The Terminal Aerodrome Forecast, the Twenty-Four-Hour Window Students Rush Past During the Briefing, and the Change Groups That Reveal What the Sky Looks Like When You Actually Arrive

The Terminal Aerodrome Forecast predicts conditions at a specific airport over a 24-hour window - sometimes 30 hours at major hubs - and it is the single most important weather document for knowing whether your destination will be accessible when you arrive. Many pilots who can decode a METAR fluently have never worked through a TAF’s change groups, which is where the forecast actually earns its value.

What Is a TAF and What Does It Cover?

A TAF is a forecast, not an observation. It is written by a human meteorologist - or a human reviewing a computer model - and reflects their best judgment about how the atmosphere will behave over the forecast window. The National Weather Service issues a fresh TAF four times each day, at approximately 0000, 0600, 1200, and 1800 Zulu. Always use the most recently issued TAF; an older one may still show the overall pattern but lacks the latest forecaster input.

One detail that surprises many students: a TAF covers only the airspace within five statute miles of the airport it describes. It tells you what to expect at the runway environment, not what conditions look like en route. For in-flight conditions along your route, read AIRMETs, SIGMETs, prog charts, and pilot reports. For whether your destination will be usable when you get there, the TAF is the document.

How Do You Read the TAF Header?

Every TAF begins with a header block containing three key elements.

Airport identifier. The four-letter ICAO identifier matches the one in METARs - Chicago Midway is KMDW, Denver International is KDEN.

Issuance time. A six-digit group followed by Z for Zulu. The first two digits are the day of the month, the next four are the Zulu time. 161200Z means the sixteenth at 1200 Zulu.

Valid period. This defines the forecast window. 1612/1712 means valid from the sixteenth at 1200 Zulu through the seventeenth at 1200 Zulu - a full 24-hour window. Every condition in that TAF applies somewhere within those bounds.

How Do You Read the Prevailing Conditions?

After the header, the TAF states prevailing conditions: what meteorologists expect for the majority of the valid period, absent any change groups modifying it.

Wind is a five-digit group. The first three digits are direction in degrees magnetic, the next two are speed in knots, and a G plus digits adds gusts. 27015G25 means winds from 270° at 15 knots, gusting to 25. Variable winds below three knots appear as VRB followed by speed - VRB02 means variable at two knots.

Visibility in a TAF is always in statute miles. P6SM means six miles or better - the practical top of the scale. Fractional values appear when conditions drop into marginal or instrument flight rules range.

Weather phenomena use the same codes as METARs: RA for rain, SN for snow, BR for mist, FG for fog, TS for thunderstorms. If no significant weather is expected, the field is omitted entirely.

Sky condition also mirrors METAR format. FEW is 1–2 eighths coverage, SCT (scattered) is 3–4 eighths, BKN (broken) is 5–7 eighths, and OVC (overcast) is 8 eighths. Heights are in hundreds of feet above ground level - OVC008 is an overcast at 800 feet, BKN040 is broken at 4,000 feet.

The prevailing section is your baseline. Change groups tell you when, how, and how much that baseline shifts.

What Are the Four Change Group Types?

There are four change group types, and each one carries a distinct meaning. Misreading any of them changes the picture.

FM - “From”

FM is the most definitive change group. When FM appears followed by a time, all prior conditions are completely replaced at that exact moment. Nothing carries over. FM signals a significant, abrupt change - a cold front passage, a sharp wind shift behind a trough. Everything after FM is the complete new weather picture from that time forward.

BECMG - “Becoming”

BECMG indicates a gradual transition over a defined window. BECMG 1418/1422 means the change begins at 1400 Zulu and is complete by 2200 Zulu on the fourteenth. Conditions are in transition within that window; you do not know the exact minute the change finishes.

An important nuance: BECMG only replaces the elements it explicitly lists. If a BECMG group shows a new wind but omits visibility and sky condition, only the wind changes. Visibility and sky carry forward from the prevailing section until another change group modifies them.

TEMPO - “Temporary”

TEMPO conditions are expected to last less than one hour at a time and to occur for less than half of the specified period. They are transient - moving in and out. The surrounding prevailing conditions still exist; TEMPO conditions are overlaid on top of them, occurring briefly and repeatedly within the window.

This does not make TEMPO insignificant. TEMPO TS means temporary thunderstorms. A TEMPO visibility of a quarter mile in heavy rain is a fundamentally different flight environment from the prevailing six miles and clear surrounding it. If TEMPO conditions are below VFR minimums and you are not instrument rated, even a brief drop matters when you are established on a seven-mile final with nowhere to go.

PROB - Probability

PROB groups appear as PROB30 or PROB40, indicating a 30% or 40% probability of those conditions occurring. The National Weather Service does not write PROB groups below 30%, so any PROB group in a TAF is at minimum at that threshold.

Thirty percent can sound manageable until you consider what is at stake. If the PROB group describes conditions below VFR minimums, that is roughly one in three - one in three times you fly that route under those conditions, the destination drops out from under you. The question to answer in preflight planning is: do you have a genuine alternate with a clear TAF, and do you have the fuel to reach it?

PROB groups can pair with TEMPO: PROB30 TEMPO is the lowest-confidence flag in the TAF, indicating a 30% chance of temporary conditions. Still worth noting when those conditions would ground a VFR pilot.

How Do Change Groups Work Together in Practice?

Consider a cross-country with a three-hour leg departing at noon local. The destination TAF shows:

• Prevailing: Winds 260° at 12 knots, visibility greater than 6 miles, sky clear.
• TEMPO 1400/1900Z: Visibility 1½ statute miles in mist, overcast at 300 feet.
• FM 2200Z: Winds shift to 030° at 8 knots, visibility 4 miles in light drizzle, broken at 1,200 feet.

In plain terms: the early afternoon looks clear, but between 1400 and 1900 Zulu there will be temporary periods of overcast at 300 feet and visibility under 2 miles - below basic VFR minimums. After 2200Z, conditions become marginal VFR with a broken ceiling at 1,200 feet and 4 miles in drizzle.

With a noon departure and a three-hour flight, arrival potentially falls inside that TEMPO window. The TEMPO is not saying conditions are bad the entire afternoon - it is saying there will be periods of below-VFR conditions within that window, and those need to be accounted for.

Options include departing earlier to land ahead of the window, identifying a divert airport with a clear TAF and confirming fuel reserves to reach it, or waiting for the next day. All are legitimate decisions. The TAF makes them available on the ground, not in the cockpit two miles from a runway that is barely visible.

Does Your Alternate Airport’s TAF Matter?

Yes - and it matters as much as the destination TAF. If conditions deteriorate en route, the alternate has to be genuinely usable. That means pulling its TAF and confirming that its change groups do not overlap your arrival window in the same way the destination’s do. An alternate with a TEMPO below VFR during your planned arrival window is not a real alternate.

What Is an Amended TAF?

TAFs can be updated before the next scheduled issuance if conditions deviate significantly from what was forecast. An amended TAF carries AMD in the header. When pulling an in-flight weather update and the TAF has been recently amended, that is a meaningful signal: the atmosphere did not behave as forecasters expected. It warrants a closer look at what else might be different than predicted.

How Does TAF Reading Apply to the Private Pilot Checkride?

The Airman Certification Standards (ACS) require demonstrating a thorough weather briefing and defending a go/no-go decision. The examiner is not looking for a candidate who cancels every flight with any uncertainty in the forecast - they are looking for a candidate who identifies the relevant factors and makes a reasoned call.

Being able to name the destination TAF, explain what each change group describes, and articulate how those conditions factor into your decision is exactly the kind of weather literacy that separates a confident, ready candidate from one who memorized codes without understanding the underlying tools.

Leidos Flight Service - the briefing provider behind ForeFlight and most other aviation weather apps - includes the TAF for your destination and planned alternates in any standard briefing. Read them every time. Not just the METAR. Not just the current conditions. The full 24-hour forecast with every change group. That is what tells you what the sky looks like when you actually arrive.

Key Takeaways

• A TAF forecasts conditions at a specific airport within a 5-statute-mile radius for the next 24 hours (up to 30 hours at major hubs), issued four times daily by the National Weather Service.
• FM replaces all prior conditions at a precise time; BECMG transitions gradually over a window and only modifies elements it explicitly lists; TEMPO overlays brief, repeating conditions on the prevailing picture; PROB30/40 flags conditions with a 30–40% probability of occurring.
• BECMG only changes the elements it lists - all other elements carry forward from prevailing conditions until another change group acts on them.
• A PROB30 group describing below-VFR conditions means roughly a one-in-three chance your destination drops below minimums during that window. That is a number to plan around, not dismiss.
• Read the full TAF for both your destination and your alternate on every cross-country briefing - current conditions at home base tell you nothing about what the sky looks like two hours away when you arrive.