The Terminal Aerodrome Forecast, the Change Groups Hidden in Plain Sight, and the Alternate Planning Window Every Cross-Country Student Misreads

The Terminal Aerodrome Forecast (TAF) is one of the most misread documents in preflight planning. Pilots frequently scan the prevailing conditions at the top, see acceptable numbers, and close the app - missing the change groups that contain the most operationally significant information. Reading a TAF correctly requires reading all of it, including everything that follows the initial good-looking numbers.

What Is a TAF and What Does It Actually Cover?

A TAF is a point forecast. It covers a specific airport within a five nautical mile radius of that airport’s reference point - not your route, not the surrounding terrain. That distinction matters when briefing a cross-country, because conditions between your departure and destination can differ significantly from what either TAF describes.

Most TAFs cover 24 hours. Airports with significant commercial traffic typically issue 30-hour TAFs. The National Weather Service updates every forecast every six hours, meaning the TAF you’re reading is never more than six hours old under normal circumstances. When conditions change faster than expected, an amended TAF is issued.

How Is a TAF Structured?

A TAF contains four main components: a header, a valid time period, a prevailing conditions group, and change groups.

The header includes the four-letter ICAO station identifier (the same one on a METAR), the issuance time, and the valid period - a date-time group showing when the forecast starts and ends.

The prevailing conditions group describes wind, visibility, weather phenomena, and sky conditions at the opening of the forecast period. It looks like a METAR and reads like one, which is exactly why pilots stop there. The prevailing conditions describe only how the airport is expected to begin the forecast period. The hours where you’re actually flying live in the change groups.

How Do I Read Wind and Visibility in a TAF?

Wind is given as a three-digit direction in degrees magnetic, followed by speed in knots. 18015 means wind from the south at 15 knots. 27012G22 means wind from the west at 12 knots, gusting to 22 knots. The gust value represents the expected maximum - instantaneous peaks beyond that figure are possible. The TAF is a probabilistic forecast, not a ceiling on what the atmosphere will do.

Visibility is in statute miles. P6SM means greater than six statute miles. 2SM puts you in IMC territory for most approaches. Below 1 statute mile, IFR approach minimums and alternate planning become serious math.

Weather phenomena use standard METAR codes: RA for rain, SN for snow, BR for mist, FG for fog, TS for thunderstorms, TSRA for thunderstorms with rain, HZ for haze.

Sky conditions follow the same coding as METARs. FEW is 1–2 eighths coverage. SCT (scattered) is 3–4 eighths. BKN (broken) is 5–7 eighths. OVC (overcast) is full coverage. The number following is the cloud base in hundreds of feet above field elevation. BKN010 means a broken ceiling at 1,000 feet - your ceiling for approach planning purposes.

What Are the Four Change Groups in a TAF?

FM - From

FM is the sharpest change group in a TAF. FM1400 means that at 1400Z, the entire forecast resets. Everything forecast before that line is no longer relevant - FM is a full replacement. New wind, new visibility, new sky conditions begin at that exact time. Draw a mental line: before that time is one picture; after it is a completely different picture.

BECMG - Becoming

BECMG indicates a gradual transition over a specified window, such as BECMG 0800/1000. Conditions will change from the previous values to the new ones sometime during those two hours. The forecaster is confident the change will happen but less certain of the exact moment. By the end of the window, conditions should have settled into the new values.

TEMPO - Temporary

TEMPO describes conditions expected to occur intermittently - for less than one hour at a time and for less than half of the total stated period. A TEMPO 1200/1600 group showing 800 overcast and 2 miles in rain means those conditions may briefly occur during that four-hour window. They won’t be sustained, but they are possible.

Temporary does not mean ignorable. If your estimated arrival falls inside a TEMPO window and conditions drop during your approach, you are dealing with those numbers at a moment with limited options. The TEMPO group is a flag - the TAF’s way of saying these conditions can happen.

PROB - Probability

PROB30 indicates approximately a 30% probability. PROB40 indicates approximately 40%. You will never see PROB50 or higher in a TAF - at 50%, the forecaster folds that condition into the prevailing forecast instead.

PROB groups are often written in combination with TEMPO: PROB40 TEMPO means roughly a 40% chance those temporary conditions occur during the stated period. FAA guidance holds that PROB30 groups should not serve as the primary basis for alternate planning decisions - the probability is considered too low to drive major operational choices. A PROB40 TEMPO group showing thunderstorms or low IFR conditions is a different matter entirely.

What Is the 1-2-3 Rule for IFR Alternate Planning?

The 1-2-3 rule determines when filing an alternate airport is required. If your destination is forecast to be below a 2,000-foot ceiling or 3 statute miles visibility from one hour before to one hour after your estimated time of arrival (ETA), you must file an alternate for IFR operations.

Evaluate that window by reading the entire TAF and identifying any FM, BECMG, or TEMPO groups that fall within that range. If any of those groups show conditions below 2,000 feet or 3 miles, file an alternate.

Example: Your ETA is 1500Z. Your planning window is 1400Z to 1600Z. The prevailing conditions look fine, but a TEMPO 1300/1700 group shows 800 overcast and 2 miles in rain. That TEMPO overlaps your window. Conditions could drop below the 1-2-3 threshold while you’re inbound on the approach. An alternate is required.

What TAF Requirements Apply to the Alternate Airport Itself?

Your alternate airport must forecast conditions meeting alternate minimums during that same one-hour-before to one-hour-after window.

• Precision approach alternate: ceiling of 600 feet and 2 statute miles visibility
• Non-precision approach alternate: ceiling of 800 feet and 2 statute miles visibility

Your alternate cannot be just any nearby airport. It has to be an airport where the TAF gives reasonable confidence the approaches will be usable when you need them. That means comparing the alternate’s TAF against those minimums - not glancing at the numbers at the top of the forecast.

What Is an Amended TAF and When Does It Change Everything?

When actual conditions diverge significantly from what was forecast, or when new weather develops that the original forecast didn’t capture, the National Weather Service issues an AMD TAF. You will see AMD in the header. An AMD supersedes the original forecast completely. The previous forecast no longer applies.

AMD TAFs are not a sign of forecaster error - they are the system doing exactly what it is designed to do. The amendment represents a real-time correction from someone watching current observations. The appropriate response is to re-evaluate your planning from scratch using the new forecast.

This is why a single preflight briefing isn’t sufficient for a flight that is several hours away. Brief again closer to departure. Brief again if there is a delay. An AMD can be issued any time conditions warrant, and the TAF from two hours ago may bear no resemblance to the one that exists now.

How Do I Use the ATIS to Verify the TAF While Airborne?

As you approach your destination, listen to the ATIS (Automatic Terminal Information Service). The ATIS is built from the current METAR - the actual observation, not the forecast.

Compare what you hear to what the TAF said would be happening at that time. If the TAF showed improving conditions by your arrival but the ATIS is still reporting IFR, that BECMG group hasn’t materialized on schedule or that FM change hasn’t taken hold yet. Your alternate is becoming more relevant, not less. Actively tracking whether the weather is following the forecast puts you ahead of the situation instead of reacting to it.

Key Takeaways

• Read the entire TAF, not just the prevailing conditions. Change groups are where the operationally significant information lives.
• FM is a full forecast replacement at a specific time. BECMG is a gradual transition over a window. TEMPO is brief but possible - take it seriously if it falls within your arrival window.
• Apply the 1-2-3 rule: any forecast below 2,000 feet ceiling or 3 statute miles visibility from one hour before to one hour after your ETA requires filing an alternate.
• PROB30 is generally too low to drive alternate decisions. PROB40, especially combined with TEMPO showing low IFR or thunderstorms, demands attention.
• Check for AMD every time you re-brief. If you see it, start your planning over with the new forecast.