How do I read the valid period in a TAF?

The valid period is formatted as Y1Y1G1G1/Y2Y2G2G2, where the first two digits are the day of the month the forecast starts, the next two are the starting hour in UTC, and the pair after the slash gives the ending day and hour. For example, 1512/1612 means the TAF is valid from the 15th at 1200Z until the 16th at 1200Z. If the period ends at midnight UTC, it is written with 24 rather than 00 to prevent date confusion.

What is the difference between TEMPO and PROB30 in a TAF?

TEMPO indicates that the listed conditions are expected to occur temporarily — lasting less than one hour at a time and covering less than half of the stated TEMPO period. PROB30 means there is a 30% probability that the described conditions will occur during the specified window. Both supplement rather than replace the base forecast, but PROB30 reflects lower confidence than TEMPO.

When does a TAF AMD (amended TAF) take effect?

A TAF AMD takes effect immediately upon issuance and cancels the previous TAF entirely. Pilots should not wait until the start of the valid period listed in the amendment before acting on it. The NWS issues a TAF AMD any time the current forecast becomes unrepresentative of existing or expected conditions, especially those significant to flight operations.

Decoding the TAF: Terminal Aerodrome Forecast Format Explained

The Terminal Aerodrome Forecast (TAF) is one of the most important weather documents a pilot consults before every flight. Issued by National Weather Service (NWS) Weather Forecast Offices (WFOs) for nearly 700 U.S. airports, a TAF packages a concise, standardized prediction of meteorological conditions expected within 5 statute miles of the center of the airport's runway complex. Because TAFs use the same weather codes found in METARs, a pilot who can read a routine weather observation can decode a forecast with only a little additional knowledge about TAF-specific groups and change indicators.

The sections below walk through every element of a TAF in the order it appears in the text, explain what the codes mean in plain language, and highlight the operational decisions each element supports. All information is grounded in the FAA Aviation Weather Handbook (FAA-H-8083-28B), Chapter 27.

Type of Report and Header

The very first element of every TAF is the report-type header, which appears in one of three forms:

TAF — a routine, scheduled forecast.
TAF AMD — an amended forecast issued any time the forecaster judges the existing TAF to be unrepresentative of actual or expected conditions, particularly regarding elements significant to flight operations. A TAF AMD immediately supersedes and cancels the previous TAF; pilots should begin using it right away rather than waiting for its stated valid period to begin.
TAF COR — a corrected forecast issued to fix a clerical or transmission error in a previously issued TAF.

Immediately after the header line comes the four-letter ICAO location identifier. U.S. airports start with K (e.g., KDFW for Dallas-Fort Worth), while Alaskan airports start with PA (e.g., PANC for Anchorage) and Hawaiian airports start with PH (e.g., PHNL for Honolulu). This identifier tells you exactly which airport the forecast covers.

Date/Time of Forecast Origin (YYGGggZ)

Following the location identifier is the date and time the forecast was completed and transmitted, coded as YYGGggZ — two digits for the day of the month (YY), four digits for the time in hours and minutes UTC (GGgg), and Z to confirm UTC. For example, 061737Z means the TAF was issued on the 6th at 1737 UTC. Routine TAFs are issued 20 to 40 minutes before the beginning of their valid period. This timestamp is entered by the forecaster and tells you how fresh the forecast is.

Valid Period (Y1Y1G1G1/Y2Y2G2G2)

The valid period group defines the window during which the forecast applies. Scheduled TAFs are issued four times daily — at 0000, 0600, 1200, and 1800Z — and cover either 24 or 30 hours depending on the airport. The format is two digits for the start day, two digits for the start hour, a slash, two digits for the end day, and two digits for the end hour, all in UTC.

1512/1612 — Valid from the 15th at 1200Z to the 16th at 1200Z (24-hour TAF).
2306/2412 — Valid from the 23rd at 0600Z to the 24th at 1200Z (30-hour TAF for a major airport).
0600/0624 — Valid from the 6th at 0000Z to the 6th at 2400Z, which is the same as the 7th at 0000Z. Note that midnight ending times are written as 24, not 00, to avoid ambiguity.

Knowing the valid period is critical: a TAF cannot be legally used to satisfy IFR alternate airport weather minimums unless the forecast covers the estimated time of arrival at the alternate.

Wind Group (dddffGfmfmKT)

Every initial forecast period and every subsequent FM (from) group begins with the surface wind forecast. The direction (ddd) is the three-digit true-north direction from which the wind is blowing, rounded to the nearest 10°. Wind speed (ff) is in knots, and KT is always appended. If gusts are expected — defined as rapid fluctuations of 10 knots or more between peaks and lulls — the letter G followed by the peak gust speed is inserted between the mean speed and KT. For example, 27018G30KT means wind from 270° at 18 knots gusting to 30 knots. Calm winds are coded as 00000KT. Wind direction in TAFs (and METARs) is always referenced to true north, which is important to remember when comparing to a runway's magnetic heading.

Visibility Group

Prevailing visibility in a TAF is reported in statute miles (and fractions). The group may include a fraction (e.g., 1/2, 3/4) combined with a whole number (e.g., 1 1/2SM). A visibility of 6 statute miles or greater is reported simply as P6SM (plus 6 statute miles). Visibility directly drives your IFR vs. VFR decision and your alternate planning calculations.

Significant Weather Group (w'w')

TAFs use the same present-weather codes as METARs, covering precipitation type, intensity, and descriptors. Common codes include RA (rain), SN (snow), TS (thunderstorm), FG (fog), BR (mist), and SHRA (rain showers). The special notation NSW (No Significant Weather) is used in a change group to indicate that significant weather previously forecast is expected to end. If no significant weather is expected, this group is simply omitted.

Sky Condition Group

Cloud layers are reported using the same three-letter contractions as METARs: FEW (1–2 oktas), SCT (3–4 oktas), BKN (5–7 oktas), and OVC (8 oktas). Heights are in hundreds of feet AGL using three digits. Obscured skies with a vertical visibility are coded as VVhhh, where hhh is the vertical visibility in hundreds of feet. SKC (sky clear) indicates no cloud layers are forecast. Cumulonimbus (CB) or towering cumulus (TCU) may be appended to a layer abbreviation when convective clouds are expected, which is particularly operationally significant for turbulence and icing avoidance.

Change Indicator Groups

Change groups are the heart of TAF interpretation. They modify the base forecast for specific sub-periods and come in three varieties:

FM (From): A complete replacement of all forecast elements beginning at the specified time. Coded as FM followed by a six-digit day-hour-minute group (e.g., FM151400 means from the 15th at 1400Z). Every FM group restates wind, visibility, weather, and sky condition for the new period.
TEMPO (Temporary): Conditions expected to last less than one hour at a time and to occur during less than half of the TEMPO period. TEMPO does not replace the base forecast; it supplements it. For example, TEMPO 1514/1518 BKN010 tells you that temporarily between 1400Z and 1800Z on the 15th, ceilings may drop to 1,000 feet broken, but this will not persist.
PROB30 (Probability 30%): Indicates a 30% probability of the specified conditions occurring. A PROB40 group indicates 40% probability. These are used for conditions that are possible but not expected to be the dominant forecast. In U.S. TAFs, only PROB30 is used (PROB40 is not issued by the NWS for U.S. domestic TAFs).

Non-Convective Low-Level Wind Shear (LLWS) Group

When non-convective low-level wind shear is forecast below 2,000 feet AGL, a WS group is appended. It provides the height of the wind shear, wind direction, and wind speed at that height. For example, WS020/23045KT indicates wind shear at 2,000 feet AGL with the wind at that altitude from 230° at 45 knots. This group alerts crews to potential airspeed fluctuations during approach and departure phases, which are the most critical flight regimes.

Why the TAF Matters Operationally

Under 14 CFR Part 91, pilots conducting IFR flight must consult all available weather reports and forecasts. The TAF is the primary source for destination and alternate planning. For IFR alternate airport requirements, the forecast ceiling and visibility at the estimated time of arrival determine whether an alternate is required and whether a specific airport qualifies. TAFs are also consulted for departure planning — a field forecast below takeoff minimums may delay departure, and an improving trend visible in the TAF supports a go decision.

For cross-country VFR flight, a TAF showing deteriorating visibility or lowering ceilings at the destination mid-flight is a critical piece of go/no-go information. The TEMPO and PROB30 groups deserve special attention here: they reveal the worst-case scenarios a forecaster considered significant enough to mention, even if not expected to dominate the forecast period.

Key Numbers and Rules

TAFs cover conditions within 5 statute miles of the airport runway complex.
Issued four times daily at 0000, 0600, 1200, and 1800Z.
Most TAFs cover 24 hours; select major airports receive 30-hour TAFs.
Issued 20–40 minutes before the beginning of the valid period.
Wind direction is referenced to true north.
Gusts are included when speed fluctuations are 10 knots or more between peaks and lulls.
Visibility of 6 SM or greater is reported as P6SM.
Midnight ending times use 24, not 00, to avoid date ambiguity.
A TAF AMD supersedes the previous TAF immediately upon issuance.
TEMPO conditions are expected to last less than one hour at a time and occur during less than half of the TEMPO period.

Common Test Traps

True vs. magnetic wind: TAF wind directions (like METAR) are referenced to true north, not magnetic north. Exam questions sometimes ask you to compare forecast wind to a runway's magnetic heading — remember to account for magnetic variation.
TEMPO means temporary, not guaranteed: Students often treat a TEMPO group as a certainty. It describes fluctuating conditions expected to be brief and intermittent, not persistent.
Valid period midnight notation: A TAF valid through midnight is coded with a 24 ending hour (e.g., 0600/0624), not 00. Confusing these can cause you to misread the coverage window.
TAF AMD is effective immediately: An amended TAF is not future-dated — it supersedes the old TAF the moment it is issued, even if its stated valid period hasn't started.
P6SM does not mean exactly 6 SM: P6SM means visibility is forecast to be greater than 6 statute miles. The actual value could be 8, 10, or more — the forecast only guarantees it will exceed 6 SM.