7.2 Terminal Aerodrome Forecasts (TAF) & Aviation Forecasts

Terminal Aerodrome Forecasts (TAF) & Aviation Forecasts

Quick Answer: A TAF is a concise terminal forecast valid for a 5-statute-mile (SM) radius from the center of an airport runway complex, issued 4 times daily (00Z, 06Z, 12Z, 18Z) with a validity period of 24 or 30 hours. The change group FM (From) denotes a rapid, permanent change starting at the exact hour and minute, completely replacing all preceding lines. TEMPO denotes temporary conditions lasting < 1 hour at a time and covering < 50% of the forecast window. PROB30 indicates a 30% probability of hazardous weather. The Graphical Forecasts for Aviation (GFA) web tool has replaced legacy text Area Forecasts (FA) for en route weather analysis across the CONUS.

While METARs reflect what has already occurred, the Terminal Aerodrome Forecast (TAF) is the primary operational tool used by instrument pilots to forecast terminal flight conditions at their estimated time of arrival (ETA). Under 14 CFR § 91.169, TAF weather parameters dictate whether an alternate airport must be designated in the IFR flight plan and whether a selected alternate meets legal filing minimums.

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TAF Issuance Schedule & Terminal Radius

• Issuance Frequency: TAFs are generated and issued four times daily by National Weather Service (NWS) Weather Forecast Offices (WFOs) at 0000Z, 0600Z, 1200Z, and 1800Z.
• Validity Periods:
  • 24-Hour TAFs: Standard for most regional and domestic airports.
  • 30-Hour TAFs: Issued for approximately 32 large international hub airports (e.g., KJFK, KORD, KLAX, KATL, KDFW) to facilitate long-haul international dispatch planning.
• Geographical Scope: A TAF is valid strictly within a 5-statute-mile (SM) radius of the center of the airport's runway complex. Conditions forecast beyond 5 SM are designated with the vicinity code VC (5 to 10 SM).

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|                                 STANDARD TAF SYNTAX & STRUCTURE                                   |
|                                                                                                   |
|  TAF  KPIT  181120Z  1812/1918  22010KT  5SM  HZ  BKN020                                          |
|  [Type] [ID]   [Issue]    [Valid]    [Wind]    [Vis] [Wx]  [Sky/Ceiling]                          |
|                                                                                                   |
|    FM181500 28015G25KT 2SM TSRA OVC010CB                                                          |
|    TEMPO 1818/1822 1/2SM +TSRA FG BKN005CB                                                        |
|    FM190100 32008KT P6SM SKC                                                                      |
|    PROB30 1908/1912 2SM BR BKN015                                                                 |
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TAF Change Groups & Transition Logic

Because weather evolves continuously, TAFs use four standardized change groups to describe the timing, probability, and duration of meteorological transitions.

1. FM (From) Group — Rapid Permanent Transition

• Syntax: FMDDHHMM (e.g., FM181500 = From the 18th day at 1500 UTC).
• Operational Meaning: Used when a rapid, permanent change in weather occurs within minutes.
• Critical Rule: The FM group completely supersedes and resets all previous forecast parameters. Every subsequent condition (wind, visibility, weather, sky condition) must be explicitly restated on the FM line. Any omitted parameter is no longer forecast.

2. TEMPO (Temporary) Group — Short-Lived Fluctuations

• Syntax: TEMPO DDHH/DDHH (e.g., TEMPO 1818/1822 = Temporary conditions between 1800Z and 2200Z on the 18th).
• Operational Meaning: Used to forecast temporary fluctuations that:
  1. Last less than 1 hour during any individual occurrence; and
  2. Cover less than 50% (half) of the total specified time window.
• Baseline Retention: During periods outside the temporary events, prevailing weather reverts to the base forecast conditions established in the preceding line.

3. PROB30 (Probability 30%) Group

• Syntax: PROB30 DDHH/DDHH (e.g., PROB30 1908/1912 = 30% probability of occurrence between 0800Z and 1200Z on the 19th).
• Operational Meaning: Indicates a 30% chance of thunderstorms, low ceilings, or restricted visibility occurring during the specified window.
• Usage Constraints: PROB30 is never used during the first 9 hours of a TAF. (Note: NWS guidelines prohibit PROB40 in domestic US TAFs; conditions with $\ge 40%$ probability are written directly as prevailing or TEMPO groups).

4. BECMG (Becoming) Group — Gradual Transition

• Syntax: BECMG DDHH/DDHH (e.g., BECMG 1814/1816 = Gradual transition occurring between 1400Z and 1600Z).
• Operational Meaning: Denotes a slow, continuous change occurring at a constant rate over a 2-hour window. Widely used in international and military TAFs (rare in domestic US NWS TAFs, which prefer FM).

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Non-Convective Low-Level Wind Shear (WS)

When non-convective low-level wind shear is anticipated within 2,000 feet Above Ground Level (AGL) that would significantly affect aircraft approach or departure performance, a WS group is added.

$\text{Format: } \mathbf{WS[Altitude]/[Wind Direction][Wind Speed]KT}$

• Example: WS020/24045KT
  • WS: Wind shear group designator.
  • 020: Altitude of the shear layer in hundreds of feet AGL (2,000 ft AGL).
  • 24045KT: Wind at that altitude is from $240^\circ$ True at 45 knots.
• Interpretation: If the surface wind is 04010KT and the TAF includes WS020/24045KT, an aircraft climbing through 2,000 feet will experience a $200^\circ$ directional shear and a 35-knot velocity change, requiring aggressive pitch and power management.

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Applying TAFs to IFR Flight Planning & Alternate Airport Rules

Under 14 CFR § 91.169, an instrument pilot filing an IFR flight plan must determine whether a destination alternate airport is legally required using the 1-2-3 Rule:

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|                    THE 1-2-3 ALTERNATE AIRPORT RULE (14 CFR 91.169)               |
|                                                                                   |
|   An alternate airport is NOT required if the destination TAF indicates:          |
|                                                                                   |
|   1. From 1 HOUR BEFORE to 1 HOUR AFTER your Estimated Time of Arrival (ETA)...   |
|   2. The CEILING is at least 2,000 FEET AGL...                                    |
|   3. The VISIBILITY is at least 3 STATUTE MILES.                                  |
|                                                                                   |
|   If either the ceiling (<2,000 ft) or visibility (<3 SM) is forecast below       |
|   these minimums at any point during ETA ± 1 hr, an alternate MUST be filed.      |
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Impact of TEMPO and PROB Groups on Legal Alternate Requirements

• Destination Requirements: If a TEMPO or PROB30 group at the destination forecasts conditions below 2,000 ft ceiling or 3 SM visibility during ETA $\pm 1\text{ hour}$, the pilot must file an alternate.
• Alternate Suitability Requirements: For an airport to qualify legally as a filed alternate, its forecast weather at your ETA must meet the published alternate minimums (standard minimums: $600\text{ ft} / 2\text{ SM}$ for precision approaches, $800\text{ ft} / 2\text{ SM}$ for non-precision approaches). If a TEMPO group brings conditions below alternate minimums at the estimated arrival time, that airport cannot be legally filed as an alternate.

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Graphical Forecasts for Aviation (GFA)

The Graphical Forecasts for Aviation (GFA) interactive web tool (aviationweather.gov/gfa) replaced the legacy text Area Forecast (FA) across the contiguous United States. It provides high-resolution, continuous graphic weather forecasts from the surface up to FL450 (45,000 feet MSL) with an 18-hour forecast horizon updated every hour.

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|                      GFA INTERACTIVE DISPLAY TABS & LAYERS                        |
|                                                                                   |
|  1. Clouds Tab: Depicts cloud cover coverage, cloud bases (MSL & AGL), and tops.   |
|  2. Surface Tab: Surface wind velocity, gusts, visibility, and precipitation.    |
|  3. Weather Tab: Precipitation types (rain, snow, freezing rain) and fog/haze.   |
|  4. Thunderstorms Tab: Convective potential, lightning probability, storm tops.  |
|  5. Icing Tab: Freezing levels, icing probability, and structural icing severity.|
|  6. Turbulence Tab: Low-level and high-level clear air turbulence (CAT) & EDR.   |
|  7. Winds Aloft Tab: Upper-air wind speeds and streamlines from 3,000 to FL450.  |
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