The Terminal Aerodrome Forecast, the TEMPO Group Most Pilots Misread, and the Alternate Planning Math That Lives Inside the Change Groups

The Terminal Aerodrome Forecast is the most commonly rushed document in a preflight briefing - and the most consequential. Understanding each change group isn’t just weather awareness; for IFR operations, the language inside a TAF has direct legal implications for alternate airport planning under FAR 91.169.

What Is a TAF and Who Issues Them?

A Terminal Aerodrome Forecast (TAF) covers conditions at a specific airport and the surrounding area within approximately a 25-statute-mile radius. The National Weather Service issues TAFs for roughly 700 airports in the United States - generally those with airline service or significant traffic volume. If your destination has no TAF, build your weather picture from the area forecast and the nearest reporting station.

TAFs are issued four times daily, cycling at approximately 0000, 0600, 1200, and 1800 Zulu. Standard coverage is 24 hours; some higher-traffic airports receive 30-hour forecasts. When conditions develop faster than anticipated, forecasters issue an amendment marked AMD in the header - meaning the original forecast needed revision. Always check the issue time. A TAF issued six hours ago is already showing its age when weather is developing.

How Do You Read the TAF Header and Base Forecast?

Every TAF opens with an identifier line: the four-letter airport identifier, issue time in day-hour-minute Zulu, and the valid period. For example:

TAF KMKE 091200Z 0912/1012

This reads: Milwaukee Mitchell International, issued at 1200Z on the 9th, valid from 1200Z the 9th through 1200Z the 10th - a 24-hour window.

The base forecast follows, describing expected conditions at the start of the valid period: wind direction and speed, visibility in statute miles (preceded by P if greater than 6), weather phenomena (RA = rain, BR = mist, TS = thunderstorm), and sky condition in hundreds of feet AGL.

Sky coverage codes matter for determining ceilings. FEW = 1–2/8 coverage; SCT (scattered) = 3–4/8; BKN (broken) = 5–7/8 and counts as a ceiling; OVC (overcast) = complete coverage, also a ceiling; CLR = no clouds below 12,000 feet.

What Does FM Mean in a TAF?

FM (“from”) marks a rapid, permanent shift in conditions beginning at a specific time. Rapid means within 30 minutes or less. Once that timestamp passes, everything before it is gone - the previous forecast no longer applies.

FM2100 means conditions change quickly at 2100Z and hold from that point forward. FM is a hard line. Most pilots read it correctly because it functions like a clean timeline: conditions are one thing, then at a defined moment they become something else.

What’s the Difference Between BECMG and FM?

BECMG (pronounced “becoming”) describes a gradual transition over a defined window - a start time and an end time. During that window, conditions are in transition and can fall anywhere between the pre-BECMG and post-BECMG state. After the window closes, the new conditions are permanent.

BECMG 1400/1600 means conditions are gradually shifting between 1400Z and 1600Z. An arrival at 1500Z - in the middle of that window - could encounter anything between the two states.

The most common mistake is treating BECMG like FM: seeing the start time and assuming conditions flip right then. They don’t. BECMG is a transition, not a switch. And once that window closes, those conditions replace everything before them permanently.

What Does TEMPO Actually Mean - and Why Do Pilots Misread It?

TEMPO has a precise technical definition in TAF language. A TEMPO group describes conditions expected to occur intermittently, where each individual occurrence lasts less than 60 minutes, and the total combined duration is less than half of the TEMPO period.

A TEMPO 1400/1800 covers a four-hour window. Each occurrence of those conditions should last under an hour, and the aggregate total shouldn’t exceed two hours across the entire window. The forecaster is signaling rough patches - not a sustained wall of bad weather.

Critically, TEMPO does not replace the base forecast. Prevailing conditions remain whatever came before the TEMPO group. During the window, you might encounter base conditions, TEMPO conditions, or something between them.

Pilots go wrong in two directions. Some assume the TEMPO conditions are what they’ll find. Others dismiss it as “just temporary, probably fine.” Both are wrong. TEMPO describes what’s possible during that window - and planning to what’s possible, not what’s most likely, is the standard.

What Do PROB30 and PROB40 Mean?

PROB groups indicate probability. Forecasters use only two values: PROB30 and PROB40.

PROB30 means the described conditions are unlikely but possible. PROB40 means a meaningful but not dominant probability. Conditions more likely than 40% go into a TEMPO group; conditions less likely than 30% don’t appear in the TAF at all.

PROB can combine with TEMPO. PROB40 TEMPO 1600/1900 means a 40% chance of those conditions occurring on a temporary basis during that window. Worth noting - and not worth dismissing.

How Do You Apply All Four Change Groups to a Real Flight?

Consider this scenario: VFR cross-country, estimated arrival 1700Z. The destination TAF reads:

• Base: Winds 250° at 10, visibility greater than 6 miles, sky clear
• BECMG 1400/1600: Winds shifting to 290° at 20, gusting 28
• TEMPO 1600/2000: Visibility 1½ miles in light rain and mist, broken ceiling at 900 feet
• FM2200: Winds 210° at 8, visibility greater than 6, scattered at 4,000 feet

Your arrival is 1700Z. The BECMG window closed at 1600Z, so the wind transition is complete: 290° at 20 gusting 28. Check the crosswind component against the destination’s runway layout.

Your arrival also falls inside the TEMPO window running 1600Z to 2000Z. That puts visibility at 1½ miles with a broken ceiling at 900 feet as a possible condition at your ETA. That’s instrument meteorological conditions for most pilots in most airspace.

Is it guaranteed? No - it’s TEMPO. But it’s possible. For a VFR pilot, possible IMC at the destination without a contingency plan is how accidents happen. The questions stack immediately: What’s the weather behind you? What’s your fuel state for a divert? What are conditions at nearby airports? Has anyone filed a PIREP along this route in the last hour?

How Does the TAF Affect IFR Alternate Planning Requirements?

FAR 91.169 requires an alternate when the destination weather is forecast below 2,000 feet ceiling or 3 statute miles visibility during the period from one hour before to one hour after your estimated time of arrival.

That two-hour window makes TEMPO groups critical. A TEMPO with a ceiling below 2,000 feet falling anywhere inside that window may legally require an alternate - even if the base forecast looks fine and even if the FM group that follows shows clear skies. Many instrument students look at a solid base forecast, miss the TEMPO sitting inside the arrival window, file without an alternate, and arrive to find a ceiling at 600 feet with limited options.

Once the alternate requirement is established, that airport has its own minimums. For an airport with a precision approach, standard alternate minimums are 600 feet ceiling and 2 statute miles visibility. For a non-precision approach, the standard is 800 feet and 2 miles. Some airports publish non-standard alternate minimums in the terminal procedures front matter - always check the Chart Supplement.

What Are the Practical Habits for Reading TAFs Effectively?

Get comfortable with the raw text TAF alongside decoded displays. Tools like ForeFlight, 1800wxbrief.com, and aviation.weather.gov offer decoded versions - valuable for learning. When you call flight service, they read the raw text. When you’re preflight-rushing, you’ll skim the raw text. Know both.

Watch for AMD in the header. Compare the amendment to the original forecast and understand what changed. A forecaster who revised their thinking is telling you something.

No single product gives you the complete picture. Layer the TAF with G-AIRMETs, winds aloft forecasts, surface analysis, and PIREPs along your route. A TAF tells you what forecasters expected when they issued it. PIREPs tell you what someone actually found an hour ago. Radar shows what’s happening now. Good weather decision-making assembles all of it - then decides honestly.

Key Takeaways

• FM is a hard line: conditions change rapidly and permanently at a specific timestamp; everything before it no longer applies.
• BECMG is a gradual transition over a time window; conditions can fall anywhere between start and end states during that period, and the new state is permanent after the window closes.
• TEMPO describes intermittent conditions - each occurrence under 60 minutes, total duration less than half the TEMPO period - and does not replace the base forecast; plan to what’s possible, not most likely.
• FAR 91.169 requires a filed alternate when ceiling or visibility falls below 2,000 feet / 3 miles within the one-hour-before to one-hour-after arrival window; a TEMPO inside that window triggers the requirement even when the surrounding forecast looks clear.
• Always check the TAF issue time, watch for AMD flags, and build your weather picture from multiple sources - TAF, PIREPs, radar, G-AIRMETs, and winds aloft together.