Aviation Weather Forecasts

Meteorology · Chapter 26 · DGCA CPL and ATPL

How a forecaster writes the next 9 to 30 hours of an airfield, an area or a route in a few lines of code, and how to read it back hour by hour.

Remember

TAF: valid 9-30 h. 9 h every 3 h (domestic, VOLMET, not international); 12-30 h every 6 h from 0000 UTC. Validity DDHH/DDHH.
TAF-only groups: PROB30/40, TX/TN with time, TAF AMD. Change groups BECMG, TEMPO, FM as in a TREND.
ARFOR (area) and ROFOR (route): figure code. Cloud like METAR (1/8, 3/8, 5/8, CB). Heights in hundreds of feet.
Indicators: 7 freezing level, 6 icing (I), 5 turbulence (B), 4 wind and temperature, 2 tropopause; 11111 jet, 22222 max wind and shear.
W1W1W1: 111 TS, 222 TC, 333 line squall, 444 hail, 555 mountain waves, 666 sandstorm, 777 duststorm, 888 freezing rain.

How do you read a TAF hour by hour?

A TAF on a timeline

A 9-hour TAF for Lucknow, laid out along a time line. Tap Next.

The header A TAF is a forecast for one aerodrome. VILK is Lucknow. 241800Z says it was issued on the 24th at 1800 UTC, which is 2330 IST. 2500/2509 is the validity: the 25th, from 0000 to 0900 UTC. So this is a 9-hour TAF.
The main forecast Next comes the main forecast. 09008KT: wind from 090° at 8 kt. 0800 FG: visibility 800 m in fog. It holds from the start of the TAF until a change group says something else. Here that means a foggy early morning, until about 04 UTC.
BECMG 2504/2505 BECMG means "becoming": a lasting change that happens some time inside the given hours. Between 04 and 05 UTC the fog clears for good. The wind picks up to 090/15 and the visibility rises to 6000 m. Cloud: SCT at 800 ft and BKN at 12 000 ft. That SCT008 is the lowest cloud in the whole TAF.
TEMPO 2506/2508 TEMPO means short spells that come and go. Between 06 and 08 UTC expect spells of wind 120/15 gusting 30 kt and 3000 m in a thunderstorm with rain (TSRA). Cloud in the spells: FEW012, FEW025CB, BKN100, so a few CB with bases at 2500 ft. Between the spells the BECMG weather holds.
BECMG AT 0800 From 0800 UTC the weather settles: wind 090/10, visibility 7000 m, cloud FEW030 SCT120 BKN280. The top layer is at 28 000 ft. To read any hour, slide along the time line, as the yellow marker does. The latest main or BECMG forecast holds, unless a TEMPO spell is on.

Memory hook: Main forecast → BECMG (lasts) → TEMPO (spells) → AT/FM (from a time).

Takeaway: Read a TAF in order. First the header: station, issue time and validity. Then the main forecast. Then the change groups in time order: BECMG for a lasting change, TEMPO for short spells, FM or AT for a change from a set time. At any hour the main forecast holds unless a change group says otherwise.

What will the weather be at Lucknow at 0600 UTC?

Read a TAF

Pick a TAF. Tap any group for its meaning, then slide through the hours and watch the forecast change.

Lucknow, fog to storms
Fog at dawn lifts between 04 and 05 UTC; thunderstorms with gusts to 30 kt come and go from 06 to 08 UTC; by 08 UTC it settles at 7 km.
TAF VILK 241800Z 2500/2509 09008KT 0800 FG BECMG 2504/2505 09015KT 6000 SCT008 BKN120 TEMPO 2506/2508 12015G30KT 3000 TSRA FEW012 FEW025CB BKN100 BECMG AT 25/0800 09010KT 7000 FEW030 SCT120 BKN280 =
Kolkata, a 30% fog
A 30 h TAF. Note PROB30 TEMPO: a 30% chance of fog spells around midnight, then heavy showers become the main weather.
TAF VECC 031500Z 0318/0424 09005KT 5000 HZ SCT015 SCT080 TEMPO 0321/0322 3000 HZ SCT008 SCT250 PROB30 TEMPO 0400/0403 0800 FG BKN004 SCT010 BECMG 0403/0404 2000 +SHRA SCT008 SCT250 BECMG 0406/0408 6000 SCT015 BKN080=
Delhi, a winter night
Calm winds after dusk let mist form, fog spells near dawn, then the haze slowly thins as the sun and a north-west wind arrive.
TAF VIDP 030900Z 0312/0418 VRB02KT 3000 HZ NSC BECMG 0316/0318 00000KT 1500 BR TEMPO 0400/0402 0800 FG BECMG 0403/0404 32005KT 1000 BR FEW020 BECMG 0404/0406 33005KT 3000 HZ BECMG 0408/0410 4000 HZ NSC BECMG 0412/0414 30010KT 6000 =
Mumbai, FM and TX/TN
Our own example. TX33/1209Z: 33 °C at 09 UTC on the 12th. FM130200: from 02 UTC on the 13th the whole forecast is replaced.
TAF VABB 120500Z 1206/1312 25012KT 6000 SCT020 BKN080 TX33/1209Z TN26/1300Z TEMPO 1210/1214 25020G32KT 3000 TSRA FEW025CB BKN080 FM130200 VRB03KT 2500 BR NSC PROB40 TEMPO 1303/1305 0800 FG
Kolkata, worked example
TAF VECC 102100Z 1100/1109 09012KT 2500 BR FEW040 SCT150 BECMG 1102/1104 16015KT 6000 TEMPO 1107/1109 FEW030CB =
Hyderabad
TAF VOHS 030900Z 0312/0418 07010KT 5000 HZ NSC TEMPO 0400/0403 1500 BR FEW020 SCT200 BECMG 0405/0406 09010KT 6000 SCT020 SCT200 BECMG 0415/0416 09005KT 3000 HZ NSC=
Coimbatore
TAF VOCB 251500Z 2518/2624 12010KT 3000 HZ SCT012 FEW025CB BKN100 TEMPO 2600/2603 1500 BR SCT010 FEW025CB OVC080 BECMG 2604/2605 27010KT 6000 TEMPO 2610/2615 2000 TSRA SCT012 FEW025CB BKN090 BECMG 2615/2616 14010KT 6000 =

Memory hook: A TAF is a story told in code: what the weather is when it starts, and every change after that.

Takeaway: Read the main forecast first. BECMG and FM change it for good; TEMPO and PROB only add spells on top. At any hour, the worst of the main and the temporary groups is what you plan for.