How often are TAFs issued and how long do they cover?

TAFs are issued **four times daily** by the National Weather Service and cover a **24- to 30-hour window**. Always confirm the valid period at the top of the TAF covers your entire planned flight, including a buffer for delays.

What is the difference between FM, BECMG, and TEMPO in a TAF?

**FM** marks a permanent, instantaneous switch to new conditions at a specific time - everything before that timestamp no longer applies. **BECMG** (becoming) describes a gradual transition to new conditions over a defined window. **TEMPO** describes intermittent conditions expected to last less than one hour and occur during less than half the specified period; they are brief episodes, not continuous deterioration.

What does PROB30 mean in a TAF and should I take it seriously?

**PROB30** indicates a **30 to 39 percent probability** of the listed conditions occurring. One-in-three odds are not negligible - treat PROB groups as real probabilities that belong in your go/no-go analysis, not fine print to skip.

If there is a TEMPO group showing IFR conditions during my arrival window, can I still fly VFR?

You can legally attempt the flight if VFR conditions exist along your route, but you should not depart without a specific, pre-planned diversion option already identified. Select an alternate with acceptable forecast weather, confirm your fuel load supports reaching it, and decide in advance at what point you will divert rather than press.

How should I use the METAR alongside the TAF during a cross-country flight?

Compare METARs at your destination to what the TAF predicted for that same hour. If actual conditions are already worse than the forecast, the front may be moving faster than expected. Process that information while you still have fuel and route flexibility - not when you are ten miles out with no margin left.

The TAF, the FM Group That Rewrites Your Arrival, and the Forecast Every Cross-Country Pilot Needs to Read All the Way to the End

The Terminal Aerodrome Forecast (TAF) is the most actionable weather product in any preflight briefing - and one of the most frequently misread. Most pilots check the base conditions, see good weather, and close the tab. That is not a complete briefing; it only describes what the airport looks like at the start of the forecast, not what it will look like when you arrive.

What Is a TAF and What Does It Cover?

TAFs are issued four times daily by the National Weather Service. Each covers a 24- to 30-hour window and applies to conditions within roughly five statute miles of the airport. That radius matters - the TAF describes the airport environment specifically, not conditions ten or fifteen miles up the road. Weather near terrain or coastlines can vary dramatically over short distances.

Before reading anything else, verify the valid period covers your entire planned flight, including arrival time and a buffer for delays. A TAF that expires at 1800Z when you land at 1745Z offers almost no margin.

How Do You Read the TAF Header?

Every TAF begins with the letters TAF, followed by the four-letter ICAO station identifier. In the contiguous United States, most identifiers start with K - KORD for Chicago O’Hare, KDEN for Denver, KMCI for Kansas City International.

After the identifier comes the issuance date and time in Zulu (UTC), then the valid period. A header reading TAF KMCI 091720Z 0918/1018 means the forecast covers Kansas City, issued on the 9th at 1720Z, valid from 1800Z on the 9th through 1800Z on the 10th - a full 24-hour window.

How Do You Decode TAF Weather Codes?

The base conditions block uses the same coding as a METAR, so this section should feel familiar if you have been reading METARs in training.

Wind is a three-digit magnetic direction followed by speed in knots. 18012KT is wind from the south at 12 knots. VRB means variable direction. A G indicates gusts - 15012G22KT is 150° at 12, gusting 22 knots.

Visibility is in statute miles. P6SM means greater than six statute miles - the clear-and-a-million indicator.

Weather phenomena use standard abbreviations. A minus sign means light; a plus means heavy; no symbol means moderate:

RA - rain | SN - snow | DZ - drizzle
BR - mist | FG - fog | HZ - haze | TS - thunderstorm

These codes combine: -RA is light rain; TSRA is a thunderstorm with moderate rain.

Sky condition uses four descriptors, each followed by a three-digit cloud height in hundreds of feet AGL:

FEW / SCT / BKN / OVC - few, scattered, broken, overcast

BKN030 is a broken ceiling at 3,000 feet. OVC008 is overcast at 800 feet. VV (vertical visibility) replaces sky condition when an obscuration is too dense to determine cloud bases. SKC means sky clear.

What Are TAF Change Groups and Why Do They Matter?

The base conditions describe only the start of the valid period. Change groups describe everything that follows - and skipping them means skipping the substance of the TAF. There are four: FM, BECMG, TEMPO, and PROB.

FM - From

FM marks a permanent, instantaneous change in conditions. The moment you cross an FM timestamp, all previous conditions are completely replaced. FM092130 means the new conditions take effect at 2130Z on the 9th. Everything earlier in the TAF no longer applies after that time.

BECMG - Becoming

BECMG describes a gradual transition over a defined window. BECMG 0800/1000 means conditions shift between 0800Z and 1000Z. The exact timing within the window is uncertain, but the endpoint is what follows the BECMG group. Use BECMG groups to understand trends - when VFR conditions will establish after a system, or how long you have before things deteriorate.

TEMPO - Temporary

TEMPO is the change group that trips up even experienced pilots. It describes conditions expected to last less than one hour at a time and occur during less than half of the specified period. The conditions are intermittent, not continuous.

A TEMPO group showing three-quarter-mile visibility in thunderstorms over a four-hour window is not predicting four straight hours of embedded storms. It is predicting one or more brief episodes - with base conditions in between. The critical question: if conditions could temporarily drop below VFR minimums right when you are on a ten-mile final, what is your plan? Do you have fuel to hold? Is your alternate already chosen?

PROB - Probability

PROB30 means a 30 to 39 percent chance of those conditions occurring. PROB40 means 40 to 49 percent. When forecasters believe conditions are more than 50 percent likely, they use a standard change group instead of a PROB qualifier.

One-in-three odds are not odds to dismiss. PROB also combines with TEMPO - PROB30 TEMPO during a specified window means there is a 30 percent chance those temporary conditions occur during that period. Both qualifiers modify the same event; read them together.

How Do Change Groups Apply to a Real Cross-Country?

Consider a flight from Denver to Kansas City - roughly 400 nautical miles at 115 knots, approximately three and a half hours of flight time. Departure at noon Mountain time (1800Z), estimated arrival around 1430 Mountain (2030Z).

The KMCI TAF base conditions look excellent: visibility greater than six statute miles, few clouds at 2,500 feet, wind 170° at 10 knots. Most pilots would stop reading here.

Continuing through the TAF:

FM092130 - Wind 200° at 15 knots. Visibility 2 statute miles in mist. Broken ceiling at 800 feet.

That FM group activates at 2130Z. Planned landing is 2030Z - exactly one hour of buffer before Kansas City drops to near-instrument conditions.

TEMPO 092000/092200 - Visibility less than 1 statute mile. Heavy rain and thunderstorm.

That TEMPO window runs from 2000Z to 2200Z. The planned landing at 2030Z falls squarely inside it.

The real picture: a front arrives in the early afternoon. Ceilings drop to 800 feet by 2130Z. During the two-hour TEMPO window that includes the planned arrival, there is a temporary possibility of near-zero visibility in thunderstorms. The pleasant morning base conditions tell you nothing about what you will fly into.

How Do You Make the Go/No-Go Decision From a TAF?

For a VFR flight into this scenario, the realistic options are:

Delay departure to arrive earlier, ahead of the TEMPO window
Choose an alternate destination not in the front’s path
Select a divert airport, carry extra fuel, and commit to a specific diversion plan before leaving the ground

“Maybe the front is running late” is not a plan. It is wishful thinking with a headset on.

The Airman Certification Standards require pilots to obtain a complete weather briefing, interpret it accurately, and make decisions based on what the weather actually says. This type of scenario - base conditions fine, change groups telling a different story - is exactly what examiners walk through during the private pilot oral.

What Are Good Habits for Reading Any TAF?

Build a mental timeline every time you open a TAF:

Start with the header. Does the valid period cover your entire flight, including a reasonable delay buffer?
Read the base conditions. What does the airport look like at the start of the forecast?
Work through every change group in time order. What do conditions look like at departure, en route, at arrival, and one hour after arrival?
Assess the trend. A TAF where each FM group tightens conditions is a fundamentally different situation from one where conditions progressively improve through the afternoon.
Flag any TEMPO group overlapping your arrival window. If it describes sub-VFR conditions, have a specific diversion plan in place before you depart.
Treat every PROB group as a real probability. Because it is one.

How Does the TAF Work Alongside the METAR?

The TAF tells you what is expected. The METAR tells you what is actually happening. When they diverge, the METAR is telling the truth.

As a flight progresses, compare METARs at the destination to what the TAF predicted for that same hour. If actual conditions are already worse than forecast, that is a signal - the front may be arriving early. Ten miles from the airport is not the time to start processing that information. Process it while options and fuel still exist.

No single product tells the whole story. A complete briefing also includes winds and temperatures aloft for cruise altitude, any AIRMETs or SIGMETs along the route, and PIREPs from pilots who recently flew through the area.

Where Can You Find TAF Data?

1800wxbrief.com - the official FAA weather briefing service
Garmin Pilot
ForeFlight
Aviation Weather Center at aviationweather.gov

Complete format and interpretation guidance is published in the FAA Pilot’s Handbook of Aeronautical Knowledge and the Airman Certification Standards.

Key Takeaways

TAFs are issued four times daily, cover a 24- to 30-hour window, and apply within five statute miles of the airport - not the broader area
Reading only the base conditions means reading the beginning of the forecast and ignoring everything that follows
FM marks a permanent switch; BECMG a gradual transition; TEMPO describes brief, intermittent conditions; PROB quantifies the likelihood of occurrence
Any TEMPO group covering your arrival window with sub-VFR conditions demands a specific, committed alternate plan before departure - not a hope that conditions hold
When forecast and actual METARs diverge, the METAR is ground truth - monitor it en route so you can act while options remain