Airport forecasting reports are important weather tools for pilots.
By Karsten Shein
Comm-Inst Climate Scientist
Based on the terminal aerodrome forecast (TAF) from EUG (Eugene OR), the pilots figured that they’d have no problem getting in. The forecast for their arrival in a couple of hours had 3 miles of visibility with a broken scud at 400 ft, deteriorating over the following 12 hours as a fog developed over the region – not ideal, but not a problem for the experienced crew.
Fortunately, the pilots noted the issue time was already nearly 3 hours old, and the latest metar said the airport was already down to a 0.5-mile visibility in freezing fog and 100 ft broken ceiling.
Waiting just a few minutes, they refreshed their TAF list and got the latest forecast, now just a few seconds old. Sure enough, the fog was now expected to build faster than previously forecast. The 2 pilots, based at EUG, knew they needed to plan for several alternates.
Bringing up the TAFs for PDX (Intl, Portland OR) and AST (Astoria OR), they felt confident that conditions at PDX would remain above minimums for the next few hours. As it turned out, they were able to shoot the ILS at EUG and land safely just a few minutes before the airport went zero-zero in a thick fog. TAFs play an important role in flight planning and anticipating conditions over the next several hours at departure, arrival, and alternate airports.
But, like any meteorological information, it is important for pilots to know and understand their strengths and limitations. TAFs are rudimentary forecasts made specifically for many, but not all, military and civilian airports and their immediate vicinity. In the US, they are valid for a 5-statute-mile radius around the airport, while elsewhere it is 5 nm (~9 km).
Most TAFs are issued every 6 hours on a routine schedule at 0000, 0600, 1200, and 1800Z in most places. However, at core airports, and where forecasters can do so, TAFs may be issued on a 3-hr schedule instead. While there are international standards for TAFs, each national aviation or weather authority has some discretion in how they are produced.
At military airfields and at many international airports, TAFs are prepared and issued by forecasters located on site. In the US, TAFs are issued by meteorologists at the airport’s relevant National Weather Service (NWS) Weather Forecast Offices. Some offices, such as those serving major metropolitan areas, may have the responsibility for issuing regular TAFs at more than a dozen airports.
Because the purpose of a TAF is to provide the most relevant forecast weather conditions significant to aviation, and to do so quickly and efficiently, they are intentionally limited to just a few basic parameters and formatting. For example, the US NWS instructs its meteorologists to limit TAFs to no more than 6 lines, not including any TEMPO (temporary weather) statements, and each line is limited to no more than 69 characters.
Military forecasters often include much more forecast information. For example, forecast temperature and altimeter settings are rarely included in civilian TAFs, but are common in military-issued TAFs.
Nearly all TAFs contain the same basic structure of information. They begin on the 1st line with the airport identifier – leading with the regional code, such as K for CONUS US stations, P for Pacific (including Hawaii and Alaska), E for Europe, C for Canada, T for the eastern Caribbean and Atlantic, and M for Mexico.
The US NWS also makes it a point to drop the last letter of an airport designator in its Pacific region, adding in the second position an H for Hawaiian airports, an A for Alaskan airports, and a G for Pacific Island territory airports, except where the airport designator begins with that subregional letter.
While it works out for airports such as ANC (Anchorage AK), which becomes PANC, an airport such as OGG (Maui HI) becomes PHOG. This can be confusing for pilots who do not frequent these airports. Every now and then, the code AMD will follow an airport identifier.
This simply means that the TAF has been amended by the forecaster because the original TAF forecast conditions are no longer representative of the expected weather. Similarly, COR indicates a corrected TAF, suggesting some error in the original forecast.
Corrections are normally issued within an hour of the original issuance, while amendments occur if an update is made more than an hour after issuing the original TAF. AMD and/or COR will be repeated at the end of the TAF, followed by the 4-digit UTC (Z) time at which the amendment or correction was made.
Amended or corrected TAFs always supersede the original TAF. The airport identifier (or AMD or COR) is followed by the origination day and time (eg, 011153Z – 1st day of the month at 1153 UTC time). Pilots should note that the origination time is the time the forecast was actually made, not the time it was issued, amended, or corrected.
TAFs can be issued as much as 30 minutes after origin because the issuing office often bundles the TAFs for a single transmission at the prescribed issuance time. Following the origin time are the valid times, meaning the times covered by the forecast. Routine TAFs are always valid for 24 hrs, and the valid times may be displayed as a single-day identifier followed by a start and stop hour or by 2 days and Z-times.
You might see a valid time displayed as 211818 or 2118/2218. Both mean the TAF is valid from 1800Z on the 21st day of the month until 1800Z on the 22nd. Subsequent TAF lines will normally begin with FM (meaning “from”), a 2-digit day, and a 4-digit Z-time (eg, FM281700).
Importantly, at the few TAF-listed airports where weather observations are not available 24 hrs a day, TAFs are only valid to the end of the observing period, and the TAF at those airports, may end with AMD NOT SKED followed by the period of service interruption.
Where a short-term weather condition – usually less than an hour or 2 – may be significant to aviation at some time during the valid period, the forecaster will include the phrase TEMPO (or begin a line with TEMPO), for temporary. The duration of the temporary condition will follow in the same format as a valid period.
So, TEMPO 0115/0117 is a forecast of certain following conditions occurring between 1500 and 1700Z on the 1st of the month. Similarly, the forecaster may also include the phrase BECMG and a 4-digit time to indicate the start and end hour over which the conditions will gradually become those forecast following the BECMG entry.
For example, BECMG 2023 OVC020 suggests that between 2000 and 2300Z, an overcast ceiling at 2000 ft will form. Finally, a TAF might use the term PROB40 (or in some cases PROB30). This just means that there is a moderate likelihood (around a 40% probability) that the following forecast conditions will occur.
If the probability of forecast conditions exceeds the 30–50% range, it is simply reported as the forecast. If it is less than 30%, it is not reported in the TAF.
After the timing qualifiers, the remainder of the forecast line is a forecast of wind, visibility, weather, sky condition, and, optionally, any important conditions such as wind shear, icing, or probability of convection.
TAF weather coding is mostly identical to METAR coding. Wind is forecast in direction (first 3 digits) and speed (last 2 digits). Gusts are forecast with a G and the 2-digit gust speed. KT follows the wind forecast to indicate that the speed is in kts (some countries may report in meters/second). Calm wind is forecast as 00000KT.
If winds are forecast to shift by more than 30º, the direction digits are replaced by VRB (although meteorologists normally avoid using VRB in TAFs). Visibility will be reported either in statute miles or in meters. In the latter, visibility will be a 4-digit number with the units implied.
Statute mile visibilities will be given in whole and quarter mile amounts (up to 6 miles) followed by SM. If visibility is forecast to exceed 6SM, the letter P will precede the visibility. In meters, 9999 indicates any visibility forecast greater than 9000 m (~7SM). The phrase CAVOK is still in wide use internationally, and simply indicates that ceiling and visibility are good, with values exceeding the maximum reportable.
Familiar METAR codes are used in TAFs to forecast the likely weather conditions for the forecast period. Common codes include FG (fog), RA (rain), TS (thunderstorm), and NSW (no significant weather). Precipitation qualifiers may be added to indicate showers (SH), freezing (FZ), or intensity (+/-).
Sky condition is the forecast state of clouds, reported in coverage and height in hundreds of feet (eliminating the 2 trailing zeros). There may be multiple cloud levels and the qualifier of CB to indicate vertically developing clouds. A TAF sky report might state BKN001 OVC030, forecasting a broken ceiling at 100 ft AGL and an overcast deck at 3000 ft. In lieu of cloud decks, some TAFs will show vertical visibility (VV) in hundreds of feet, often when fog is also forecast.
SKC means a forecast of clear skies. TAFs may also forecast icing or turbulence by following the sky group with a 6-digit number beginning with 6 (icing) or 5 (turbulence). The 2nd digit indicates the type of icing/turbulence. The next 3 digits are the base of the forecast layer in hundreds of feet, and the last digit is the layer depth in thousands of feet.
Because TAFs are generally limited to immediate airport operation considerations, these groups are rarely included. Military TAF forecasters will also frequently include temperature and altimeter settings, but civilian forecasters do not. Where present, forecast altimeter settings will begin with QNH (or just Q where space is limited) and a 4-digit altimeter setting followed by INS (inches).
Temperature forecasts start with T followed by the maximum temperature (°C)/hour (Z) and minimum temperature/hour for the forecast period. Despite their basic nature, TAFs are often more accurate than the area forecasts issued for the region surrounding the airport.
This is because they take into account observations made at the airport, as well as localized nuances in geography and weather patterns that the forecasters know affect the airport’s weather.
Trend type forecasts
A cousin to the TAF is the trend type forecast (TTF), normally only produced by a professional forecaster located at the airport itself, and issued as part of a METAR to alert pilots to a likely significant change in weather conditions over the next 2 hours.
TTFs are most commonly issued in the UK and Europe, but may be issued anywhere. The TTF forecast appears in the METAR following the code TEMPO. When a TTF appears in a METAR, pilots should consider the TTF to be better information than what is contained in the current valid TAF for the airport.
A TTF might look something like TEMPO 0500 +TSRA VV002 RED. This TTF suggests that a developing strong thunderstorm is expected to drop visibility to 500 m and ceiling to 200 ft. The term RED is a so-called “color state.” These are included in TAFs and TTFs in some places, and were added to improve the speed at which a pilot could interpret ceiling and visibility conditions.
There are 8 colors to this index, and they represent the worst conditions forecast for the valid period, ranging from blue (BLU – best, with minimum 2500 ft ceiling and 8 km visibility) to red (RED – worst, with min ceiling less than 200 ft and visibility less than 800 m).
If there is another reason that an airfield is unusable, the word BLACK will precede the applicable color code.
TAFs are a good information source for pilots, both during flight planning and enroute. Ahead of the flight, they are a way to quickly sort out the meteorological viability of alternate airports. Many pilots don’t consider the individual weather forecasts of airports they are considering as alternates, preferring to simply choose a nearby airport that can handle their aircraft and may have the right facilities to get the boss home and turn the flight as soon as the weather at the destination improves.
However, ignoring the TAFs at potential alternate airports may mean that, when you divert, you find your designated alternate is in even worse shape than your original destination. A quick look at the TAF for the alternate can have you deciding instead on a different alternate a little further out where the forecast is not quite as dire.
Enroute, looking at TAF updates along with the current METAR of your destination can help you validate the accuracy and timing of the forecast. For example, you may notice that the old forecast had arrival time winds of 10 kts down the runway, ramping up to a strong crosswind later that day.
But halfway through the flight, the new TAF advances the frontal wind shift to your arrival time, meaning you’ll face a stiff 25-kt crosswind gusting to 32. Similarly, you might have seen a TAF call for a ceiling dropping from 5000 ft broken to a 100 ft overcast after your arrival time, but the METARs are showing a more rapid deterioration, giving you time to prep for an instrument landing to the minimums.
Karsten Shein is cofounder and science director at ExplorEiS. He was formerly an assistant professor at Shippensburg University and a climatologist with NOAA. Shein holds a commercial license with instrument rating.