Terminal Aerodrome Forecasts (TAFS) — Page 492, Lesson 469

I want to walk you through the change indicators used in Terminal Aerodrome Forecasts, or TAFs. These are the codes that tell us when and how the weather is expected to change during the forecast period. There are three main groups: the FROM group, the BECOMING group, and the TEMPORARY group. Let's start with the FROM group, introduced by the code FM. In a TAF, the FM group marks the fact that a rapid change in the forecast conditions is expected, which will lead to the appearance of a new set of prevailing conditions becoming established at the aerodrome. So it's not a gradual shift—it's a sharp, defined change. The change indicator FM is followed by a six-digit date and time group. The first two digits are the day of the month, followed by the hours and minutes, to indicate the time at which the change is expected to begin. For example, in the TAF: `TAF EGTK 130600Z 1307/1316 31015KT 8000 –SHRA SCT010 BKN018 FM 131220 27017KT 4000 BKN010=`, the code FM 131220 means that certain weather changes will occur from the 13th at 1220 UTC. This weather forecast following the code FM supersedes the TAF forecast prior to 1220 UTC. So the FM indicator introduces what is effectively a new forecast, associated with a new weather situation, and it supersedes the previous forecast. The FM group contains all the elements of a complete TAF forecast: wind, visibility, weather, and cloud. In our example, from the 13th at 1220Z until the end of the TAF period, the wind will change to be 270° true at 17 knots, with a prevailing visibility of 4000 metres, and broken cloud at 1000 feet. The forecast following the FM indicator continues either to the end of the current TAF, or until another change indicator occurs in the TAF. Now let's move to the BECOMING group, introduced by the code BECMG. This means "becoming." It is followed by an eight-figure date and time group which indicates the period during which there will be a permanent change in the forecast conditions. The key point here is that the forecast change introduced by BECMG will occur at an unspecified time within the time period stated. So we know it will happen, but we don't know exactly when—just that it will be permanent once it does. For example, consider this TAF: `TAF EGTK 130600Z 1307/1316 31015KT 8000 –SHRA SCT010 BKN018 BECMG 1309/1311 5000 –RA=`. This indicates that, at some time on the 13th between 0900 UTC and 1100 UTC, but definitely by 1100 UTC, the prevailing conditions will give 5000 metres visibility, in light rain. Notice there is no new wind information after BECMG, so the inference is that the wind will be as previously forecast: 310° true at 15 knots. Finally, we have the TEMPORARY group, introduced by the code TEMPO. This means "temporarily." TEMPO indicates that a change in meteorological conditions will occur at any time within the specified time period, but is expected to last less than one hour each time, and, in aggregate, will last no longer than half the time period of the complete forecast. So it's a short-lived fluctuation, not a permanent shift. The TEMPO indicator is followed by an 8-digit date and time group indicating the hours between which the temporary conditions are expected to begin and end. To summarise: FM marks a rapid, permanent change at a specific time, with a full new forecast. BECMG marks a permanent change that happens gradually within a time window, but at an unspecified moment. TEMPO marks a temporary fluctuation that lasts less than an hour each time and less than half the total forecast period in aggregate. Each of these gives us a different way to describe how the weather will evolve at the aerodrome.