How is a TEMPO group different from a BECMG group in a TAF?

BECMG describes a gradual, permanent transition to new conditions over a defined time window. TEMPO describes brief, temporary dips in conditions expected to last less than 60 minutes each and occur during less than half the stated time period - after which conditions return to the base forecast.

Does a TEMPO group mean conditions will definitely deteriorate?

No, but it means there is meaningful probability of deterioration. Forecasters include TEMPO groups when conditions could dip to the described level. "Temporary" refers to the duration of each occurrence, not the likelihood of one happening - treat it as a contingency trigger, not background noise.

What does PROB40 mean in a TAF, and should I factor it into planning?

PROB40 means the forecaster assigns a 40% chance of the following conditions occurring - close to a coin flip. TAFs only use PROB30 or PROB40; anything above 50% should already appear in the base forecast. If the conditions described would close your destination or force a divert, PROB40 deserves serious weight in your planning.

If the METAR at my destination looks fine, do I still need to read the TAF?

Yes. The METAR reports current conditions only. The TAF shows how conditions are expected to evolve over the next 24–30 hours. A clean METAR at departure tells you nothing about the ceiling and visibility three hours later when you arrive.

What should I do if I see AMD or COR at the start of a TAF?

Treat your previous brief as invalid and pull the current forecast from scratch. AMD means conditions changed enough to warrant an updated forecast; COR means an error in the original was fixed. Either way, the TAF you reviewed earlier is no longer current.

The TAF, the TEMPO Group, and the Two-Hour Window That Tells You Whether Your Destination Will Be There When You Arrive

The Terminal Aerodrome Forecast (TAF) is one of the most actionable weather products available during preflight planning - and one of the most commonly misread. Understanding how TAF change groups work, particularly TEMPO, is the difference between a routine cross-country and arriving at a field that’s gone IFR with no contingency plan.

What Is a TAF and What Does It Actually Cover?

The TAF is an official weather forecast issued by the National Weather Service for specific airports. Not every airport has one - you’ll find them at larger airports, busy general aviation fields, and airports near Class C and Class B airspace. The absence of a TAF for a destination is itself useful planning information.

TAFs are issued four times a day, every six hours, typically at 0000, 0600, 1200, and 1800 Zulu. They cover a 24 or 30-hour window from the issue time and apply to conditions within a five statute mile radius of the airport.

That radius matters. The TAF tells you what to expect at the field. It tells you nothing about the 200 miles of airspace between you and your destination.

How Do You Read the TAF Header?

Every TAF starts with a header line. Here’s an example:

TAF KORD 250530Z 2506/2606

KORD is the airport identifier for Chicago O’Hare. 250530Z is the issue time - the 25th of the month at 0530 Zulu. 2506 is when the forecast period begins; 2606 is when it ends. This is a 24-hour forecast covering from 0600 today through 0600 tomorrow.

The body reads like the back half of a METAR - wind, visibility, weather, and sky coverage in that order.

Wind: 18015G24 = winds from 180° at 15 knots, gusting to 24
Visibility: P6SM = greater than 6 statute miles
Sky: SKC = sky clear; BKN030 = broken clouds at 3,000 feet

What Do the Four TAF Change Groups Mean?

There are four change group indicators: FM, BECMG, TEMPO, and PROB. Using them interchangeably is how you end up surprised sixty miles from a field that was supposed to be VFR.

FM (From) marks a permanent, hard cutover. Everything before the FM time uses prior conditions; everything after uses the new conditions - no blending, no gradual shift. FM1400 means the conditions that follow completely replace everything previously stated, starting at 1400 Zulu.

BECMG (Becoming) describes a gradual transition over a defined window. BECMG 1200/1400 means conditions are gradually transitioning between 1200 and 1400 Zulu. By the second time code, the new conditions are expected to be fully in place. This group appears when a front is moving through slowly, a sea breeze is developing, or clouds are expected to build or dissipate.

TEMPO (Temporary) is the one that trips pilots up most. The formal definition: conditions listed after TEMPO are expected to occur for periods of less than 60 minutes at a time, and those conditions should cover less than half of the total time period listed.

TEMPO 1400/1800 TSRA BKN010 means between 1400 and 1800 Zulu, there may be brief intervals of thunderstorms with a broken ceiling at 1,000 feet. Each occurrence should last under an hour. The conditions shouldn’t be present for more than two of those four hours total.

PROB (Probability) appears as PROB30 or PROB40 - the only two values used in TAFs. PROB30 is lower confidence; PROB40 is close to a coin flip. Anything above 50% probability is supposed to already be baked into the base forecast. Neither value should be dismissed if the described conditions would close your destination.

Why Does TEMPO Get Misread?

Many pilots see the word “temporary” and mentally file it as background noise. That’s the wrong read. A forecaster included a TEMPO group because there is meaningful probability of conditions deteriorating during that window. Temporary describes the duration of each occurrence - not the likelihood of it happening.

Plan your flight around the base conditions. Build your contingency around the TEMPO. That means an alternate that looks clean during the same window and enough fuel to divert and still land with legal reserves. If TEMPO conditions are active when you arrive, you need somewhere to go and fuel to get there.

How Do You Use the TAF to Plan a Flight?

The most important step - and the one most students skip - is time alignment. Map every change group against your actual flight timeline before you file.

Here’s a worked example:

Departure: 1300 Zulu
Flight time: 3 hours
Planned arrival: 1600 Zulu
Alternate (30 min beyond destination): 1630 Zulu in a divert scenario

With that timeline established, go through the TAF systematically:

What are conditions from 1300 through 1630Z?
Is there an FM group changing conditions during your en route phase?
Is a BECMG transition happening while you’re still airborne?
Are there TEMPO groups covering your arrival window?
Are there PROB flags anywhere in that stretch?

If the TAF shows solid VFR from issue time through your arrival with no change groups until after 2000Z, the picture looks good. If there’s a BECMG starting at 1500 that takes the ceiling below VFR limits by 1600, you may be arriving into an active transition.

That’s the scenario that catches pilots. The METAR at departure looked clean. The TAF had a BECMG group beginning two hours before estimated arrival, dropping the ceiling below VFR. The information was there - it just wasn’t read all the way through.

What Do AMD and COR Mean in a TAF Header?

AMD (Amended) means the forecast has been updated since it was originally issued. This happens when conditions change significantly enough that the original forecast is no longer valid. If you briefed weather in the morning and several hours passed before departure, check whether the TAF was amended in that window.

COR (Corrected) indicates a fix to an error in the original forecast rather than a change in actual conditions. In both cases, the action is the same: pull the new TAF from scratch and discard the previous brief. The old forecast is no longer valid.

What Are the Limits of TAF Coverage?

A TAF applies within a five statute mile radius of the airport. Localized effects inside that ring - a fog bank sitting at the threshold, a valley that traps moisture, a sea breeze boundary right at the field - may not be fully captured.

Pilot reports (PIREPs) from aircraft that recently flew into or out of the area provide the ground truth that the TAF can’t. When you complete a flight, consider filing a PIREP for the conditions you actually encountered. It takes two minutes and feeds the system that other pilots are depending on.

What Will the Checkride Examiner Ask About TAFs?

The Airman Certification Standards for the private pilot certificate require demonstrating the ability to obtain and analyze weather information for flight planning and decision-making. The word analyze is doing significant work in that sentence.

The examiner wants more than a printout. Be ready to:

Identify which change groups fall within your planned flight window
Explain what each group means for your go/no-go decision
State specifically what conditions you expect at your destination and when
Describe what you would do if a TEMPO condition is occurring on arrival
Know your alternate’s weather and articulate why it’s an appropriate choice

The TAF is not a green light or a red light. It’s a forecast built with imperfect data, and the examiner is looking for a pilot who reads the full picture and plans against reality - not the best case.

Key Takeaways

TAFs are issued four times daily (0000, 0600, 1200, 1800 Zulu) and cover a five statute mile radius around the airport
FM = permanent cutover; BECMG = gradual transition to new conditions; TEMPO = brief dips covering less than half the stated window; PROB = probability flag (30% or 40% only)
TEMPO does not mean unlikely - it means brief when it occurs; build contingency, alternate, and fuel around it
Time-align every TAF change group against your actual departure, en route, and arrival times before filing
If the header shows AMD or COR, discard your previous brief and pull the current forecast
PIREPs from other pilots fill the localized gaps that a five-mile forecast radius cannot capture