Reading the TAF Line by Line - Decoding the Terminal Forecast for Your Oshkosh Arrival Window

SUMMARY: Learn to read a Terminal Aerodrome Forecast line by line so you can time your Oshkosh arrival around afternoon storms.

A Terminal Aerodrome Forecast (TAF) is a meteorologist’s prediction of weather within about five statute miles of a single airport, typically covering a 24- or 30-hour period and issued four times daily. Reading it correctly means going past the first line to decode the wind, visibility, ceiling, and - most importantly - the change groups that reveal when conditions will turn. For a high-stakes destination like Wittman Regional Airport (KOSH) during EAA AirVenture, the TAF doesn’t tell you whether to go; it tells you when to go.

What Is a TAF, and What Is It Not?

A TAF is a forecast - a human meteorologist at the National Weather Service predicting what the weather will do at one specific airport over a set window. That prediction covers only a small bubble, roughly five statute miles from the center of the runway complex. It is not a regional picture, and it will not tell you what’s happening thirty miles out on your arrival.

Contrast that with a METAR, which is an observation - what is actually happening right now, measured. The METAR is a photograph; the TAF is an educated prediction made by a professional. Respect the forecast, but never confuse a guess with a measurement.

Most TAFs cover a 24- or 30-hour period and are issued four times a day, roughly at 1130Z, 1730Z, 2330Z, and 0530Z. A forecast can also be amended whenever the weather does something unexpected. When you see a header reading TAF AMD, that’s an amended forecast - reality diverged from the plan badly enough that a forecaster rewrote it. Treat an amended TAF as a signal to pay closer attention.

How Do I Read the TAF Header and Valid Period?

The forecast opens with the station identifier - for our example, KOSH (Oshkosh). Next comes a six-digit issuance time followed by Z (Zulu). The first two digits are the day of the month; the next four are the issue time in Coordinated Universal Time.

Everything in aviation weather is in Zulu because weather doesn’t care what time zone you filed from. During summer, Central Daylight Time is Zulu minus five hours, so 1800Z is 1:00 p.m. in Oshkosh. Learn to convert in your head.

After the header comes the valid period, formatted as day/start-hour slash day/end-hour. A forecast valid from the 11th at 1800Z through the 12th at 1800Z covers a full day, and everything after that describes that window.

How Do I Decode the Wind, Visibility, and Sky Groups?

The first line after the valid period is the prevailing forecast - the base conditions expected at the start.

Wind comes first: five digits plus KT. The first three digits are the direction the wind is coming from in degrees true (not magnetic - the tower gives you magnetic, so expect an offset). The next two digits are speed in knots, and a G marks gusts. So 21012G22KT means wind from 210° true at 12 knots, gusting to 22.

That gust spread matters enormously for an Oshkosh arrival. The Fisk arrival - the VFR procedure everyone flies - keeps you low, slow, in trail, and looking out the window. Wind from the southwest gusting into the low twenties changes the runway in use, your ground speed over the railroad tracks, and how hard you’re working.

Visibility follows as a number plus SM (statute miles). P6SM means greater than six statute miles, the highest a TAF reports. When visibility drops below six, the forecast states it exactly - 3SM, 1 1/2SM - and almost always pairs it with a weather group explaining why (rain, mist, haze).

Sky condition is described by coverage and height:

• SKC – sky clear
• FEW – few
• SCT – scattered
• BKN – broken
• OVC – overcast

Each is followed by three digits representing hundreds of feet above ground level. So BKN035 is a broken layer at 3,500 feet AGL.

What Counts as a Ceiling in a TAF?

A ceiling is the lowest broken or overcast layer. Few and scattered layers do not make a ceiling, because you can see up through them. Broken and overcast layers do, because they form a lid.

If a forecast shows scattered at 3,000 and broken at 6,000, your ceiling is 6,000, not 3,000. For a VFR pilot, that distinction is the whole ballgame - the ceiling determines whether you’re getting into the airport under visual flight rules.

What Do the Change Groups (FM, BECMG, TEMPO, PROB) Mean?

Weather isn’t static, so the forecaster uses keywords to say “and then this happens.”

FM (From) is a clean break. FM followed by a six-digit time means everything changes rapidly and completely at that hour, and the new conditions hold until the next change group. Read FM lines like the chapters of the forecast.

BECMG (Becoming) is gentler - a gradual transition over a stated window, usually a couple of hours. It’s followed by two time groups marking the start and end of the transition, so BECMG 2224 means the weather eases into the new state between 2200Z and 2400Z.

TEMPO (Temporary) is the one to circle in red. Tempo means temporary fluctuations expected to last less than an hour each, and less than half the total period. The base conditions hold, but briefly and repeatedly expect it to get worse. A TAF can show beautiful VFR all afternoon with a tempo group buried three lines down calling temporarily 3 miles in thunderstorms and rain with a broken ceiling at 2,000. That’s the difference between a smooth arrival and holding over Ripon watching gray build to your west. Never read the top line and stop - the trap is in the change groups.

PROB30 / PROB40 (Probability) tells you there’s a real but not dominant chance - 30 to 40 percent - of something significant, usually thunderstorms (TSRA, thunderstorm with rain). For a July afternoon in Wisconsin, that is not a number to wave off. It’s the number that tells you to arrive in the morning.

How Do I Use the TAF to Time My Oshkosh Arrival?

Picture flying to AirVenture from 300 miles south in mid-July, loaded with camping gear, aiming to land in time for the afternoon airshow. You pull the KOSH TAF the night before. The morning looks gorgeous - light wind, visibility better than six, a few clouds at 4,000. But you keep reading, and around your target arrival time sits a PROB40 group with thunderstorms and rain, plus a tempo group dropping a broken ceiling to 2,000.

That forecast isn’t telling you not to go. It’s telling you when to go. Arrive early and you’re tied down on the North Forty by 10 a.m., eating a bratwurst while the storms roll through. Sleep in, and you’re holding over Ripon in deteriorating conditions with 200 other airplanes while a thunderstorm builds over the field.

Same forecast, two completely different days. The TAF doesn’t make the decision - you do. But you can’t make it if you only read the first line.

What Else Should I Check Alongside the TAF?

1. Never read a TAF in isolation. It’s one instrument in the panel. Pair it with the winds and temperatures aloft for your route, the AIRMETs and SIGMETs for hazards in between, the Graphical Forecast for Aviation for the big picture, and the METARs along the way to check whether reality matches the forecast. When observations start diverging from the prediction, the forecast is losing the plot - time to get conservative.

2. Get a real briefing. Call Flight Service at 1-800-WX-BRIEF or pull a standard briefing through your electronic flight bag. A standard briefing includes TAFs for your departure, route, and destination, plus everything around them. The Airman Certification Standards for both the private and instrument ratings expect you to obtain and interpret this information - the examiner wants a pilot who can look at a real forecast and say what they’ll do about it.

3. For AirVenture specifically, favor an early arrival. Summer convection in Wisconsin builds in the afternoon and settles at night, so the Oshkosh weather window is a moving target during the show. Morning air is smoother, visibility is usually better, and traffic is often more manageable before the afternoon rush stacks up over Ripon and Fisk.

Read the TAF for a Story, Not a Verdict

The forecaster is telling you a story about the next 24 hours over the field. There’s a beginning - the base conditions. There are chapters - the FM lines. And there are the side plots that might bite you - the tempos and probs. Your job as pilot in command is to read the whole story and decide where in it you want your airplane to be. Arrive in the good chapter. Be tied down before the storm chapter starts.

Key Takeaways

• A TAF is a forecast within about five statute miles of one airport; a METAR is a measured observation. Know the difference.
• A ceiling is the lowest broken (BKN) or overcast (OVC) layer - few and scattered layers never count.
• The dangers hide in the change groups: FM (clean break), BECMG (gradual), TEMPO (brief worsening), and PROB30/40 (30–40% chance of significant weather).
• TAF winds are in degrees true; the tower gives magnetic, so expect an offset.
• For a July Oshkosh arrival, PROB40 thunderstorms in the afternoon means launch early - morning air is smoother and traffic is lighter.