Decoding the TAF and using Terminal Aerodrome Forecasts to make real go or no-go decisions

The Terminal Aerodrome Forecast (TAF) is the most important weather product for planning a cross-country flight, yet many pilots struggle to decode it. A TAF covers the expected weather within a five statute mile radius of an airport over a 24- to 30-hour period, using nearly the same codes as a METAR but with time groups and change indicators that reveal how conditions will evolve. Mastering it turns weather briefings from guesswork into informed decision-making.

What Is a TAF and Why Does It Matter More Than a METAR?

A METAR is a snapshot of current conditions. A TAF is a forecast of what’s coming. That distinction is critical for flight planning because the weather at your destination two hours from now matters far more than the weather right now.

TAFs are issued by the National Weather Service four times daily: at 0040Z, 0640Z, 1240Z, and 1840Z. Each covers a 24-hour period, though major airports may receive 30-hour TAFs. They can be amended anytime conditions change significantly — an amended TAF will show TAF AMD in the header.

For IFR flights, you need TAFs for your departure, destination, and alternate airports. For VFR cross-countries, pull TAFs for every airport along your route that has one, plus any potential diversion airports.

How Do You Read a TAF Line by Line?

If you can read a METAR, you already know roughly 70% of the TAF. Wind format, visibility, and cloud layers are identical. The complexity lies in the structure: the header, valid period, and change groups.

Here’s a real-world example for San Antonio International (KSAT):

TAF KSAT 121740Z 1218/1318 18010KT P6SM SKC TEMPO 1220/1300 5SM SCT040 BKN080 FM130200 21015G25KT 3SM SCT025 BKN050 OVC100

Breaking it down:

• TAF KSAT — Terminal Aerodrome Forecast for San Antonio International
• 121740Z — Issued on the 12th at 1740Z
• 1218/1318 — Valid from 1800Z on the 12th through 1800Z on the 13th (24 hours)
• 18010KT P6SM SKC — Baseline conditions: wind from 180° at 10 knots, visibility greater than 6 statute miles, sky clear

Everything after the baseline describes how conditions are expected to change.

What Do FM, TEMPO, BECMG, and PROB Mean?

These four change indicators are where most pilots get lost — and where the real decision-making information lives.

FM (From) signals a permanent shift to new conditions at a specific time. Everything before it is replaced. FM130200 means starting at 0200Z on the 13th, the new conditions take over and the old baseline is gone.

TEMPO (Temporary) indicates fluctuating conditions between two times. The weather may briefly shift to the TEMPO values, then return to baseline. Think summer convective buildups — clear, then a cell rolls through, then clear again. TEMPO 1220/1300 means these fluctuations may occur between 2000Z on the 12th and 0000Z on the 13th.

BECMG (Becoming) describes a gradual transition. At the start of the BECMG window, you have the old weather. By the end, you have the new weather. A classic example is morning fog slowly burning off.

PROB (Probability) appears as PROB30 or PROB40, indicating a 30% or 40% chance of conditions occurring during a specified window. You will never see PROB50 or higher in a TAF — if confidence reaches 50%, the forecaster upgrades it to a TEMPO or FM group. The Airman Certification Standards expects you to interpret PROB groups and factor them into your decisions.

How Do You Decode Visibility, Clouds, and Wind in a TAF?

Visibility is reported in statute miles. P6SM means greater than six miles. A number like 3SM means three miles. Weather phenomena causing reduced visibility follow the value: 3SM BR (three miles in mist), 2SM +RA (two miles in heavy rain), 1SM FG (one mile in fog).

Cloud layers use a three-letter descriptor plus a three-digit height in hundreds of feet AGL:

• SKC — Sky clear
• FEW — Few clouds (1–2 oktas)
• SCT — Scattered (3–4 oktas)
• BKN — Broken (5–7 oktas)
• OVC — Overcast (8 oktas)

The ceiling is the lowest BKN or OVC layer. That’s the number you compare against VFR minimums.

Wind follows METAR format: three-digit direction, two- or three-digit speed in knots, with G for gusts. VRB indicates variable direction, typically at low speeds.

How Do You Use a TAF to Make a Go or No-Go Decision?

Follow these five steps:

Step 1: Identify your full time window. Not just departure — your entire flight plus a buffer. Departing at 1800Z and arriving at 1930Z? Review everything from 1800Z through 2100Z in case of delays.

Step 2: Check your departure airport TAF. Find the forecast group covering your departure time. Compare winds, visibility, and ceilings against both regulatory requirements and your personal minimums.

Step 3: Check your destination airport TAF at your expected arrival time. Do the same for enroute airports with TAFs.

Step 4: Take TEMPO and PROB groups seriously. A TEMPO group dropping visibility to two miles in your arrival window could be happening exactly when you get there. “Temporary” doesn’t mean “ignorable.”

Step 5: Read the trend. Is weather improving or deteriorating over the course of your flight? Deteriorating trends are a yellow flag. An FM group showing worsening conditions one hour after your departure means your destination weather could be significantly worse than what you left.

What Does a Deteriorating TAF Trend Look Like?

Consider this scenario: You plan to depart at 1700Z. The TAF shows clear skies and calm winds at that time. But there’s a BECMG 1500/1700 group showing conditions transitioning to broken at 3,000 feet. An FM1800 group shows visibility dropping to four miles with overcast at 2,500 feet.

By your 1700Z departure, you’re in the middle of the BECMG window — conditions could be partway between clear and broken at 3,000. One hour later, things get worse. On a two-hour flight, you could arrive in conditions far worse than what you departed in.

This is exactly the type of analysis a checkride examiner expects. They want to hear you walk through the TAFs, identify trends, and explain your reasoning — not just confirm that current conditions meet minimums.

How Should You Handle Morning Fog in a TAF?

A common Texas scenario: The TAF shows 1½SM FG OVC004 at 0600Z, a BECMG 1400/1600 to P6SM and SCT050, then an FM1800 with clear skies and light winds.

The morning is unflyable. But the TAF tells you exactly when conditions improve. Instead of scrubbing the flight, adjust your departure time to after the BECMG window closes — launch at 1000 or 1100 local when the good weather has established itself.

This is the TAF doing its job. It didn’t just tell you the weather was bad. It told you when it would get better. The pilot who reads the TAF adjusts the plan. The pilot who doesn’t cancels a flight that didn’t need canceling.

What Are the Most Common TAF Mistakes Pilots Make?

Confusing Zulu and local time. Every time in a TAF is Zulu. An FM group at 2300Z is 6:00 PM Central during daylight saving time. Always convert to local and note it on your nav log.

Ignoring the issuance time. A TAF issued 12 hours ago may have been superseded by an amendment. Always check for the most recent version.

Treating the TAF as a guarantee. Forecast accuracy degrades over time. Conditions at hour 2 are probably reliable. Conditions at hour 22 are a best guess. Cross-reference with actual METARs as your flight approaches.

Not setting personal minimums above legal minimums. Three miles visibility and a 1,000-foot ceiling is the legal VFR minimum in controlled airspace, but a 60-hour student pilot should not fly in three-and-a-thousand conditions. Set personal minimums — perhaps five miles and 3,000 feet to start — and tighten them as experience builds. A TAF is only useful if you know what numbers you’re comparing against.

What Doesn’t a TAF Tell You?

A TAF covers only a five-mile radius around one airport. It says nothing about enroute weather at your cruising altitude. For that, you need:

• The Graphical Forecasts for Aviation (GFA) tool on aviationweather.gov
• Pilot reports (PIREPs)
• Winds and temperatures aloft forecasts

The TAF is one piece of the weather puzzle — arguably the most important piece for departure and destination decisions, but not the whole picture.

Key Takeaways

• A TAF forecasts 24–30 hours of weather within five statute miles of an airport, issued four times daily and amended as needed
• Four change indicators drive your planning: FM (permanent shift), TEMPO (temporary fluctuations), BECMG (gradual transition), and PROB (lower-confidence events)
• Read the trend, not just the snapshot — deteriorating conditions after your departure time are more important than clear skies at takeoff
• TEMPO and PROB groups demand attention — temporary doesn’t mean irrelevant when it overlaps your arrival window
• Set personal minimums higher than legal minimums and compare TAF values against those numbers, not the bare regulatory floor