Decoding the TAF and the twenty-four hour forecast hiding in plain text

A Terminal Aerodrome Forecast (TAF) is a coded weather forecast covering the next 24 to 30 hours at a specific airport. While a METAR is a snapshot of current conditions, the TAF is the full timeline — telling you what the weather is expected to do from issuance through expiration. It is arguably the single most useful piece of weather data for planning a cross-country flight, and it is the one most student pilots skip because it looks intimidating.

It doesn’t have to be. Once you understand the structure, a TAF reads like a story with a beginning, middle, and end.

What Does a TAF Look Like and Who Writes It?

A TAF begins with the station identifier, the issuance time, and a valid period. TAFs are issued four times per day (every six hours) by human forecasters at Weather Forecast Offices — not automated systems. A meteorologist reviews models and current conditions before writing each forecast, so there is professional judgment built into every line.

The valid period appears as two four-digit groups separated by a slash. The first two digits of each group are the day of the month, and the last two are the hour in Zulu time. For example, 0718/0818 covers from the 7th at 1800Z through the 8th at 1800Z — a full 24 hours.

Always confirm that your estimated arrival time falls within the valid period. If your flight lands outside that window, the TAF won’t help you. Students routinely plan afternoon flights using a TAF that expired at noon.

How Do You Read the Wind, Visibility, and Sky Condition Groups?

After the valid period, the first forecast group is the prevailing forecast — the baseline conditions expected from the start of the valid period until something changes.

Winds appear as five or six digits followed by KT. The first three digits are the direction in degrees true, the next two are speed in knots, and a “G” followed by additional digits indicates gusts. Example: 27015G25KT means wind from 270° at 15 knots, gusting to 25.

Visibility follows, reported in statute miles in the United States. 6SM means six statute miles. P6SM means greater than six statute miles — the best visibility a TAF will report. There is no “15SM” in TAF language; anything above six gets the “P” prefix.

Sky condition uses a three-letter coverage abbreviation plus a three-digit height in hundreds of feet AGL:

• FEW — 1/8 to 2/8 sky coverage
• SCT (scattered) — 3/8 to 4/8
• BKN (broken) — 5/8 to 7/8
• OVC (overcast) — 8/8

So BKN025 means broken clouds at 2,500 feet AGL, and OVC010 means overcast at 1,000 feet.

What Do the Weather Codes Between Visibility and Sky Condition Mean?

When weather phenomena beyond wind and clouds are expected, coded descriptors appear between visibility and sky condition. Common codes include:

• RA — rain
• SN — snow
• BR — mist
• FG — fog
• TS — thunderstorm

Intensity qualifiers change the meaning significantly. A minus sign (-) means light, no sign means moderate, and a plus sign (+) means heavy. So -RA is light rain, while +TSRA is heavy thunderstorms with rain — a combination that should stop you in your tracks during flight planning.

What Are FM, TEMPO, and BECMG Change Groups?

Weather changes over time, and the TAF captures those changes through three types of change indicators. Understanding these is where TAF reading goes from mechanical decoding to real flight planning.

FM (From)

FM followed by a date/time means conditions are expected to change completely at that time. Everything after the FM replaces everything before it. Think of it as a clean break — a new baseline. If the prevailing forecast shows clear skies and then FM1600 shows OVC015 3SM BR, the forecaster expects conditions to go from great to marginal at 1600Z.

TEMPO (Temporary)

TEMPO means conditions may temporarily fluctuate to the stated values during the specified time window. The key constraints: temporary conditions are expected to last less than one hour at a time and cover less than half of the total TEMPO period.

The practical implication for VFR pilots is critical. If a TEMPO group drops conditions below VFR minimums, you need to seriously evaluate whether you want to be airborne during that window. “Temporary” can still strand you 30 miles from the airport when the ceiling drops.

BECMG (Becoming)

BECMG indicates a gradual transition from current conditions to the new conditions over the specified time period. Where FM is a light switch, BECMG is a dimmer switch. BECMG 0814/0816 BKN030 means clouds are expected to gradually build to broken at 3,000 feet between 1400Z and 1600Z.

How Do Change Groups Work Together?

Think of a TAF as layers. The prevailing forecast is the foundation. Each FM group lays a new foundation, replacing everything before it. TEMPO and BECMG groups are modifications to whatever foundation is currently active — they don’t replace the baseline, they temporarily or gradually alter it.

The groups are sequential and build on each other. Reading them in order gives you the weather story from start to finish.

How Do You Apply a TAF to a Real Flight Decision?

Consider this scenario. You are planning a two-hour cross-country, departing at 1500Z with arrival at 1700Z. The destination TAF reads:

• Prevailing: 18008KT P6SM FEW080 — light winds, clear, unlimited visibility
• FM1600: 21015G25KT 5SM HZ BKN030 — wind shift, gusts, haze, ceiling at 3,000
• TEMPO 1600/2000: 3SM -TSRA BKN015CB — brief thunderstorms, ceiling at 1,500 with cumulonimbus

Your departure weather is beautiful. But one hour before your planned arrival, a front is expected to push through with gusty winds, reduced visibility, and embedded thunderstorms. That is a go/no-go decision — and the TAF gave it to you before you started the engine.

The Airman Certification Standards for the private pilot checkride require you to obtain and interpret weather information, identify trends, spot deterioration, and explain your decision. The examiner wants to see you read the TAF, apply it to your flight, and articulate why you would go, delay, or cancel.

What Doesn’t the TAF Tell You?

The TAF covers weather at the airport only — not between airports, not along your route. For en route weather, you need AIRMETs, SIGMETs, area forecasts, and prognostic charts. The TAF is your arrival and departure weather. It is one critical piece of the puzzle, but not the whole picture.

What Is the Best Way to Practice Reading TAFs?

Pull up the TAF for your home airport — even on days you are not flying. Read it, then go outside and compare the forecast to what you actually see. Do this for one week and the codes will stop looking like alphabet soup. They will start reading like weather.

The official reference for TAF formatting is FAA Advisory Circular 00-45H, Aviation Weather Services. The TAF section is worth reading at least once.

Key Takeaways

• A TAF covers 24–30 hours of forecast weather at a specific airport, issued every six hours by a human forecaster
• FM is a full replacement, TEMPO is a brief fluctuation, and BECMG is a gradual transition — know the difference
• Always verify your arrival time falls within the valid period before relying on a TAF
• TEMPO groups below VFR minimums are a red flag even though conditions are expected to be brief
• The TAF is airport-only weather — use AIRMETs, SIGMETs, and prog charts for en route conditions