8.2 Terminal Aerodrome Forecasts (TAF)

8.2 Terminal Aerodrome Forecasts (TAF)

Quick Summary: The Terminal Aerodrome Forecast (TAF) is the regulatory weather forecast utilized by aircraft dispatchers under 14 CFR § 121.613 to verify that weather conditions at destination and alternate airports will be at or above authorized landing minimums at the estimated time of arrival (ETA). A TAF applies within a strict 5 Statute Mile (SM) radius from the center of the airport runway complex (Aerodrome Reference Point, ARP), with weather between 5 and 10 SM categorized as vicinity (VC). Routine TAFs are issued four times daily (0000Z, 0600Z, 1200Z, 1800Z) for durations of 24 or 30 hours. Under standard Part 121 rules, the worst-case forecast condition at ETA controls. However, under FAA Exemption 3585 or Exemption 17347, air carriers may dispatch to destinations or list alternates when conditional remarks (TEMPO, PROB30) forecast weather below minimums, provided a strict two-tier 50% minimums test and a mandatory second alternate requirement are met.

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1. TAF Specifications, Issuance Schedule & Geographic Domain

Terminal Aerodrome Forecasts are concise, high-precision weather forecasts prepared by National Weather Service (NWS) Weather Forecast Offices (WFOs) specifically for aviation operations. Unlike broad regional forecasts, a TAF provides runway-specific meteorological predictions tailored to the critical terminal flight phases: takeoff, descent, approach, and landing.

Geographic Radius & Boundaries

• Primary Aerodrome Area: A TAF covers the terminal airspace within a 5 Statute Mile (SM) radius from the center of the airport runway complex (formally designated as the Aerodrome Reference Point - ARP).
• Vicinity (VC): Weather phenomena occurring between 5 SM and 10 SM from the ARP are designated as being in the vicinity (e.g., VCTS = thunderstorm in the vicinity).
• Beyond 10 SM: Weather occurring beyond 10 SM is not included in the body of the TAF; it is captured in regional convective advisories or upper-air charts.

Issuance Cycles & Validity Periods

• Issuance Schedule: TAFs are issued routinely four times each day at 0000Z, 0600Z, 1200Z, and 1800Z (normally transmitted between 20 and 40 minutes prior to the valid period start time).
• Validity Durations:
  • 24-Hour TAFs: Issued for the majority of commercial domestic airports.
  • 30-Hour TAFs: Issued for major international gateway hub airports (such as KJFK, KORD, KATL, KDFW, KLAX, KSFO, KDEN, KMIA). The extended 30-hour window is specifically designed to facilitate flight dispatch planning for ultra-long-haul international flights operated under 14 CFR Part 121 Flag operations, allowing dispatchers to evaluate destination and alternate forecasts for 12-to-16-hour flight durations without relying on expired forecast issuances.

+---------------------------------------------------------------------------------------------------+
|                             TAF SCHEDULE AND COVERAGE METRICS                                     |
+-----------------------+---------------------------------------------------------------------------+
| Metric                | Regulatory Standard                                                       |
+-----------------------+---------------------------------------------------------------------------+
| Issuance Frequency    | 4 times daily: 0000Z, 0600Z, 1200Z, 1800Z                                 |
| Standard Validity     | 24 hours (regional airports) or 30 hours (international hubs)             |
| Geographic Coverage   | 5 Statute Miles (SM) from the Aerodrome Reference Point (ARP)             |
| Vicinity Definition   | 5 to 10 Statute Miles (SM) from the ARP (coded as VC)                      |
| Controlling Agency    | National Weather Service (NWS) Weather Forecast Offices (WFOs)            |
+-----------------------+---------------------------------------------------------------------------+

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2. Anatomy of TAF Elements & Change Groups

A complete TAF comprises a base forecast representing prevailing conditions from the start of the period, followed by chronological change groups that reflect the evolution of weather systems across the forecast period:

TAF KORD 111720Z 1118/1224 22014KT P6SM BKN035
     FM112200 28018G28KT 3SM TSRA BKN015CB OVC030
     TEMPO 1200/1204 1/2SM +TSRA FG BKN008 OVC012CB
     FM120600 32015KT P6SM SCT025 BKN060
     BECMG 1212/1214 34010KT NSW SKC
     PROB30 1218/1222 2SM -SHRA OVC015=

Base Forecast Elements

1. Message Identifier & Station: TAF KORD (Terminal Aerodrome Forecast for Chicago O'Hare).
2. Date/Time of Issuance: 111720Z (11th day of the month at 17:20 UTC).
3. Valid Period: 1118/1224 (Valid from the 11th day at 18:00 UTC through the 12th day at 24:00 UTC; a 30-hour forecast).
4. Prevailing Conditions: 22014KT P6SM BKN035 (Wind 220° True at 14 knots, visibility greater than 6 Statute Miles [P6SM], broken cloud deck at 3,500 feet AGL).

1. The FM (From) Group: Rapid, Permanent Transition

• Syntax: FMddhhmm (e.g., FM112200 = From the 11th day at 22:00 UTC).
• Meteorological Nature: Denotes a rapid, dramatic meteorological transition occurring in less than 1 hour (e.g., a fast-moving cold front passage or abrupt wind shift).
• Regulatory Effect: The FM group is a complete reset. It completely supersedes and obliterates ALL previous forecast conditions. Every basic weather element—wind direction and speed, visibility, weather phenomena, and cloud cover/ceiling—must be restated. If an element is not listed in the FM group, it is assumed not to exist (e.g., no weather phenomena listed means no significant weather).

2. The BECMG (Becoming) Group: Gradual, Permanent Evolution

• Syntax: BECMG ddhh/ddhh (e.g., BECMG 1212/1214 = Becoming between 12:00 UTC and 14:00 UTC on the 12th).
• Meteorological Nature: Denotes a gradual, permanent meteorological change taking place over a specified time window (typically 2 to 4 hours).
• Regulatory Effect: The conditions begin changing at the start hour, and by the end hour, the transition is permanent and complete. Unlike the FM group, BECMG represents a partial replacement: only the specific elements listed are changed; any elements not mentioned carry forward unchanged from the preceding forecast group. (Note: While standard internationally under ICAO and in U.S. military TAFs, BECMG is rarely used in domestic U.S. civil NWS TAFs).

3. The TEMPO (Temporary) Group: Short-Lived Fluctuations

• Syntax: TEMPO ddhh/ddhh (e.g., TEMPO 1200/1204 = Temporary conditions between 00:00 UTC and 04:00 UTC on the 12th).
• Meteorological Nature: Denotes transient fluctuations in weather elements (such as brief summer thunderstorms, passing snow squalls, or temporary fog) that meet two strict mathematical thresholds:
  1. Individual Duration: Expected to persist for less than 1 hour in each single instance.
  2. Aggregate Duration: In total aggregate, expected to cover less than half (50%) of the specified forecast time period.
• Regulatory Effect: The prevailing base weather remains in effect throughout the window, but the temporary conditions may be encountered intermittently. Once the temporary phenomenon ceases, weather immediately returns to the prevailing baseline.

4. The PROB30 (Probability) Group: Low-Probability Occurrence

• Syntax: PROB30 ddhh/ddhh (e.g., PROB30 1218/1222 = 30% probability between 18:00 UTC and 22:00 UTC on the 12th).
• Meteorological Nature: Represents a 30% to 39% probability of occurrence of hazardous convective weather or precipitation. In U.S. civil TAFs, PROB30 is not used during the first 9 hours of the forecast period.
• PROB40 Prohibited in U.S. Civil TAFs: Under NWS policy, PROB40 is never used in U.S. civil TAFs (if confidence reaches 40% or higher, forecasters must use a TEMPO group or include the phenomenon in the prevailing group).

5. Non-Convective Low-Level Wind Shear (WS Group)

• Syntax: WS[altitude in hundreds of feet AGL]/[wind direction][wind speed]KT (e.g., WS020/22050KT = Wind shear at 2,000 feet AGL with wind from 220° True at 50 knots).
• Aviation Impact: Warns of non-convective low-level wind shear below 2,000 feet AGL (such as a nocturnal low-level temperature inversion or frontal shear). It signals severe airspeed fluctuations during approach and takeoff.

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3. Standard Part 121 Dispatch Legalities

Under 14 CFR § 121.613, the legal rule governing domestic dispatch release states:

"No person may dispatch or release an aircraft for flight... unless the weather reports or forecasts, or any combination thereof, indicate that the weather conditions will be at or above the authorized minimums at the estimated time of arrival at each airport to which dispatched."

The Standard Dispatch Rule: Worst-Case Controls

In standard Part 121 airline dispatch (operating without an approved exemption), the FAA enforces a strict worst-case legal interpretation of TAF conditions at the flight's Estimated Time of Arrival (ETA):

• Dispatchers must evaluate the prevailing forecast (main body) AND all conditional groups (TEMPO, PROB30, BECMG) valid at the ETA.
• If ANY conditional remark valid at the ETA indicates that ceiling or visibility will drop below authorized landing minimums, the flight CANNOT be legally dispatched under standard rules!
• Example: An airport's ILS approach requires 1/2 SM visibility. The TAF main body forecasts 3 SM visibility, but includes a TEMPO 1418/1422 1/4SM +TSRA. If the flight's ETA is 19:30 UTC, the flight cannot legally be dispatched under standard § 121.613 rules because the TEMPO visibility (1/4 SM) is below the required landing minimum (1/2 SM).

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4. FAA Exemption 17347 (Formerly Exemption 3585)

Recognizing that conservative "worst-case" dispatch rules caused unnecessary flight cancellations and stranded passengers due to low-probability, short-lived conditional weather, the FAA granted conditional-language exemptions such as Exemption 17347 (and the earlier Exemption 3585). This exemption provides an alternative, highly structured legal pathway for scheduled Part 121 air carriers to dispatch flights when conditional forecast language would otherwise block release.

Core Philosophy of Exemption 17347

Exemption 17347 establishes that the main body of a TAF represents the high-probability meteorological baseline, while conditional groups (TEMPO, PROB30) represent transient or low-probability risks. As long as the prevailing baseline is solid, flights may be released despite sub-minimum conditional remarks, provided a rigorous two-tier 50% test is satisfied and a second alternate airport is designated.

+---------------------------------------------------------------------------------------------------+
|                         EXEMPTION 17347 TWO-TIER MINIMUMS TEST                                    |
+-----------------------+---------------------------------+-----------------------------------------+
| Airport Category      | Main Body Forecast Requirement  | Conditional Remarks (TEMPO/PROB) Limit  |
+-----------------------+---------------------------------+-----------------------------------------+
| Destination Airport   | At or above authorized landing  | Not less than 50% of authorized landing |
|                       | minimums (ceiling & visibility) | visibility (ceiling not a factor)       |
+-----------------------+---------------------------------+-----------------------------------------+
| First Alternate       | At or above authorized OpSpecs  | Not less than 50% of authorized alternate|
|                       | C055 alternate ceiling & vis    | ceiling AND visibility minimums         |
+-----------------------+---------------------------------+-----------------------------------------+
| Second Alternate      | At or above 100% of OpSpecs     | At or above 100% of OpSpecs C055        |
| (MANDATORY)           | C055 alternate ceiling & vis    | alternate ceiling AND visibility minimums|
+-----------------------+---------------------------------+-----------------------------------------+

Detailed Breakdown of Exemption 17347 Requirements

1. Destination Airport Weather Requirements

• Main Body: Must forecast weather at or above the authorized landing minimums (both ceiling and visibility) at the ETA.
• Conditional Remarks (TEMPO / PROB): May forecast weather below authorized landing minimums at the ETA, provided the conditional visibility is not less than 50% of the authorized landing visibility required for the expected instrument approach.
• Note on Ceiling: For straight-in precision approaches (ILS, GLS), ceiling is not a regulatory minimum unless specifically mandated by the carrier's OpSpecs or published chart notes. Visibility is the sole controlling condition.

2. First Alternate Airport Weather Requirements

• Main Body: Must forecast weather at or above authorized alternate minimums derived from Operations Specification Table C055 (the 1-Nav / 2-Nav rule) at the ETA.
• Conditional Remarks (TEMPO / PROB): May forecast weather below authorized alternate minimums at the ETA, provided the conditional values are not less than 50% of the required alternate ceiling AND visibility!

3. Mandatory Second Alternate Airport

• Whenever Exemption 17347 is utilized for dispatch release, the dispatcher MUST designate a second alternate airport in the dispatch release!
• Strict Weather Standard for Second Alternate: Both the main body AND all conditional remarks of the second alternate TAF must forecast weather at or above 100% of the authorized alternate minimums (OpSpecs C055) at the ETA. No 50% reduction is permitted for the second alternate!

4. Operational Controls & Dispatcher Duties

• Dispatch Release Annotation: The dispatch release must explicitly document that the flight is being operated under Exemption 17347 (or carrier OpSpec A005).
• Fuel Requirements (14 CFR § 121.639): The aircraft must carry sufficient fuel to fly to the destination, fly to and land at the most distant alternate airport, and fly thereafter for 45 minutes at normal cruising fuel consumption.
• En Route Weather Monitoring: Under joint operational control, the dispatcher must actively track weather developments, monitor TAF amendments, and immediately notify the flight crew if conditions deteriorate beyond allowable tolerances.

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5. Practical Dispatch Scenario: Applying Exemption 17347

The Operational Problem

A flight from Atlanta (KATL) to Chicago O'Hare (KORD) has an estimated time of arrival (ETA) of 20:30 UTC.

• The expected approach at KORD is the ILS Rwy 10C, requiring minimums of 200-foot Decision Altitude (DA) and 1/2 SM visibility (2400 RVR).
• First Alternate is Milwaukee (KMKE), where OpSpecs Table C055 alternate minimums are derived as 600-foot ceiling and 1 1/2 SM visibility.
• Second Alternate is Indianapolis (KIND), where OpSpecs Table C055 alternate minimums are derived as 400-foot ceiling and 1 SM visibility.

Forecast Data Analysis at ETA (20:30 UTC)

1. Destination: Chicago O'Hare (KORD)

• TAF Excerpt: FM2000 24012KT 3SM BKN020 TEMPO 2022 1/4SM +TSRA FG OVC004
• Main Body: 3 SM visibility and 2,000 ft ceiling. Exceeds the 1/2 SM and 200 ft approach minimums. (Passes)
• Conditional (TEMPO): 1/4 SM visibility and 400 ft ceiling.
  • Required landing visibility = 1/2 SM.
  • 50% of landing visibility = 1/4 SM (0.25 SM).
  • The TEMPO visibility of 1/4 SM equals exactly 50% of required visibility. (Passes Exemption 17347 limit)

2. First Alternate: Milwaukee (KMKE)

• TAF Excerpt: FM1900 21014KT 4SM OVC015 PROB30 2023 3/4SM -SN OVC004
• Main Body: 4 SM visibility and 1,500 ft ceiling. Exceeds the C055 required 600 ft / 1 1/2 SM. (Passes)
• Conditional (PROB30): 3/4 SM visibility and 400 ft ceiling.
  • Required alternate ceiling = 600 ft; 50% limit = 300 ft. Forecast is 400 ft. (Passes)
  • Required alternate visibility = 1 1/2 SM; 50% limit = 3/4 SM. Forecast is 3/4 SM. (Passes)

3. Second Alternate: Indianapolis (KIND)

• TAF Excerpt: FM1800 20010KT P6SM SCT025 OVC050 TEMPO 2023 2SM -RA BR BKN015
• Main Body: P6SM visibility and 5,000 ft ceiling. Exceeds C055 minimums (400 ft / 1 SM). (Passes)
• Conditional (TEMPO): 2 SM visibility and 1,500 ft ceiling.
  • Must be 100% at or above C055 minimums (400 ft / 1 SM).
  • 1,500 ft >= 400 ft, and 2 SM >= 1 SM. (Passes 100% requirement)

Dispatch Conclusion

The flight may be legally dispatched under Exemption 17347. The dispatcher lists both KMKE and KIND on the release, notes compliance with Exemption 17347, and fuels the aircraft for: KATL to KORD + KORD to KIND (the most distant alternate: ~155 NM vs. KMKE ~60 NM) + 45 minutes reserve fuel.