What is the difference between TEMPO and BECMG in a TAF?

BECMG describes a gradual, permanent transition from one condition to another across a defined time window. TEMPO describes brief, temporary fluctuations that overlay the base conditions without replacing them - the base forecast still applies as the dominant scenario.

Does a TEMPO group mean conditions will definitely drop to those values?

Not with certainty, but it means a meteorologist determined those conditions would occur for some portion of that window. TEMPO is not a "maybe" - it is a forecast of brief but real deterioration. If a TEMPO covers your arrival time, you should plan for those conditions, not discount them.

How long can TEMPO conditions last?

By definition, a TEMPO event occurs for less than one hour at a time and affects less than half of the overall forecast period. A single TEMPO event can last up to 59 minutes - long enough that you may need to fly a missed approach, hold, and attempt again.

What is PROB30 or PROB40 in a TAF?

PROB indicates a 30 or 40 percent probability of the stated conditions. The National Weather Service only uses PROB30 or PROB40; anything at 50 percent or above is issued as a TEMPO instead, and anything below 30 percent is not included in the forecast.

Where can I find the Area Forecast Discussion to understand a TAF better?

The Area Forecast Discussion is available at aviationweather.gov. It is the free-text explanation meteorologists write alongside their forecasts, describing their confidence level and reasoning - particularly useful when a TAF change group seems inconsistent with the rest of your weather briefing.

The Terminal Aerodrome Forecast, the TEMPO Group That Catches Student Pilots Off Guard, and the Change Lines That Predict Whether Your Destination Will Be Open When You Land

The Terminal Aerodrome Forecast (TAF) is the most information-dense weather product in a pilot’s preflight briefing - and the most commonly misread. Most student pilots scan the base conditions, check the ceiling and visibility, and move on. The change groups that follow tell the real story of what will happen at your destination when you arrive.

What Is a TAF and What Does It Cover?

A TAF is a coded weather forecast for conditions at a specific airport, covering a 25-mile radius around that airport. The National Weather Service issues them, trained meteorologists write them, and they are updated four times a day. Depending on the airport, the forecast runs for either a 24-hour or 30-hour period.

The critical distinction between a TAF and a METAR: a METAR tells you what the weather is right now. A TAF tells you what the weather is going to do. On a cross-country flight, what matters is the forecast for when you arrive - not current conditions at your departure airport.

How to Read the TAF Header

Every TAF starts with a header. You’ll see the word TAF, sometimes followed by AMD (amended) or COR (corrected). Then comes the four-letter ICAO station identifier, followed by the issue time in day/hour/minute format in Zulu time, and finally the valid period.

A valid period shown as 2400/2418 means the forecast covers the 24th day of the month from 0000Z through 1800Z - an 18-hour window. Confirming that your arrival time falls inside the valid period is the first thing to verify before reading anything else.

What Is the Base Forecast Line?

The first line after the header is the base forecast. It gives initial conditions at the start of the valid period: wind direction and speed, visibility in statute miles, sky condition in feet AGL, and any weather phenomena. The format is similar to a METAR.

This is where most pilots stop reading. Everything that follows - the change groups - is where the forecast actually earns its place in your briefing.

What Are TAF Change Groups?

Change groups describe how conditions evolve across the forecast period. There are four types: FM, BECMG, TEMPO, and PROB. Each means something different for your flight planning, and treating them as interchangeable is how pilots end up surprised by conditions their TAF predicted.

What Does FM Mean in a TAF?

FM stands for From. When you see FM, conditions are expected to change rapidly - within about 30 minutes - and the new conditions completely replace everything that came before. FM is a hard reset on the forecast.

FM1400 means that starting at 1400Z, an entirely new set of conditions applies. Previous ceiling, wind, and visibility values are no longer relevant. FM is the most straightforward change group and the one most pilots actually read when they see it.

What Does BECMG Mean in a TAF?

BECMG stands for Becoming. It describes a gradual transition from one condition to another across a specified time window, given as a start and end time.

BECMG 1200/1400 means conditions are transitioning starting no earlier than 1200Z and reaching the new state by 1400Z. The exact moment of change isn’t specified - only the window. If your arrival falls inside a BECMG window, plan for the worst of both the old and new conditions. That’s the conservative read, and in aviation, conservative is almost always right.

What Does TEMPO Mean in a TAF - and Why Does It Catch Pilots Off Guard?

TEMPO stands for Temporary. It means conditions are expected to fluctuate briefly to the stated values - occurring for less than one hour at a time and for less than half of the forecast period. A TEMPO group does not replace the base conditions. It overlays them.

This is the distinction that bites student pilots. The base forecast remains the dominant scenario, but during the TEMPO window, conditions will temporarily drop to those lower values. A base forecast showing ceiling 3,000 feet, visibility 5 miles paired with a TEMPO group showing ceiling 500 feet, visibility 1 mile in fog is telling you that most of the time you’ll have VFR conditions - but during that window, expect periods of IMC.

TEMPO does not mean unlikely. It means brief but real. A meteorologist looked at the data and determined those conditions would exist for some portion of that window. If a TEMPO group covers your arrival time, you need an alternate, extra fuel, and a pre-planned decision point.

Temporary conditions in a TAF can last up to 59 minutes at a time. If you arrive during an active TEMPO, you may need to fly the approach, go missed, hold, and try again. Confirm in preflight planning - not while holding over the fix - that you have fuel for that scenario.

What Does PROB Mean in a TAF?

PROB stands for Probability. By National Weather Service convention, you’ll see it as either PROB30 or PROB40, representing a 30 to 49 percent probability of the stated conditions. If probability reaches 50 percent or higher, forecasters issue a TEMPO instead. Below 30 percent, the condition isn’t mentioned.

PROB40 means 4 times in 10, those conditions occur. PROB30 means 3 times in 10. That’s not negligible - especially when what’s being forecast is thunderstorms or instrument conditions. PROB TS over your planned arrival window means there is a real and quantified chance of thunderstorms. That risk deserves a deliberate decision, not a dismissal.

How Do You Build a Complete Weather Picture from a TAF?

Reading a TAF effectively is a sequence, not a scan. Here’s the process:

Check the valid period first. Confirm that your arrival window falls inside it.
Note the base conditions. This is the dominant forecast scenario.
Read every change group in order. For each one, ask: does this cover my departure, en route phase, arrival window, or approach time? If yes, those conditions apply to you.
Flag every TEMPO and PROB group that contains low ceilings, low visibility, thunderstorm language, or significant wind shifts. These are your briefing red flags.
Connect the TAF to the broader picture. If a surface analysis shows a front moving through and a TEMPO group in the TAF aligns with that front’s timing, those two products are telling the same story. That increases the likelihood the TEMPO will verify.

If a TEMPO group contradicts everything else in the briefing, that discrepancy is worth investigating - not ignoring. The Area Forecast Discussion (AFD) is a free-text product that meteorologists write to explain the reasoning behind their forecast, including where they have high confidence and where they’re uncertain. It’s available at aviationweather.gov and is one of the most underused tools in a pilot’s briefing kit.

What About the 30-Hour TAF?

Major airports and many busier general aviation fields issue 30-hour TAFs. That extra six hours matters for overnight trips and early morning departures. If you’re flying in and spending the night, the TAF may already cover your planned departure the following morning. Check it. A BECMG group describing an early-morning fog event could mean your 0700 departure won’t happen.

How Does the TAF Apply to Alternate Airport Planning?

Under FAR Part 91, VFR pilots are not required to file an alternate. But the Airman Certification Standards for cross-country flight expect you to identify one and know the weather there. If your destination goes below minimums, you need somewhere to go - and you need to know that before you’re 30 miles out with a low fuel state.

Pull the TAF at your alternate for the same arrival window. A TEMPO group covering that same window at both your destination and your alternate is not a viable plan. The alternate has to actually be viable when you need it.

The Private Pilot ACS states that applicants should be able to obtain and correctly interpret weather information. TAF change groups are precisely what that standard is testing. Reading only the base forecast line is not a complete interpretation.

Key Takeaways

A TAF forecasts conditions for a 25-mile radius around an airport, updated four times daily, covering 24 or 30 hours depending on the facility.
FM is a complete replacement of prior conditions; BECMG is a gradual transition across a defined window; TEMPO overlays the base forecast with brief but real deterioration; PROB30/40 signals a 30–49% chance of the stated conditions.
A TEMPO group does not mean conditions are unlikely - it means a trained meteorologist determined they would occur during that window. If it covers your arrival, plan for those conditions.
TEMPO events can last up to 59 minutes. Pre-plan fuel for a missed approach and hold before departure, not in the air.
Always check the Area Forecast Discussion at aviationweather.gov when a change group conflicts with the rest of your briefing - it explains the forecaster’s reasoning.