The TAF, the Four Change Groups That Tell the Real Story, and the TEMPO Window That Has Trapped More Than a Few Student Pilots

A Terminal Aerodrome Forecast is a time-based prediction of weather conditions at a specific airport, and it communicates far more than the numbers at first glance. Where a METAR is a snapshot of current conditions, a TAF is a movie - and that movie has a plot. If you don’t know how to read the change groups, you’re only watching the opening scene.

What Is a TAF and How Is It Issued?

TAFs are issued by the National Weather Service four times each day, at 0000, 0600, 1200, and 1800 Zulu. Most airports receive a 24-hour TAF. Major hub airports receive a 30-hour TAF. Some airports receive no TAF at all, which is itself useful information when planning a cross-country and selecting fuel stops.

When conditions change significantly between scheduled issue times, the forecaster publishes an amended TAF. You’ll see AMD at the beginning of the product. If you pulled a briefing an hour ago and conditions are moving fast, verify whether an amendment has been issued - your old briefing may no longer reflect the current forecast.

How Do I Read the TAF Header and Valid Period?

Every TAF opens with a header identifying the airport and the period it covers. The four-letter ICAO identifier is followed by a date-time group showing when the TAF was issued, then the valid period.

A TAF reading TAF KDEN 290600Z 2906/3006 covers Denver International, issued at 0600 Zulu on the 29th, valid from 0600 on the 29th through 0600 on the 30th - a 24-hour window.

If your planned arrival falls outside the valid period, there is no official forecast for that moment yet. That gap requires an alternate source of information or a conservative planning assumption.

What Do the Base Conditions in a TAF Mean?

After the header, the TAF opens with base conditions - the starting point for the entire forecast. This section reads almost identically to a METAR: wind, visibility, sky condition, weather phenomena. The codes are largely the same. If you can read a METAR, you can read the base conditions of a TAF.

These base conditions apply from the start of the valid period until the first change group takes effect. Everything that follows modifies or replaces them.

What Are the Four TAF Change Groups?

Change groups transform a static snapshot into a timeline. There are four types, and knowing them cold is essential to using a TAF correctly.

FM - From. An FM group signals a complete replacement of all previous conditions at a specific time. It is not an addition or a modification - it is an entirely new forecast that takes effect the moment that timestamp arrives. If you see FM1400, then at 1400 Zulu, every condition from the previous forecast is replaced by whatever follows that FM group, and those conditions persist until the next change group supersedes them.

This is the change group with the greatest planning impact. If your arrival time falls after an FM group, you are arriving under those FM conditions, not the base conditions you saw in the header.

BECMG - Becoming. A BECMG group describes a gradual transition expected to complete over a specified window, typically two hours. Unlike FM, which flips like a switch, BECMG describes conditions bleeding from one state to another. By the end of the BECMG window, the new conditions are established and they persist afterward.

TEMPO - Temporary. TEMPO describes a fluctuation, not a transition. The conditions in a TEMPO group are expected to last less than one hour at a time and to occur during less than half of the period it covers. The underlying base or FM conditions still apply outside those brief windows. TEMPO is the forecaster saying: it will probably be fine, but watch for these pockets.

PROB - Probability. PROB appears as either PROB30 or PROB40, representing a 30% or 40% chance of those conditions occurring. You will almost never see PROB50 or higher in a TAF. When a forecaster believes there is better than a 50% chance of something happening, they write it into the base forecast, or as a BECMG or TEMPO. PROB is reserved for conditions the forecaster cannot ignore but is not confident enough to make the main event.

PROB30 is one-in-three odds. If a TAF shows PROB30 for thunderstorms during your arrival window, that is not a number to dismiss. The forecaster is flagging something they’re watching. You should be watching it too.

How Do I Decode a TAF Example Step by Step?

Consider a hypothetical TAF with the following structure:

Base conditions: Wind 210 at 8 knots. Visibility 10 miles. Sky clear. Temperature 12°C. A clean, easy morning.

FM1200: Wind 290 at 22 knots, gusting to 35. Visibility 5 miles. Light rain and mist. Ceiling broken at 1,800 feet.

At noon Zulu, all of those comfortable morning conditions are gone. The wind has shifted and increased significantly, there is precipitation, and the ceiling sits at 1,800 feet broken. That is still VFR - but it is a meaningfully different flight than the morning picture suggested.

TEMPO 1500/1900: Visibility 1 mile in heavy rain. Ceiling broken at 400 feet.

For four hours in the mid-to-late afternoon, the forecast includes temporary windows where that airport drops below VFR minimums. An arrival at 1800 Zulu is outside the TEMPO window. An arrival at 1600 Zulu is squarely inside it.

The TAF is asking whether you have a plan for that TEMPO window.

What Does a TEMPO Window Mean for My Flight Planning?

For VFR pilots, a TEMPO for below-minimums conditions during the arrival window is a go, no-go, or delay decision. Can you push the departure back to arrive after the TEMPO clears? If not, do you have enough fuel to hold, divert, and land safely? Do your personal minimums allow for the ceiling and visibility in that TEMPO group?

For IFR pilots, the conversation shifts to alternates. The alternate requirement is not paperwork - it is a forecast-based decision, and the TAF is the document that makes that decision defensible or indefensible. If your destination’s TAF shows deteriorating conditions during your arrival window and your alternate’s TAF shows the same front arriving an hour later, your alternate may not be available when you need it.

The correct approach is to overlay both TAFs against your planned timeline and confirm that the divert option is genuinely available during the specific hours you would need it - not just at the time you file the flight plan.

How Reliable Are TAFs, and What Other Products Should I Use?

TAF accuracy has improved substantially as numerical weather prediction has advanced. But TAFs are point forecasts for a single airport. They do not capture localized effects at smaller scales: valley fog developing from a river drainage, a sea breeze arriving two hours early, or a microclimate that consistently behaves differently from the broader synoptic picture.

PIREPs (Pilot Reports) fill part of that gap. If the TAF shows TEMPO for low visibility during your arrival window and there are PIREPs showing 600 overcast and 1-mile visibility that have been consistent for the past two hours, the TEMPO has already started. The TAF’s timing may be off by an hour - that is not a failure of the forecast; it is why no single product is used in isolation.

The Area Forecast Discussion (AFD) is the plain-language product where the forecaster explains the reasoning behind their forecast. The TAF gives you the forecast; the AFD gives you the confidence level behind it. An AFD noting “confidence is high in FM timing due to a well-defined frontal boundary” is very different from one that reads “timing of the FM change is uncertain by ±3 hours due to uncertainty in frontal progression.” Same TAF. Very different planning implications.

Good weather briefing builds a coherent picture from multiple sources. The TAF is the anchor document - most powerful when read alongside current METARs, regional PIREPs, and the forecaster’s own AFD.

What Does TAF Complexity Tell Me?

The number and type of change groups in a TAF communicates something about the complexity of the weather itself. A TAF with no change groups - just base conditions valid for 24 hours - is a confident forecast. The forecaster looked at the models and concluded conditions would be stable all day.

A TAF with three FM groups, two TEMPO blocks, and a PROB is a forecaster saying: there is a lot happening, and I’ve done my best to characterize it, but this system has moving parts.

That complexity is information. When a TAF looks complicated, slow down. Read every change group carefully, identify exactly where your planned arrival falls in the timeline, and ask whether your fuel planning and alternate selection account for the scenario where the messy part of the forecast arrives earlier than expected.

What Do Checkride Examiners Expect When They Ask About TAFs?

Oral exam examiners present weather scenarios and ask how you would use them. They are not looking for a recitation of TAF codes. They want to see decision-making.

A strong answer sounds like: My arrival falls inside a TEMPO window, so I’m planning to arrive with an additional 45 minutes of fuel beyond my required reserve, my alternate has a clean forecast through the evening hours, and my personal minimums account for the potential ceiling in that TEMPO group. That answer demonstrates genuine understanding.

A TAF does not tell you whether to go or not to go. It gives you the information to make that decision yourself, with full awareness of what you’re flying into. The pilot who sees a TEMPO window, builds a contingency, and launches anyway is not less cautious than the pilot who calls it a no-go. They are simply a pilot who has prepared for what the sky is planning.

Key Takeaways

• FM completely replaces all previous conditions at a specific time - if your arrival falls after an FM group, those are your arrival conditions.
• BECMG describes a gradual transition completing over a window, typically two hours; conditions persist after.
• TEMPO describes brief fluctuations expected to last less than one hour at a time, in less than half the covered period - base conditions still apply outside those windows.
• PROB30/PROB40 represent 30% and 40% odds respectively; you will almost never see PROB50 in a TAF because higher-confidence forecasts are written into the base forecast or change groups.
• TAF complexity itself communicates forecaster confidence - a complicated TAF is a prompt to slow down and plan for uncertainty.
• Always cross-reference the TAF with current METARs, PIREPs, and the Area Forecast Discussion for the full picture.