Understanding TAF: Terminal Area Forecast Explained

TAF VS. METAR: FORECAST VS. OBSERVATION

A METAR is generated from automated sensors or trained weather observers at an airport. It reflects actual, measured conditions at the time of issue. A TAF is generated by National Weather Service (NWS) meteorologists and represents their best forecast for that airport's surface weather. Pilots must understand that a TAF carries inherent uncertainty. It is a professional forecast, not a guarantee.

TAFs are produced for airports with sufficient aviation traffic to justify the forecasting resource. Not every airport has a TAF. Smaller general aviation airports often rely on nearby TAF stations or pilot reports (PIREPs) instead.

---

VALID PERIOD AND ISSUANCE SCHEDULE

TAFs are issued four times daily at approximately 0000Z, 0600Z, 1200Z, and 1800Z. Standard TAFs cover a 24-hour valid period. Extended TAFs, issued for larger airports, cover 30 hours. The FAA and ICAO both use the TAF format, though minor formatting differences exist between domestic and international products.

The valid period is encoded directly in the header line. A pilot must always verify the valid period before using any TAF for planning purposes. An expired TAF provides no useful forecast protection.

---

DECODING THE TAF HEADER LINE

Every TAF begins with a standard header. Consider the following example:

`` TAF KORD 151720Z 1518/1624 27012KT 9999 BKN025 ` • TAF — Product type identifier • KORD — ICAO station identifier for Chicago O'Hare • 151720Z — Issued on the 15th day of the month at 1720 UTC • 1518/1624 — Valid from the 15th at 1800Z through the 16th at 2400Z • 27012KT — Wind from 270 degrees at 12 knots • 9999 — Visibility 9999 meters (used in ICAO format; US TAFs may use statute miles such as 6SM) • BKN025 — Broken ceiling at 2,500 feet AGL

In US domestic TAFs, visibility is reported in statute miles rather than meters. An entry of 6SM means six statute miles. When visibility exceeds six statute miles, the TAF will show P6SM`, meaning "greater than six statute miles."

---

FM (FROM) GROUPS

An FM group signals a complete and permanent change in conditions beginning at the specified time. All previously forecast conditions are replaced entirely.

`` FM160600 25006KT P6SM SKC ``

This line means that from the 16th at 0600Z, winds shift to 250 at 6 knots, visibility exceeds six statute miles, and the sky becomes clear. FM groups always start at the left margin and apply until the next change group or the end of the valid period. FM is the most definitive change indicator in a TAF.

---

BECMG (BECOMING) GROUPS

A BECMG group describes a gradual transition to new conditions over a specified time window. Only the elements explicitly listed in the BECMG group change; all other conditions remain as previously forecast.

`` BECMG 1522/1524 VRB03KT BKN010 ``

This means conditions are gradually changing between 2200Z and 0000Z on the 15th/16th to variable winds at 3 knots and a broken ceiling at 1,000 feet. The change may begin at any point within the window and will be complete by the end of it. BECMG is commonly used for improving or deteriorating conditions that evolve slowly, such as sea fog filling in or a frontal passage clearing overnight.

---

TEMPO GROUPS

A TEMPO group describes temporary fluctuations in conditions expected to last less than one hour at a time and to occur during less than half of the specified period. TEMPO does not replace the prevailing forecast; it overlays it.

`` TEMPO 1520/1522 3SM TSRA BKN015CB ``

This forecasts temporary conditions of 3 statute miles visibility, thunderstorms with rain, and a broken cumulonimbus layer at 1,500 feet between 2000Z and 2200Z. Pilots must treat TEMPO conditions seriously. Even brief IMC during a TEMPO window can trap an aircraft on approach or prevent a departure.

---

PROB GROUPS

PROB30 and PROB40 indicate the probability of a condition occurring. PROB40 means a 40 percent probability; PROB30 means a 30 percent probability. The FAA does not use PROB20 or lower in US TAFs.

`` PROB30 1602/1604 1SM FG OVC002 `

This indicates a 30 percent chance of one statute mile visibility in fog with an overcast ceiling at 200 feet between 0200Z and 0400Z on the 16th. PROB groups often appear in conjunction with TEMPO:

` PROB40 TEMPO 1608/1612 2SM RASN OVC008 ``

This combination means a 40 percent probability of temporary conditions of 2 miles, rain and snow, and an overcast at 800 feet. PROB30 conditions are sometimes overlooked by pilots. They represent meaningful risk and deserve attention during alternates planning.

---

READING WIND AND VISIBILITY IN A TAF

TAF winds follow the same format as METAR winds. A calm wind is reported as 00000KT. Variable winds below 6 knots appear as VRB03KT. Gusts are appended: 27018G28KT means 270 at 18 knots gusting to 28 knots.

Visibility governs flight category for each TAF group period:

| Category | Ceiling | Visibility | |---|---|---| | VFR | Greater than 3,000 ft | Greater than 5SM | | MVFR | 1,000 to 3,000 ft | 3 to 5SM | | IFR | 500 to 999 ft | 1 to less than 3SM | | LIFR | Below 500 ft | Less than 1SM |

A BKN or OVC layer in any TAF group defines the ceiling for that period. FEW and SCT layers do not constitute a ceiling.

---

USING TAF FOR FLIGHT PLANNING GO/NO-GO DECISIONS

The TAF is a mandatory preflight planning tool under FAA guidance. Pilots should evaluate the TAF for both the destination and any alternate airport required by FAR 91.167 (IFR alternate requirements).

Key planning steps: • Identify the arrival window. Determine the expected arrival time and examine the TAF for one hour before and one hour after that time. This is the window used to determine alternate requirements under the 1-2-3 rule. • Check the alternate TAF. Under FAR 91.169, the alternate must forecast ceiling at least 600 feet and visibility at least 2 statute miles (for a precision approach) at the estimated time of arrival. • Evaluate TEMPO and PROB groups. Do not dismiss low-probability events when fuel, alternates, or approach minimums are marginal. • Cross-reference with METARs. Compare current observations at the destination and en-route airports against the TAF. If actual conditions are significantly worse than the TAF predicted, the forecast model may be off and greater caution is warranted. • Review PIREPs. Pilot reports supplement the TAF by providing real-time observations along the route and near the destination.

---

LIMITATIONS OF TAF ACCURACY

TAFs carry well-documented limitations. Forecast accuracy degrades significantly beyond 12 hours. Convective activity is particularly difficult to forecast precisely in time and location. Mountain obscuration, radiation fog, and sea fog events frequently develop faster or more intensely than TAF models predict.

Additional limitations include: • TAFs represent conditions **at