If information sources, such as surface observations, are missing, unreliable, or not complete, forecasters will append AMD NOT SKED to the end of a TAF. The use of AMD NOT SKED indicates the forecaster has enough data, using the total observation concept, to issue a forecast, but will not provide updates. This allows airport operations to continue using a valid TAF.

In rare situations where observations have been missing for extended periods of time (i.e., more than one TAF cycle of six hours) and the total observation concept cannot provide sufficient information, the TAF may be suspended by the use of NIL TAF.

For TAFs with less than 24-hour observational coverage, the TAF will be valid to the end of the routine scheduled forecast period even if observations cease prior to that time. The time observations are scheduled to end and/or resume will be indicated by expanding the AMD NOT SKED statement. Expanded statements will include the observation ending time (AFT Y Y HHmm, e.g., AFT 120200), the scheduled observation resumption time (TIL Y Y HHmm, e.g., TIL 171200Z) or the period of observation unavailability (Y Y HH/Y Y hh, e.g., 2502-2512). TIL will be used only when the beginning of the scheduled TAF valid period coincides with the time of the last observation or when observations are scheduled to resume prior to the next scheduled issuance time. When used, these remarks will immediately follow the last forecast group. If a routine TAF issuance is scheduled to be made after observations have ceased, but before they resume, the remark AMD NOT SKED will immediately follow the valid period group of the scheduled issuance. After sufficient data using the total observation concept has been received, the AMD NOT SKED remark will be removed.

Examples of Scheduled Part-Time Observations TAFs:

No amendments will be available between the 15th day of the month at 0500 UTC and the 15th day of the month at 1800 UTC due to lack of a complete observational set between those times.

Amendments are not scheduled.

TAFs for automated stations without present weather and obstruction to vision information and have no augmentation or only part-time augmentation are prepared using the procedures for part-time manual observation sites detailed in the previous section, with one exception. This exception is the remark used when the automated system is unattended. Specifically, the time an augmented automated system is scheduled to go into unattended operation and/or the time augmentation resumes is included in a remark unique to automated observing sites: AMD LTD TO CLD VIS AND WIND (AFT YYHHmm, or TIL YYhhmm, or YYHH-YYhh), where YY is the date, HHmm is the time, in hours and minutes, of last augmented observation, and hhmm is the time, in hours and minutes, the second complete observation is expected to be received. This remark, which does not preclude amendments for other forecast elements, is appended to the last scheduled TAF issued prior to the last augmented observation. It will also be appended to all subsequent amendments until augmentation resumes.

The AMD LTD TO (elements specified) remark is a flag for users and differs from the AMD NOT SKED AFT Z remark for part-time manual observation sites. AMD LTD TO means users should expect amendments only for those elements and the times specified.

Example:

The amended forecast indicates that amendments will only be issued for wind, visibility, and clouds, between the 15th day of the month at 0500Z and the 15th day of the month at 1800Z.

An amendment includes forecasts for all appropriate TAF elements, even those not reported when the automated site is not augmented. If unreported elements are judged to be crucial to the TAF and cannot be adequately determined (e.g., fog versus moderate snow), the TAF will be suspended (i.e., an amended TAF stating “AMD NOT SKED”).

AWOS systems with part-time augmentation, which the forecaster suspects are providing unreliable information when not augmented, will be reported for maintenance and treated the same as part-time manual observation sites. In such cases, the AMD NOT SKED AFT YY/aaZ remark will be used.

The TAF issued for automated observing stations with no augmentation may be suspended in the event the forecaster is notified of, or strongly suspects, an outage or unrepresentative data. Forecasters may also suspend TAF amendments when an element the forecaster judges to be critical is missing from the observation and cannot be obtained using the total observation concept. The term AMD NOT SKED will be appended, on a separate line and indented five spaces, to the end of an amendment to the existing TAF when appropriate.