Pilots: New Terminal Aerodrome Forecast Format Begins November 4 5:30 pm CST

Worldwide, the TAF or Terminal Aerodrome Forecast is the method and format used to convey weather forecasts in meteorology and aviation.  In the United States, TAF forecasts are prepared for nearly 625 airport terminal sites by the National Weather Service.  These forecasts contain information critical to aircraft, including cloud coverage and cielings, visibility, precipitation, and wind.  The U.S. definition of a terminal is the area within 5 statue miles of the center of an airpport's runway complex.  TAFs are used by a veriety of aviation users, including domestic and internation comcerial airlines, general aviation, civilian, and military operators.

Beginning 5:30 pm CST on November 4 (effective with the 00 UTC 11/5/2008 issuance of TAFS), all TAFs issued by the NWS will change the date and time indicator format in the TAF from the present 24 hour format to a new 30-hour format consistent with new international Civil Aviation Organization (INCO) rules.  This format change is part of an internation effort to implement a consistent TAF format even as current TAF valiud times vary from a few hours up to 30 hours.

In addition, TAFs for 32 larger airports will become valid for 30 hours.  This includes Chicago-O'Hare International Airport.

The change affects the date/time groups in the TAFs. The valid time of a TAF becomes ddhh/ddhh from the present ddhhhh. TEMPOs and PROBS will also change, hhhh in both groups' changes to ddhh/ddhh. FROM groups change from hhmm to ddhhmm.
 
dd is the date of the month ie 17 or 17th.
hh is the hour ie 22 or 2200GMT
mm is minutes 15 or 15 minutes after the hour
 
Example: The present form
KABC 131128Z 131212 14005KT P6SM SCT025 OVC040
          TEMPO 1216 OVC025
     FM1600 13015G23KT P6SM OVC015
     FM2100 13015G22KT P6SM OVC008
          TEMPO 2101 1SM -SN
     FM0100 09015KT 3SM BR OVC006
          TEMPO 0105 2SM -SN BLSN
     FM0500 01015KT 5SM BR OVC006=
 
            Becomes ...

            30-hr TAF
KABC 131128Z 1312/1418 14005KT P6SM SCT025 OVC040
          TEMPO 1312/1316 OVC025
     FM131600 13015G23KT P6SM OVC015
     FM132100 13015G22KT P6SM OVC008
          TEMPO 1321/1401 1SM -SN
     FM140100 09015KT 3SM BR OVC006
          TEMPO 1401/1405 2SM -SN BLSN
     FM141500 01015KT 5SM BR OVC006=
 
            Or ...

            24-hr TAF
KABC 131128Z 1312/1412 14005KT P6SM SCT025 OVC040
          TEMPO 1312/1316 OVC025
     FM131600 13015G23KT P6SM OVC015
     FM132100 13015G22KT P6SM OVC008
          TEMPO 1321/1401 1SM -SN
     FM140100 09015KT 3SM BR OVC006
          TEMPO 1401/1405 2SM -SN BLSN
     FM140500 01015KT 5SM BR OVC006=



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