4.2 Terminal Aerodrome Forecasts (TAF): Groups & Timing

4.2 Terminal Aerodrome Forecasts (TAF): Groups & Timing

Terminal Aerodrome Forecasts (TAF) provide certified forecasts of meteorological conditions expected to exist within a 5 statute mile radius of an airport runway complex. For the aircraft dispatcher, the TAF is the primary legal basis for issuing a flight release under 14 CFR 121.613, evaluating destination alternate requirements under 14 CFR 121.619, and applying regulatory relief such as FAA Exemption 3585. Mastering the precise timing, structure, and legal interpretation of main body versus conditional change groups is fundamental to operational control.

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1. Purpose, Issuance Schedule, and Geographical Coverage

A Terminal Aerodrome Forecast (TAF) is a concise statement of expected meteorological conditions significant to aviation for a specified aerodrome during a specified period.

• Geographic Scope: The forecast applies specifically to the aerodrome and the airspace within a 5 statute mile (SM) radius from the center of the airport runway complex. Weather expected between 5 and 10 SM from the center is designated as "in the vicinity" (VC).
• Issuance Cadence: In the United States, National Weather Service (NWS) Weather Forecast Offices (WFOs) issue routine TAFs four times daily:
  • 0000Z;
  • 0600Z;
  • 1200Z;
  • 1800Z. TAFs are prepared and transmitted between 20 and 40 minutes prior to the start of the valid period (e.g., the 1200Z TAF is transmitted around 1140Z).
• Valid Period Length:
  • Standard domestic airports: 24-hour validity.
  • Major international transport hubs (e.g., ORD, ATL, JFK, DFW, LAX, SFO, SEA): 30-hour validity. Thirty-hour TAFs were introduced to accommodate long-haul international flights where flight times exceed 12 to 14 hours, ensuring pilots and dispatchers have certified forecasts covering the destination at ETA without relying on outdated forecast cycles.

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2. Standard TAF Structure & Initial Forecast Period

The architecture of a TAF consists of the header line, the base (initial) forecast group, and subsequent change groups:

TAF KDFW 121130Z 1212/1318 16012KT P6SM VCSH SCT025 BKN080
    WS020/21045KT
    FM121500 18018G28KT 4SM SHRA OVC015
    TEMPO 1218/1222 1 1/2SM +TSRA BKN008CB
    FM130200 32015KT P6SM SKC

Header Decoding

• TAF: Identifies the report type. TAF AMD indicates an amended forecast; TAF COR indicates a corrected forecast.
• KDFW: ICAO aerodrome identifier.
• 121130Z: Issuance timestamp (12th day of the month at 1130 UTC).
• 1212/1318: Valid period group. Formatted as DDHH/DDHH. Here, the forecast is valid from the 12th day at 1200Z through the 13th day at 1800Z (a 30-hour forecast).

Base Forecast Group

Following the valid period group is the base forecast representing the prevailing atmospheric conditions expected at the start of the valid period:

• Wind: 16012KT ($160^\circ\text{ true}$ at 12 kt).
• Visibility: P6SM (greater than 6 statute miles, meaning unrestricted visibility).
• Significant Weather: VCSH (showers in the vicinity, between 5 and 10 SM).
• Sky Condition: SCT025 BKN080 (scattered clouds at 2,500 ft AGL, broken ceiling at 8,000 ft AGL).
• Non-Convective Low-Level Wind Shear (LLWS): WS020/21045KT denotes wind shear at 2,000 ft AGL with winds forecast from $210^\circ$ at 45 knots. When surface winds are light and strong low-level jet winds exist aloft, this group warns crews of severe airspeed and pitch deviations on approach or departure.

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3. Change Groups: FM, TEMPO, PROB30 & BECMG

Atmospheric conditions rarely remain constant over 24 or 30 hours. Meteorologists use specific change groups to delineate temporal shifts:

From Group (FM)

The FM group indicates a rapid and permanent change in prevailing conditions expected to occur within a one-hour window.

• Format: FM followed by a six-digit date/time group (DDHHMM). In FM121500, the change begins rapidly at 1500 UTC on the 12th day.
• The "Clean Slate" Rule: The FM group completely supersedes and overwrites all previous forecast conditions. Every meteorological parameter (wind, visibility, weather phenomena, and cloud layers) must be explicitly re-stated in the FM group. Any condition not listed in the FM group is assumed to be unrestricted. If a prior group forecasted rain and a 1,000-foot ceiling, and the FM group states only FM121500 28010KT P6SM SKC, the rain and low ceiling are completely eliminated from 1500Z onward.

Temporary Group (TEMPO)

The TEMPO group is used for temporary fluctuations in prevailing conditions that are expected to:

1. Last for less than 1 hour in each individual instance; and
2. In the aggregate, represent less than half of the total time window specified in the group.

• Format: TEMPO DDHH/DDHH (e.g., TEMPO 1218/1222 indicates fluctuations between 1800Z and 2200Z on the 12th day).
• Interpretation: The conditions listed in the base forecast or preceding FM group remain prevailing. The TEMPO group only specifies parameters that temporarily deviate from the base conditions. For instance, if base conditions are P6SM SCT040 and TEMPO 1218/1222 2SM TSRA BKN015CB is appended, the predominant weather remains 6+ SM with scattered clouds, but brief, discrete thunderstorms will intermittently lower visibility to 2 SM and ceilings to 1,500 ft.

Probability Group (PROB30)

The PROB30 group indicates a 30% probability of occurrence of hazardous convective weather or low visibility during a specified time window:

• Format: PROB30 DDHH/DDHH (e.g., PROB30 1308/1312 1SM TSRA OVC008CB).
• NWS Civil Restrictions: NWSI 10-813 (October 30, 2024) removed the former nine-hour PROB30 exclusion. PROB30 expresses a 30 percent probability of the specified precipitation or thunderstorm event and related conditions; read its actual validity window. 2. PROB40 Prohibition: PROB40 (40% probability) is strictly unauthorized in US domestic civil NWS TAFs. If a phenomenon has a 40% or greater chance of occurring, NWS forecasters must use a TEMPO group or include it in the prevailing forecast. PROB40 appears only in military (USAF/USN) or foreign international ICAO TAFs.

Becoming Group (BECMG)

The BECMG group indicates a gradual change in atmospheric conditions occurring at a constant or variable rate over a specified duration (typically 2 to 4 hours).

• International vs. Domestic Rule: BECMG is widely used in international ICAO forecasts worldwide. However, BECMG is strictly NOT used in US domestic civil NWS TAFs. In the United States, forecasters must use FM groups for transitions. Dispatchers handling international flag flights must understand BECMG, but domestic US dispatches will never see BECMG in an NWS product.

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4. Part 121 Dispatch Rules: Main Body vs. Conditional Language

The legal standard governing domestic and flag air carrier flight release is codified in 14 CFR 121.613:

14 CFR 121.613 Dispatch or flight release under IFR or over-the-top: No person may dispatch or release an aircraft for flight under IFR or over-the-top unless appropriate 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 the airport or airports to which dispatched or released.

The Controlling Factor at ETA

When evaluating a TAF under § 121.613, the dispatcher must project the flight's Estimated Time of Arrival (ETA) and inspect the corresponding forecast group:

• Main Body Prevailing Conditions: The baseline forecast established by the initial forecast or the prevailing FM group.
• Conditional Remarks: Any TEMPO or PROB30 groups whose valid time window encompasses the aircraft's ETA.

The Strict Regulatory Rule

Under the literal FAA statutory interpretation of 14 CFR 121.613, both the main body and all conditional language must be at or above authorized landing minimums at ETA. If the main body forecasts $3\text{ SM}$ visibility and a $1{,}500\text{ ft}$ ceiling, but a TEMPO group spanning ETA states 1/4SM +TSRA FG OVC002, the conditional remark indicates that weather could be below minimums upon arrival. Under strict Part 121 regulations without an exemption, the dispatcher cannot legally release the flight.

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5. Conditional Forecasts and Specific Dispatch Relief

Conditional forecasts cannot be dismissed merely because they are TEMPO or PROB30. Under § 121.613, use the appropriate reports/forecasts and the carrier's approved interpretation procedures to establish destination conditions at ETA. Separately apply the relevant alternate-selection rule and the alternate's required weather window.

Some carriers hold FAA exemptions historically discussed using numbers such as 3585 or 11060. These are specific grants with conditions, expiration dates, eligible holders, and implementation requirements, not a universal rule that every airline may use. Before relying on relief, verify the carrier's applicable current grant and authorized procedures. Do not assume that a remembered half-minimum formula applies to all destinations, approach types, or alternates.

For a basic exercise with no exemption, suppose destination landing visibility is 1 SM and an overlapping TEMPO group forecasts 1/2 SM at ETA. The main prevailing group meeting 1 SM does not by itself establish an authorized release. Obtain a forecast meeting the applicable rule, change the plan, delay, or demonstrate a specific applicable authorization. If an exemption is supplied in an exam packet, apply its destination, first-alternate, second-alternate, fuel, and inflight monitoring conditions exactly as written.

The current policy change allowing an early PROB30 group does not create regulatory dispatch relief. Forecast syntax and permission to dispatch are different questions. Include source issue times in the briefing, reassess after an ETA shift, and document why the selected forecast periods support the release.

6. Selecting Destination Alternates: The Part 121 1-2-3 Rule (§ 121.619)

Under 14 CFR 121.619, for domestic operations, an alternate airport is required for the destination unless:

1. Time Window: For at least 1 hour before to 1 hour after the estimated time of arrival (ETA);
2. Ceiling: The ceiling will be at least 2,000 feet above the airport elevation; and
3. Visibility: Visibility will be at least 3 statute miles.

If either the ceiling or the visibility in the TAF is forecast to drop below 2,000 feet or 3 statute miles at any point within the ETA $\pm 1\text{ hour}$ window, at least one destination alternate must be specified in the dispatch release. When evaluating the 1-2-3 rule, both the main body and TEMPO groups within that two-hour window must be analyzed.

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7. Practical Traps & Operational Scenarios

Practical Trap 1: The PROB40 Illusion

Dispatch candidates sometimes believe that a TAF containing PROB40 can be used for domestic dispatch. In the United States, civil NWS TAFs never use PROB40. If a candidate encounters PROB40 on an exam question, it is either an international aerodrome forecast or a military observation, where civil OpSpecs exemptions like Exemption 3585 cannot be automatically applied without specific international authorizations.

Practical Trap 2: The Overwrite Scope of FM Groups

When an FM group appears, candidates often assume unmentioned clouds or weather from the previous line carry forward. This is incorrect: an FM group is an absolute structural reset. If FM141600 24010KT P6SM SKC appears, all previous cloud layers, visibilities, and weather phenomena cease to exist at 1600Z.

Operational Scenario: ETA Shifts and TAF Change Groups

Flight 884 is dispatched from Atlanta to Boston with an original departure at 1300Z and ETA of 1530Z. The Boston TAF shows:

FM121500 22010KT P6SM BKN040
FM121600 04020G35KT 1/2SM +SN FG OVC003

Due to a mechanical ground delay, departure is pushed by 40 minutes, shifting the new ETA to 1610Z. At 1530Z, weather was unrestricted. At 1610Z, the flight arrives after the FM121600 change, into blizzard conditions ($1/2\text{ SM}$ and 300-ft ceiling). The dispatcher must recalculate destination fuel, check alternate weather, and ensure an alternate is listed under the 1-2-3 rule before permitting the delayed flight to depart.

Official References

Source check: 2026-10-12. 2026 FAA Aviation Weather Handbook; Current NWS TAF policy (October 2024); AWC current products; Dispatcher weather information § 121.601. For operational use, apply current issued authorizations and the assigned aircraft’s approved data.