8.2 Terminal Aerodrome Forecasts (TAF) & Graphical Forecasts (GFA)

Terminal Aerodrome Forecasts (TAF) & Graphical Forecasts (GFA)

While METARs and SPECIs provide critical snapshots of present surface observations, safe flight planning demands reliable predictive tools. Ground instructors must ensure airmen understand the exact spatial boundaries, valid periods, and conditional change syntax of aviation forecasts. The primary aerodrome forecast utilized across civil aviation is the Terminal Aerodrome Forecast (TAF), prepared by National Weather Service (NWS) Weather Forecast Offices (WFO). For en route and regional preflight planning, the modern digital standard across the contiguous United States is the web-based Graphical Forecasts for Aviation (GFA), which completely replaced the legacy text Area Forecast (FA).

---

Terminal Aerodrome Forecast (TAF) Architecture

A Terminal Aerodrome Forecast is a concise statement of expected meteorological conditions at an airport during a specified time window. Ground instructors must emphasize three fundamental parameters tested heavily on FAA knowledge exams:

1. Geographic Coverage Area: A TAF applies strictly to an area within a 5-statute-mile radius from the center of the airport runway complex.
2. Issuance Frequency: Routine civilian TAFs are issued four times daily at six-hour intervals:
   • 0000Z, 0600Z, 1200Z, and 1800Z.
3. Valid Forecast Periods: Standard civilian TAFs are valid for 24 hours. However, for major international air carrier hub airports (such as Chicago O'Hare KORD, Dallas/Fort Worth KDFW, Atlanta KATL, Denver KDEN, New York JFK KJFK, and Los Angeles KLAX), TAFs are issued with 30-hour valid periods to accommodate long-haul international flights and extended flight dispatch planning.

Baseline TAF Format Elements

The structure of a TAF closely mirrors the alphanumeric syntax of a METAR:

• Header & Station: Type of report (TAF), station identifier (4-letter ICAO code, e.g., KORD), and issuance date/time (DDHHMMZ, e.g., 261130Z = issued on the 26th day at 1130 UTC).
• Valid Period Group: Coded as DDHH/DDHH (e.g., 2612/2718 = valid from the 26th day at 1200Z through the 27th day at 1800Z, representing a 30-hour forecast period).
• Baseline Forecast Line: Immediately follows the validity group, stating prevailing winds (direction in True North and speed in knots), visibility in statute miles (P6SM indicates visibility greater than 6 statute miles), significant weather phenomena, and cloud layers in hundreds of feet AGL (BKN030, OVC080).
• Non-Convective Low-Level Wind Shear Group (WS): When significant non-convective low-level wind shear is expected below 2,000 feet AGL, a wind shear group is inserted following the sky condition: WS[altitude AGL]/[direction true][speed KT].
  • Example: WS020/22045KT denotes low-level wind shear at 2,000 feet AGL with winds forecast from 220° True at 45 knots.

---

TAF Change Groups in Depth

Conditions rarely remain constant over a 24- or 30-hour valid period. Forecasters convey expected atmospheric transitions using four change groups: FM, BECMG, TEMPO, and PROB30. (NWS TAFs generally omit BECMG, which appears more often in military and international TAFs.)

1. The FM (From) Group

The FM (From) group indicates a rapid, significant, and permanent change in prevailing meteorological conditions expected to occur within less than one hour.

• Syntax: FM followed by a 6-digit day, hour, and minute group (e.g., FM261600 = From the 26th day at 1600Z).
• The Complete Replacement Rule: The occurrence of an FM group completely supersedes and replaces all previous forecast lines. All previous forecast conditions (winds, visibility, weather, clouds) are nullified. Therefore, an FM line must always contain a complete restatement of all required forecast parameters: wind, visibility, weather, and sky condition.

2. The BECMG (Becoming) Group

The BECMG (Becoming) group indicates a gradual, permanent change in conditions occurring over a specified two-digit hour span.

• Syntax: BECMG DDHH/DDHH (e.g., BECMG 2614/2616 = Becoming between 1400Z and 1600Z on the 26th).
• Conditions transition progressively during the specified window and remain in effect permanently until the next change group or the end of the TAF. Unlike FM, only the specific meteorological elements that are changing are listed in the BECMG line; unchanged baseline conditions carry forward.

3. The TEMPO (Temporary) Group

The TEMPO (Temporary) group denotes temporary fluctuations in conditions that are expected to:

1. Last less than 1 hour in each individual occurrence; and
2. In the aggregate, occur during less than half of the time period designated by the group.

• Syntax: TEMPO DDHH/DDHH (e.g., TEMPO 2618/2622 2SM TSRA BKN020CB).
• Operational Decoding: Between 1800Z and 2200Z on the 26th, transient episodes of 2 SM visibility in thunderstorms with rain and broken cumulonimbus at 2,000 feet AGL are expected. Outside of these brief passing showers, the primary baseline forecast conditions remain fully intact.

4. The PROB30 (Probability 30%) Group

The PROB30 (Probability 30%) group indicates a 30% probability of hazardous weather (thunderstorms or precipitation) and associated visibility or ceiling reductions occurring during the designated time window.

• Syntax: PROB30 DDHH/DDHH (e.g., PROB30 2702/2706 3SM -SN BKN020).

The PROB30-Only Rule for NWS TAFs: Candidates frequently confuse civilian FAA TAFs with military or international formats. In civilian FAA CONUS TAFs, PROB40 is NEVER used! If the forecaster evaluates a 30% likelihood of weather, PROB30 is coded. If the forecaster evaluates a 50% or greater probability, the phenomenon must be forecast using standard baseline, FM, BECMG, or TEMPO groups. (A 40% probability is not encoded under FAA civilian standards; PROB40 appears strictly in military or foreign ICAO TAFs). Furthermore, PROB30 is not used in the first 9 hours of a TAF's valid period, including amendments (NWS Instruction 10-813).

---

Graphical Forecasts for Aviation (GFA)

In October 2017, the legacy text Area Forecast (FA) for the contiguous United States was retired, replacing it with the interactive web-based Graphical Forecasts for Aviation (GFA). Ground instructors must ensure students understand why this transition occurred: legacy Area Forecasts provided coarse, multi-state text paragraphs that were often ambiguous and difficult to apply to specific flight routes. The GFA provides continuous, high-resolution visual meteorological data across the entire contiguous United States.

GFA Structure & Operational Scope

• Geographic Domain: Covers the contiguous United States (with views for Alaska, Hawaii, the Gulf, the Caribbean, and nearby oceanic areas) from the surface up to FL480.
• Temporal Resolution: Displays observations up to 18 hours in the past and forecasts in hourly steps up to 18 hours ahead.

Core GFA Interactive Layers & Tabs

The GFA web interface provides specialized interactive layers accessible via altitude slider bars and regional drill-down windows:

1. Clouds Tab: Depicts cloud coverage (scattered, broken, overcast), cloud base altitudes (in hundreds of feet AGL or MSL), cloud tops, and significant convective cloud formations.
2. Precipitation & Weather Tab: Integrates composite radar reflectivity with surface weather phenomena, differentiating liquid rain, freezing rain, mixed precipitation, and snow.
3. Icing Tab: Depicts structural icing potential, severity (trace, light, moderate, heavy), and Supercooled Large Droplets (SLD) risk from the surface to FL480, allowing pilots to slice through specific cruising altitudes.
4. Turbulence Tab: Displays forecast Clear Air Turbulence (CAT), Low-Level Turbulence (LLT), Mountain Wave activity, and Eddy Dissipation Rate (EDR) across selectable flight levels.
5. Winds Tab: Maps surface wind vectors, streamlines, isotachs, and upper-level winds aloft.
6. Visibility & Ceiling Tab: Translates surface visibility and cloud ceilings into standard flight category classifications.

Standard Flight Category Classifications

The GFA displays flight categories using standardized FAA color-coding recognized across civilian aviation:

• LIFR (Low Instrument Flight Rules) — MAGENTA:
  • Ceiling: Less than 500 feet AGL; and/or
  • Visibility: Less than 1 statute mile.
• IFR (Instrument Flight Rules) — RED:
  • Ceiling: 500 feet to less than 1,000 feet AGL; and/or
  • Visibility: 1 statute mile to less than 3 statute miles.
• MVFR (Marginal Visual Flight Rules) — BLUE:
  • Ceiling: 1,000 feet to 3,000 feet AGL; and/or
  • Visibility: 3 statute miles to 5 statute miles.
• VFR (Visual Flight Rules) — GREEN:
  • Ceiling: Greater than 3,000 feet AGL; and
  • Visibility: Greater than 5 statute miles.

Note: Either the ceiling or the visibility can drive the flight category. If the ceiling is 800 feet (IFR) but visibility is 10 miles (VFR), the overall flight category is IFR.

Vertical Cross-Section Profile Tool

A premier capability of the GFA is the Vertical Cross-Section tool. By inputting a flight route between waypoints (e.g., KORD to KDEN), the tool generates a 2D vertical cut of the atmosphere displaying terrain elevations, cloud bases and tops, forecast icing bands, and turbulence layers directly along the planned route and altitude profile.

---

Practical TAF Decoding Scenario

Analyze the following 30-hour terminal aerodrome forecast for Chicago O'Hare (KORD):

TAF KORD 261130Z 2612/2718 20014KT P6SM BKN040 WS020/24045KT
  FM261600 24018G28KT 5SM -SHRA OVC025
  TEMPO 2618/2622 2SM TSRA BKN015CB
  FM262300 31015KT P6SM SCT050
  PROB30 2702/2706 3SM -SN BKN020

1. 2612/2718 20014KT P6SM BKN040 WS020/24045KT: Valid starting on the 26th at 1200Z. Wind 200° True at 14 knots, visibility greater than 6 SM, ceiling broken at 4,000 feet AGL. Low-level wind shear present at 2,000 feet AGL with winds from 240° True at 45 knots.
2. FM261600 24018G28KT 5SM -SHRA OVC025: At 1600Z on the 26th, rapid and complete change. Wind shifts to 240° True at 18 knots gusting to 28 knots, visibility decreases to 5 SM in light rain showers, ceiling drops to overcast at 2,500 feet AGL (MVFR). The low-level wind shear group from the previous line is now completely cancelled.
3. TEMPO 2618/2622 2SM TSRA BKN015CB: Between 1800Z and 2200Z, temporary fluctuations lasting under 1 hour with visibility 2 SM in moderate thunderstorms with rain, and broken cumulonimbus ceiling at 1,500 feet AGL.
4. FM262300 31015KT P6SM SCT050: At 2300Z, rapid cold frontal passage. Wind shifts to 310° True at 15 knots, visibility improves to greater than 6 SM, sky scattered at 5,000 feet AGL (no ceiling; VFR conditions).
5. PROB30 2702/2706 3SM -SN BKN020: Between 0200Z and 0600Z on the 27th, a 30% probability of visibility dropping to 3 SM in light snow with a broken ceiling at 2,000 feet AGL.