A terminal aerodrome forecast, or TAF, is a forecast of weather conditions at an airport, which is usually valid for 24 hours. TAF’s are normally issued four times per day, at 0000Z, 0600Z, 1200Z, and 1800Z.
Example:
KPWM 030520Z 0306/0406 14004KT 1/4SM -DZ FG VV001 FM030800 27005KT 2SM BR SCT002 BKN006 FM030900 27005KT P6SM SCT006 BKN040 FM031300 30005KT P6SM SKC FM031700 18008KT P6SM SKC
The first item is a TAF is the location identifier, followed by the issuance and valid dates and times.
This example tells us this is a TAF for Portland, Maine, issued on the third day of the month at 0520 Zulu. It is valid from the third day of the month at 0600 Zulu to the fourth day of the month at 0600 Zulu.
Winds are expected to be from 140 at 4 knots, with a visibility of one quarter statute mile in light drizzle and fog, with a vertical visibility of 100 feet.
The next line reads: From 0800 Zulu on the third day of the month, the weather will be, wind from 270 degrees at 5 knots, with a visibility of 2 statute miles in mist, scattered clouds at 200 feet with a broken ceiling at 600 feet.
The term from indicates a rapid and significant weather change is expected within a short time period.
Becoming and temporary conditions may also be forecast in a TAF.
Example:
KPWM 030520Z 0306/0406 14004KT 1/4SM -DZ FG VV001 BCMG 0307/0308 27005KT 2SM BR SCT002 BKN006 FM030900 27005KT P6SM SCT006 BKN040 FM031300 30005KT P6SM SKC
If the Portland TAF were written like this, the second line would read becoming between the third day of the month at 0700 Zulu and the third day of the month and 0800 Zulu, winds 270 at five, visibility two, mist, scattered clouds at 200 feet, ceiling 600 broken.
Becoming means to a more gradual change is expected over a period of time.
If conditions are forecast to exist only temporarily, TEMPO is coded into the TAF.
Example:
KSEA 030536Z 0306/0412 26006KT P6SM SKC FM031200 21004KT P6SM SCT005 TEMPO 0314/0317 BKN005 FM031900 28006KT P6SM SKC
This example TAF for Seattle tells us that from the third day of the month at 1200 Zulu, the winds are expected to be from 210 degrees at four knots, with more than six miles visibility, and scattered clouds at 500 feet.
“P” is printed in from if the visibility to mean greater than.
The next line forecasts a temporary weather condition will exist between the third day of the month at 1400 Zulu and the third day of the month at 1700 Zulu. This temporary condition is a 500 foot broken cloud ceiling.
After 1700 Zulu, this temporary condition should end, and the weather once again become winds 210 at four, with plus 6 miles of visibility, and scattered clouds at 500 feet.
From 1900 Zulu, light west winds are forecast at 6 knots, with plus six miles of visibility, and clear skies.
Example:
KEYW 131155Z 1312/1412 VRB03KT P6SM SCT025CB BKN250 PROB30 1316/1318 2SM TSRA BKN020CB FM131800 VRB03KT P6SM SCT025 BKN250
In this example, a probability of weather occurring is given in the second line of the forecast. Probabilities are given in percent.
This TAF decodes to read:
Forecast for Key West airport, issued on the 13th day of the month at 1155 Zulu time, valid from the 13th day of the month at 1200 Zulu to the 14th day of the month at 1200 Zulu, winds variable at three knots, visibility greater than six, scattered cumulonimbus clouds at 2,500 feet, ceiling 25,000 broken.
There is a 30% probably between the 13th day of the month at 1600 Zulu and the 13th day of the month at 1800 Zulu that the visibility will be 2 statute miles, with thunderstorms and moderate rain, with a 2,000 foot cumulonimbus ceiling.
From the 13th day of the month at 1800 Zulu, winds variable at three knots, visibility greater than six, scattered clouds and 2500, ceiling 25,000 broken.