A Terminal Aerodrome Forecast (TAF) is far more than a single snapshot of expected weather. It is a time-sequenced document that covers 24 or 30 hours of anticipated conditions at an airport, and the real power of a TAF lies in its change groups. These coded groups—FM (From), TEMPO (Temporary), PROB (Probability), and BECMG (Becoming)—describe how and when the forecast meteorological picture is expected to evolve. Each group carries a precise, standardized meaning, and confusing one for another can lead to a pilot either dismissing a serious threat or over-reacting to a minor fluctuation.
According to the FAA Aviation Weather Handbook (FAA-H-8083-28B), Chapter 27, TAFs are issued by National Weather Service Weather Forecast Offices (NWS WFOs) for nearly 700 U.S. airports. They cover conditions within 5 statute miles of the center of an airport's runway complex and use the same weather codes found in METARs. Routine TAFs are issued four times daily at 0000, 0600, 1200, and 1800 UTC and are valid for either 24 or 30 hours depending on the airport's designation. What follows is a thorough breakdown of every change group a pilot will encounter.
The FM (From) Group
The FM group signals a complete, clean break in forecast conditions. When an FM group appears, it completely replaces every element of the previous forecast period—wind, visibility, weather, and sky conditions—starting at the specified time. Think of it as flipping a switch: the old forecast is entirely discarded, and everything that follows the FM code applies until the next change group or the end of the valid period.
The format is FMYYGGgg, where YY is the day of the month, GG is the hour (UTC), and gg is the minute (almost always 00 in practice). For example, FM1814 means: starting on the 18th at 1400 UTC, use only the conditions listed after this code. If the TAF previously forecast 3SM in mist and then an FM group shows P6SM SKC, the 3SM and mist are completely gone from that point forward—there is no carryover.
FM is used when a forecaster anticipates a significant, distinct shift in conditions—typically associated with a frontal passage, sea breeze boundary, or other well-defined meteorological transition. Because FM creates a sharp dividing line, it is the most definitive of the change groups and requires no hedging language or probability qualifier. A flight planned to arrive or depart near an FM time should treat both sides of that boundary as separate meteorological environments.
The BECMG (Becoming) Group
Where FM describes an abrupt change, BECMG describes a gradual transition. The change is expected to occur at an unspecified point within a defined time window, and the new conditions are expected to persist once the transition is complete. The format is BECMG Y1Y1GG/Y2Y2GG, where the slash-separated pair defines the window during which the change will take place.
For example, BECMG 1822/1824 BKN010 means ceilings are expected to lower to broken at 1,000 feet at some point between 2200 and 0000 UTC on the 18th/19th, and once they reach that level they will stay there. Critically, BECMG does not mean conditions oscillate during that window—it means the transition happens somewhere inside the window and then the new conditions are the forecast. The window is typically two hours but should not exceed four hours per NWS practice.
Pilots planning flights that overlap a BECMG window should plan for the possibility that the change occurs at the worst moment within that window. BECMG is commonly used for gradual sea-breeze effects, slowly developing fog, or afternoon heating clearing morning low clouds.
The TEMPO (Temporary) Group
The TEMPO group forecasts temporary fluctuations in one or more conditions that are expected to last less than one hour in each instance and cover less than half of the period specified. The format is TEMPO Y1Y1GG/Y2Y2GG, followed by the elements expected to deviate temporarily from the base forecast.
For example, if the base forecast shows 5SM -SHRA BKN030 and then TEMPO 1815/1820 2SM TSRA OVC010 appears, it means that between 1500 and 2000 UTC on the 18th, thunderstorm conditions with 2SM visibility and an overcast at 1,000 feet could temporarily interrupt the otherwise better conditions. The base forecast conditions are still the primary expectation; TEMPO flags intermittent excursions below that baseline.
A key rule: TEMPO conditions, by definition, should occur during less than half the total TEMPO period. If a forecaster expects deterioration for more than half the time window, the proper tool is either FM or BECMG, not TEMPO. From a pilot's perspective, TEMPO is a yellow flag—the conditions listed are possible and could be significant, but they are not the dominant expected picture. However, if TEMPO conditions would make the flight illegal (e.g., below alternate minimums), pilots must still account for them in planning.
The PROB (Probability) Group
The PROB group explicitly assigns a percentage likelihood to a set of conditions. In NWS domestic U.S. TAFs, only PROB30 is used—representing a 30 to less than 50 percent probability of the stated conditions occurring. (PROB40 is used in some international TAFs but not in standard NWS domestic products.) The format is PROB30 Y1Y1GG/Y2Y2GG followed by the elements.
For example, PROB30 1810/1814 1/2SM +TSRA means there is a 30 percent chance of visibility dropping to one-half statute mile in heavy thunderstorms between 1000 and 1400 UTC on the 18th. PROB is used when a forecaster believes the event is possible but below the threshold of confidence needed to include it in the base forecast or a TEMPO group. Per NWS policy, PROB30 is not used in the first six hours of a TAF, reflecting lower near-term forecast uncertainty expected during that window.
PROB30 can be combined with TEMPO—written as PROB30 TEMPO—indicating that a low-probability event, if it occurs, will itself be temporary in nature. This combination is particularly common for low-probability but high-impact phenomena like brief severe thunderstorm conditions.
Why These Groups Matter for Flight Planning
Change groups are the mechanism by which a TAF communicates risk across time. For instrument flight rules (IFR) planning under 14 CFR Part 91, pilots must consult the TAF for destination and alternate airports and understand how conditions are forecast to change throughout the flight. An FM group that crosses below alternate minimums during the planned arrival window may legally require naming an alternate that would otherwise not be needed. A TEMPO or PROB30 containing ceiling and visibility below minimums does not automatically trigger the alternate requirement, but good aeronautical decision-making demands the pilot assess whether those temporary or low-probability conditions represent an acceptable risk.
Understanding which change group applies also tells you how confident the forecaster was. FM and BECMG represent high confidence that a change will occur; TEMPO represents moderate confidence in temporary deviations; PROB30 represents meaningful but below-50-percent confidence in an event. Treating a PROB30 the same as an FM is an overreaction; ignoring a PROB30 for severe thunderstorms is an underreaction.
Key Numbers and Rules
- FM: Completely replaces all prior forecast elements from the stated time onward; no carryover from previous period.
- BECMG window: Transition occurs at an unspecified time within the stated window; new conditions persist afterward; window typically ≤4 hours.
- TEMPO duration: Each occurrence must last less than one hour; conditions must prevail for less than half of the stated TEMPO period.
- PROB30: 30–49% probability; not issued in the first 6 hours of a TAF; PROB40 is not used in standard NWS domestic TAFs.
- TAF coverage: Within 5 statute miles of the center of the runway complex.
- Valid periods: Issued 4 times daily (0000, 0600, 1200, 1800 UTC); standard TAFs valid 24 hours; select major airports valid 30 hours.
- TAF AMD: An amended TAF supersedes the previous TAF immediately upon issuance—do not wait for the start of the AMD's valid period to apply it.
Common Test Traps
- FM vs. BECMG confusion: Students often think BECMG is like FM with a ramp. Wrong—FM means total replacement instantly at a specific time; BECMG means gradual change within a window, and once complete, conditions stay changed.
- TEMPO duration misread: TEMPO does not mean conditions last throughout the entire window. Each occurrence is brief (under one hour) and must total less than half the window's duration. If the forecast calls for deterioration more than half the time, it should be in the base forecast or an FM, not TEMPO.
- PROB30 in the first 6 hours: NWS policy prohibits PROB30 in the first 6 hours of a TAF. Test questions sometimes imply it can appear anywhere—it cannot.
- PROB40 in domestic TAFs: PROB40 does not appear in standard NWS domestic U.S. TAFs. Only PROB30 is used. Confusing this with international practice is a common error.
- Alternate planning with TEMPO/PROB: Students sometimes assume any forecast condition below minimums triggers an alternate requirement. For regulatory purposes under Part 91, the specific wording of the applicable rule and the forecast element type (FM vs. TEMPO) matter. Always read the regulation carefully—TEMPO and PROB conditions require aeronautical judgment, not just automatic application of alternate rules.