Terminal Aerodrome Forecasts (TAFS) — Page 492, Lesson 471

I want to walk you through Terminal Aerodrome Forecasts — TAFs — and I’ll start with a full example so you can see how the pieces fit together before we break each part down. Here’s a TAF for EGTK, which is the ICAO code for Oxford/Kidlington in the UK. The message reads: TAF EGTK 130600Z 1307/1316 31015KT 8000 –SHRA SCT010 BKN018 TEMPO 1312/1314 4000 TSRA BKN010CB= Let me walk you through what that tells us. The TAF is issued on the 13th day of the month at 0600 UTC. It covers a forecast period from 1307/1316 — that means from 0700 UTC on the 13th to 1600 UTC on the 13th. The surface wind will be from 310 degrees at 15 knots. The prevailing visibility is 8000 metres. The weather is light rain showers, coded –SHRA. The cloud layers: there’s a scattered layer at 1000 feet, coded SCT010, and a broken layer at 1800 feet, coded BKN018. Now, the TEMPO group tells us that temporarily, between 1200 UTC and 1400 UTC on the 13th, the visibility will drop to 4000 metres, and the weather will become thunderstorms with moderate rain, coded TSRA. There will be 5 to 7 oktas of cumulonimbus cloud at 1000 feet, coded BKN010CB. After 1400 UTC, the weather returns to the conditions specified in the first part of the message — the prevailing visibility of 8000 metres, light rain showers, and the scattered and broken cloud layers. So that’s the basic structure: a prevailing forecast, then a TEMPO period where conditions change temporarily and then revert. Now let’s look at the Probability indicator, PROB. The code PROB in a TAF indicates the probability of occurrence of specified weather phenomena. It’s a percentage probability — a 30% probability is considered low, while a 40% probability indicates that it is highly likely that the weather being forecast will actually occur. PROB can be followed by a time group of its own, and/or by an indicator such as BECMG or TEMPO. Here’s an example TAF that uses PROB: TAF EGTK 130600Z 1307/1316 31015KT 8000 –SHRA SCT010 BKN018 PROB40 TEMPO 1310/1314 +TSRAGR SCT005CB= This tells us there is a high probability — 40% — that between 1000 UTC and 1400 UTC, there will be thunderstorms with heavy rain and hail, coded +TSRAGR, and from 3 to 4 oktas of cumulonimbus clouds at 500 feet, coded SCT005CB. The storms will not last longer than one hour at a time and less than two hours in total — that’s one half of the period to which the TEMPO applies, since the TEMPO period is four hours from 1000 to 1400. Next, let’s talk about temperature. Some meteorological authorities include forecast maximum and minimum temperatures in a TAF. The format is: TX15/2016Z — this means the maximum temperature is expected to be 15°C at 201600Z, which is the 20th day of the month at 1600 UTC. TN09/2105Z — this means the minimum temperature is expected to be 9°C at 210500Z, which is the 21st day of the month at 0500 UTC. Now, amendment. When a TAF requires an amendment, the amended forecast may be indicated by the code AMD, highlighted in red, after the TAF identifier. For example: TAF AMD EGTK 130600Z 1307/1316 31015KT 8000 –SHRA SCT010 BKN018 PROB40 TEMPO 1310/1314 +TSRAGR SCT005CB= That AMD tells you this is a corrected or updated forecast. Finally, the end of message. An equals sign (=) appears at the end of the TAF to denote that the message is complete. You’ll see it right after the last group in both examples. Used correctly, TAFs will enable a pilot to make accurate and informed decisions about a planned flight, including the expected conditions en-route, and at destination and alternate aerodromes. That’s the whole point — you’re not just reading codes; you’re building a picture of what the weather will do during your flight.