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

BECMG describes a gradual, permanent transition to new conditions - once the window closes, the new conditions persist. TEMPO describes temporary fluctuations that overlay the existing base forecast without replacing it, where each individual event lasts less than one hour.

Does a PROB40 group require me to file an alternate airport?

No. Under the IFR alternate airport rule (the 1-2-3 rule), only the base forecast - main body, FM, and BECMG groups - triggers the alternate requirement. PROB and TEMPO groups do not legally require an alternate. However, a PROB40 indicating low IFR conditions should still drive a judgment question about fuel, divert options, and proficiency.

How do I know what conditions apply at my exact arrival time when reading a TAF?

Work through the change groups in order from the start of the valid period to your ETA, applying each group's rules: FM groups replace all prior conditions; BECMG transitions are complete by the end of the window; TEMPO groups overlay (not replace) whatever base conditions currently apply. The conditions in effect at your ETA are the most recently applied base conditions, with any active TEMPO or PROB group considered as a possible overlay.

What does FM mean in a TAF, and how is it different from BECMG?

FM (from) marks an abrupt, instantaneous replacement of all prior forecast conditions at a specific time. BECMG describes a gradual transition over a defined time window. With FM, the old conditions are completely gone the moment that time arrives; with BECMG, the shift happens gradually and unpredictably within the stated window.

Where can I practice decoding TAFs?

Pull daily TAFs for a familiar airport from **aviationweather.gov**, decode each one fully hour by hour, then check the following day's observations to see how the forecast verified. Repeating this over a week builds intuition for how coded groups map to real conditions.

The Terminal Aerodrome Forecast, the Change Groups That Trip Every Cross-Country Student, and the Probability Code That Sits Between the Legal Minimum and the Safe Choice

The main body of a Terminal Aerodrome Forecast tells you where conditions stand at the start of the forecast period. The change groups tell you where conditions are going. Both sections live in the same document, and skipping the change groups means flying with an incomplete picture.

What Are TAF Change Groups and Why Do They Matter?

A Terminal Aerodrome Forecast (TAF) covers conditions within five statute miles of a specific airport. Most commercial-service airports publish them, and many general aviation fields do too. Each TAF covers a 24- or 30-hour window depending on the station, and updates four times a day - roughly every six hours.

The TAF opens with a station identifier, issue time, and valid period. The main body follows, listing wind, visibility, significant weather, and sky condition. After that come the change groups: FM, BECMG, TEMPO, and PROB. These are not fine print. They describe what the weather is about to do.

How Does the FM Group Work in a TAF?

FM stands for “from.” It marks the point at which conditions change completely and permanently within the forecast period.

Everything after an FM group replaces everything that came before it - it is not an addition. If the main body shows 3,000 feet broken and you hit an FM group timed for 1500Z showing 800 overcast, then from 1500Z forward you are looking at 800 overcast. The 3,000 broken no longer exists in the forecast.

A common mistake is blending conditions from before and after an FM group. Don’t. FM is a clean break.

What Does BECMG Mean in a TAF?

BECMG stands for “becoming.” It describes a gradual transition from one set of conditions to another, and it always comes with a time window written as two four-digit UTC times. BECMG 0912 means the change begins at 0900Z and will be complete by 1200Z.

That window guarantees the transition will be complete by the end time. It does not tell you when within that window the change actually occurs - it could happen at 0905Z or at 1150Z. If your planned arrival falls inside a BECMG window, you have to account for the possibility of landing in the old conditions, the new conditions, or somewhere in between.

BECMG is also permanent. Once the window closes, those new conditions persist until the next forecast group supersedes them. Becoming means transitioning to a new state that then stays - it is not temporary.

How Do You Interpret a TEMPO Group?

TEMPO means temporary, and two specific rules define what temporary means in a TAF.

Rule one: each individual occurrence of the described conditions lasts less than one hour. Rule two: those conditions occur for less than half of the total TEMPO time period.

So a TEMPO 0915 describing visibility dropping to one and a half miles in rain showers means that during the six-hour window from 0900Z to 1500Z, you may encounter patches of reduced visibility - but each individual patch lasts under an hour, and total time in those conditions stays under three hours.

TEMPO does not replace the base conditions. It overlays them. Whatever the base forecast says - from the main body or the most recent FM group - that forecast still runs underneath a TEMPO.

The practical question every time you see TEMPO: does my planned arrival fall inside this window, and if those temporary conditions materialize during my approach or landing, can I handle them?

What Do PROB30 and PROB40 Mean for Flight Planning?

PROB30 and PROB40 represent the probability in percent that the described conditions occur during the stated time window. You will not see PROB50 or higher in a TAF - when probability reaches 50% or above, forecasters place those conditions directly into the base forecast or a TEMPO group. Probability groups exist specifically for the uncertain zone between possible and probable.

For IFR alternate airport planning, the relevant regulation uses the 1-2-3 rule: one hour before and one hour after your estimated time of arrival (ETA), if the destination is forecast to have a ceiling below 2,000 feet or visibility below 3 statute miles, you need an alternate on your IFR flight plan.

That calculation uses the base forecast - main body, FM groups, and BECMG groups. TEMPO and PROB groups do not legally trigger the alternate requirement.

When Does a PROB40 Require More Than a Legal Check?

When you see PROB40 describing a 500-foot ceiling and one mile visibility at your destination, you cannot close the briefing just because it doesn’t trigger the alternate requirement. The regulation sets the legal minimum. It does not make the decision.

PROB40 means a trained forecaster, using real meteorological data, is telling you that four times out of ten in similar setups, those conditions verify. The questions to ask:

If those conditions materialize, can I execute this flight safely?
Do I have enough fuel to divert?
Is there an airport within range with better forecast conditions?
Am I current and proficient for what this might turn into?

How Do You Read a TAF from Start to Finish?

A repeatable five-step process builds a coherent picture faster than memorizing definitions alone.

Step 1: Identify the valid period and locate your ETA within it. Know exactly where you sit in the forecast timeline before reading a single change group.

Step 2: Read the main body for opening conditions - wind, visibility, weather phenomena, sky condition.

Step 3: Walk through each change group in order. For each FM, note what replaces prior conditions and when. For each BECMG, note the transition, when the window opens, and when it closes. For each TEMPO, note what could temporarily overlay the base conditions and whether your arrival falls inside the window. For each PROB40 group, ask the judgment questions.

Step 4: Examine the roughly three-hour window centered on your ETA. What is the full range of conditions you might encounter? Best case, worst case. Do your minimums fit comfortably inside the worst case?

Step 5: Look at the three hours after your planned landing. If you arrive late, need to go around, or weather moves faster than the forecast anticipated - is there margin in the picture?

That final step reflects what the Airman Certification Standards evaluates during cross-country planning. The examiner isn’t checking whether you can decode individual codes. The examiner is looking for whether you can build a coherent picture of the airspace and atmosphere your flight moves through - and make a sound decision from it.

How Can You Get Better at Reading TAFs?

Pull a real TAF from aviationweather.gov for your home airport every day for a week. Decode it completely, step by step, writing out the conditions at every hour of the forecast period. Then check back the following morning and compare against what actually occurred.

You will develop a feel for how coded language maps to real conditions. You’ll notice when a BECMG window closed and the forecasters got it right. You’ll catch a TEMPO that materialized - and one that didn’t. You’ll see a PROB40 group verify and understand why that number deserves respect.

The FAA’s Aeronautical Information Manual, Chapter 7 and the Aviation Weather Handbook both cover TAF structure in solid detail. Sitting with both documents alongside real-world examples is time well spent.

Key Takeaways

The main body of a TAF gives opening conditions; change groups (FM, BECMG, TEMPO, PROB) describe where conditions are going - both sections are required reading.
FM is a clean replacement of all prior conditions at a specific time; never blend conditions across an FM boundary.
BECMG is a gradual but permanent transition; if your arrival falls inside the BECMG window, any state of that transition is possible.
TEMPO overlays the base forecast temporarily; each event lasts less than one hour and total time in those conditions stays under half the TEMPO period.
PROB40 doesn’t legally require an alternate, but it represents a 40% probability of those conditions from a trained forecaster - it demands a judgment call, not just a legal check.