updates
will be issued by the NHC at a minimum of every six hours at the regular
advisory issuance time of 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). TCVs for
special advisories will be issued for the same circumstances that apply for a
standard advisory.