TAFs are similar to weather forecasts shown on tv. However, the information is presented in a textual format and generally requires a higher skill level to comprehend. The weather information that TAFs provide is tailored to the needs of the aviation and can include forecasted:
The weather information is only officially accurate for the area within a five statute mile radius from the issuing airport center. However, if forecasters believe a significant weather event will occur within the area between 5 and 10 statute miles, they can list the weather event as "in the vicinity." Weather in the vicinity will be proceeded by the text code "VC" before the type of weather, i.e., VCFG, VCSH, VCTS.
Forecasters will list weather phenomena within the 5-10 statute mile range if the weather has an equal to or greater than 50% probability of occurring for at least half of the forecasted period.
TAFs will also typically include more detail about expected weather conditions within the first 12 hours of the forecast. Times beyond the first 12 hours may contain less detail. However, they will still highlight significant weather changes for airline dispatchers and other aviation groups' strategic planning.
TAFs are typically grouped as either "Routine" or "Amended." However, a better way of grouping TAFs is as either "Routine" or "Unscheduled."
The difference between a "Routine" and "Unscheduled" TAF is that routine TAFs are issued routinely at set time periods. Unscheduled TAFs are issued on an as-needed basis.
When an unscheduled TAF is issued, it will be issued as either an amended, delayed, or corrected TAF.
Amended TAFs are issued when the current TAF and its forecasted weather do not match the actual weather. The benefit of issuing the amendment to the TAF is that the quality of the TAF will improve. The whole purpose of the TAF is to allow the end-user: airlines, dispatchers, airports, and you, to plan for the future. A TAF that deviates from the actual weather conditions creates a rippling effect that increases delays and creates a rippling effect across the entire country. Therefore, the sooner forecasters can amend TAFs, the better.
For an Amended TAF to be issued, the forecasters' assessment of existing conditions and expectations must differ from the current conditions. However, small differences in the forecast to current observed conditions do not call for minor adjustments to the TAF. Changing a TAF for small differences to the observed weather currently occurring is referred to as "Chasing the observation" and is not beneficial to end-users.
Instead, Amended TAFs will be issued when significant operational criteria are met. Such as:
These criteria are considered significant because a change from one to the other requires different operating and regulatory requirements. Such as Fuel requirements between VFR and IFR, requirements for alternates, flight cancellations, delays, and traffic flow, to name a few.
Delayed TAFs are issued when a problem occurs. Potential causes of delayed TAF might include power outages, equipment failures, and mechanical errors. Delayed TAFs are issued as soon as possible after the cause of the problem is fixed.
Corrected TAFs are issued when a typographical error or other mistake is discovered. Once discovered, a corrected TAF is issued as soon as possible.
A Routine TAFs is issued every 6 hours. Therefore, a routine TAF will be issued four times daily.
Routine TAFs are issued at:
Even though Amended TAFs are considered an "unscheduled" TAF, amended TAFs can be routinely issued at set times after the initial TAF issuance.
Routinely scheduled amended TAFs are issued at:
Generally, routinely scheduled amended TAFs will be found at the FAA's core airports.
For each 6 hour period is a scheduled issuance window for when the TAF for that period must be issued.
The issuance window for every time period starts 40 minutes before issuance and ends 20 minutes before issuance.
Issuance Time: 00z. Issuance window: 2320z - 2340z.
Issuance Time: 06z. Issuance window: 0520z - 0540z.
Issuance Time: 12z. Issuance window: 1120z - 1140z.
Issuance Time: 18z. Issuance window: 1720z - 1740z.
On top of these scheduled times, weather forecasts can choose to issue a TAF more frequently than every 6 hours as a method of keeping the TAF as most representative of current conditions as previously described.
The issuance of a new TAF cancels any previous TAF for the same time and location.
Routine TAFs may also be amended before the top of the hour without being marked as an Amended TAF. Therefore, an 18z TAF may be amended between 1720z to 1759z.
Generally, each TAF is valid for 24 hours. However, high-impact airports (those with large amounts of domestic and international traffic) are valid for 30 hours.
As of this writing, 30 hour TAFs can be found at the following locations: