Reading the TAF, the TEMPO Group and the PROB30 Trap That Fools VFR Pilots on Cross-Countries

A Terminal Aerodrome Forecast (TAF) is a forecaster’s plain-spoken prediction of the weather within about five statute miles of a single airport, and reading the whole thing - especially the TEMPO and PROB30 groups buried near the bottom - is what separates a safe go/no-go decision from a bad one. The trap most VFR pilots fall into is treating a TEMPO condition as a “maybe” and a PROB30 as “probably won’t happen,” when in fact TEMPO means this will occur and PROB30 means here’s the bad thing that might show up. This guide walks through a real TAF piece by piece so you can spot the ugly parts on purpose instead of skimming past them.

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

A TAF is a forecast for one specific airport - not a region, not a state - covering the weather expected from the surface up within roughly five statute miles of the field. A human forecaster at the National Weather Service writes each one, and it covers a 24- or 30-hour window depending on the airport.

Most TAFs are updated four times a day, roughly every six hours, and are amended in between when the weather does something the forecaster didn’t expect.

The little airplane symbol and the green “VFR” dot on your tablet are summaries of a summary. The raw TAF is where the forecaster tells you the actual story of the day, including the parts your weather app quietly rounds off.

What If My Destination Doesn’t Have a TAF?

TAFs are issued only for a limited number of airports - mostly larger fields with enough traffic to justify a forecaster’s attention. Your grass strip or quiet Class Delta may not have one at all.

The skill here is learning to borrow. Find the nearest reporting airport that shares your weather, read its TAF, and apply judgment about the terrain and distance between you and it. That isn’t cheating - it’s exactly what your instructor does.

How Do I Read the TAF Header?

The header packs the whole timeline into two numbers, and this is where people get tripped up.

It starts with K followed by the airport identifier, then a six-digit issue time. The first two digits are the day of the month; the next four are the issue time in Zulu (UTC). So 151120Z means the forecast was issued on the 15th at 1120 Zulu.

Next comes the valid period: day and hour it becomes valid, a slash, then day and hour it expires. 1518/1618 means the forecast is good from the 15th at 1800Z until the 16th at 1800Z - a 24-hour window.

Everything after the header is the forecaster describing what the sky will do across that window.

What Does a FROM (FM) Group Mean?

A FROM group is the backbone of the TAF. It signals a rapid, significant, lasting change. When you hit an FM line, erase everything before it - the new conditions completely replace the old ones and carry forward until the next change group.

Picture this body. From the start: wind 240 at 10 knots, visibility greater than 6 statute miles (written P6SM, the “P” meaning plus), and few clouds at 25,000 feet. A severe-clear afternoon.

Then: FM2300. At 2300 Zulu the forecaster now calls wind 180 at 8, visibility still greater than 6, but sky broken at 800 feet (BKN008). The ceiling has fallen to essentially 800 overcast - below basic VFR at a controlled field.

Just by reading the FROM groups in order, you can build the plan: gorgeous afternoon, then the ceiling drops around 2300Z. Land well before that and you’re golden. Arrive right around it and you’d better be paying attention.

What Does the TEMPO Group Actually Tell Me?

TEMPO stands for temporary. It describes conditions the forecaster expects to come and go, each occurrence lasting less than an hour and, in total, covering less than half the time period.

You might see TEMPO 1520/1523, then visibility 3 statute miles in mist and broken at 500 feet (BKN005). Read correctly: the prevailing FROM conditions are still the main story, but between 2000 and 2300 Zulu, the visibility will temporarily sag to 3 miles and the ceiling to 500 feet. It won’t stay there - it’ll bounce - but it will happen.

Here’s the mistake, and it’s an easy one to make: reading that TEMPO group and thinking, “It’s only temporary, the real forecast is fine, I’ll just time it.” A TEMPO condition is not a maybe. The forecaster is telling you it will occur - you just don’t know the exact minute. “I’ll time it” assumes you can predict something the professional meteorologist just told you is unpredictable enough to require the temporary group.

On your checkride, the examiner wants to hear you treat a TEMPO 500-and-3 as a real possibility you have a plan for, not a footnote you wave away. The Airman Certification Standards, under preflight preparation, require you to interpret weather reports and forecasts and make a competent go/no-go decision - which means reading the whole TAF, including the ugly parts, and saying out loud what you’d do if the ugly part shows up while you’re overhead.

What Does PROB30 Mean - and Why Is It a Trap?

PROB stands for probability, and the number is a percentage. PROB30 means a 30% chance of the conditions that follow; PROB40 means 40%. You’ll basically never see PROB50 or higher - if the forecaster is more than half sure, they’ll just forecast it outright with a FROM or TEMPO - and you won’t see it below 30, because below that they generally don’t bother.

Say the TAF ends with PROB30 0200/0600, then visibility one-half statute mile, thunderstorm, and vertical visibility 200 feet (VV002). Thunderstorms and near-zero visibility.

The trap is the word “only.” Our brains hear “30 percent” and quietly round it down to zero. Flip it around: a 30% chance of a thunderstorm sitting on your destination is roughly one in three. Would you drive off if there were a one-in-three chance the bridge was out? At minimum you’d have another way home figured out first.

Notice, too, what the forecaster put behind that PROB30 - not light rain, but a thunderstorm and a 200-foot ceiling. The PROB group is often where the forecaster hides the day’s worst realistic case. So when you see PROB30, don’t read the number first. Read the conditions first and ask, “What is the forecaster worried about here?” Then decide whether a 30% shot at that is something you can live with.

How Do Probability and Severity Work Together?

Probability and severity must be weighed as a pair. A 30% chance of a broken layer at 3,000 feet is a shrug. A 30% chance of a thunderstorm is a planning problem. Same number, completely different decision. The error is treating the percentage as if it means the same thing every time.

How Should I Handle a PROB30 for Thunderstorms on My Arrival?

Imagine you’re a fresh private pilot on a two-hour cross-country to visit family. The main body of the TAF is VFR the whole way, but at the bottom sits a PROB30 for the exact four-hour window you’d be arriving in, calling for thunderstorms and half-mile visibility.

A good decision often isn’t “cancel” - it’s “go, but.” Build outs into the plan:

• Go, but move your departure earlier to arrive well ahead of the window.
• Go, but pick an alternate out from under the storm risk and carry extra fuel to reach it.
• Go, but set a hard personal rule: if the destination is reporting worse than your personal minimums when you’re 30 minutes out, divert - no debate.

Get-there-itis does its worst damage in the last 30 minutes, when you’re close enough to smell the barbecue and you start bargaining with a sky that isn’t negotiating. The examiner is looking for the pilot who reads a scary forecast, understands it, and plans around it - not the one who folds and not the one who ignores it.

What’s the Right Workflow for Reading a TAF Before a Flight?

Get a real weather briefing - either from Leidos Flight Service at 1-800-WX-BRIEF or through their website - then work the TAF deliberately:

1. Read it out loud. Reading silently lets your eye skip the parts you don’t want to see. Reading aloud forces you through every group.
2. Confirm it’s current. Check the issue time and valid period against your actual departure and arrival times, and make sure your whole flight - plus a comfortable pad - fits inside the valid window. An old TAF is a trap all by itself.
3. Read the FROM groups in order and build the shape of the day in your head: sunny, then a front, then clearing - whatever it is.
4. Go hunting for the TEMPO and PROB groups on purpose. For each one, ask two questions: Does this fall during my flight? and If it happens, can I handle it? If the first is yes and the second is no, you don’t have a flight yet - you have more planning to do.
5. Cross-check against the big picture. The TAF covers one airport; AIRMETs, SIGMETs, graphical forecasts, and prog charts describe the whole airmass. If the TAF says PROB30 thunderstorms and the convective outlook is lit up across your route, that 30% starts to feel like a promise.
6. Read the wind lines, not just the ceiling and visibility. A TAF that quietly ramps the wind from 10 knots to 22 gusting 30 over your arrival window is warning you the crosswind you practiced for may not be the crosswind you get.

The Bottom Line

The TAF isn’t trying to scare you or bless you. The FROM groups are the forecaster’s best guess at the plan. The TEMPO group says, “this will happen, on and off - I just can’t tell you exactly when.” The PROB group says, “here’s the bad thing that might show up, and here’s how worried I am about it.” Your job isn’t to outsmart the forecast - it’s to read all of it, especially the parts that don’t fit the flight you already decided to take. The 30% isn’t permission to ignore it. It’s an invitation to have a plan.

Key Takeaways

• A TAF forecasts weather within about 5 statute miles of one airport, over a 24- or 30-hour window, updated four times a day.
• A FROM (FM) group means a lasting change - erase everything before it. A TEMPO group means conditions that will occur temporarily (each under an hour, less than half the period).
• PROB30 = 30% chance; PROB40 = 40%. You won’t see PROB50+ or anything below 30.
• Read the conditions behind a PROB group before the number - it’s where the forecaster hides the day’s worst realistic case.
• Weigh probability and severity together, build outs (earlier departure, alternate, extra fuel, a hard divert rule) into your plan, and always check the wind lines.