Change groups are used to indicate a change in any or all elements from the predominant forecast and are followed by the description of all the elements that are forecast to change. Each change group indicates the time during which the changes are forecast to occur. When used, a change group will begin on a new line. The TAF code uses four change groups; RAPID GRADU, prevailing conditions which will individually last for less than 1 hour.

RAPID GG is used to describe a change in prevailing conditions that will take place during a period of time less than 1/2 hour. The GG is the last whole hour UTC before the change begins. This group must be followed by a complete description of the conditions that will prevail after the change. The wind, visibility, weather, cloud, and minimum altimeter setting groups must all be included. Remarks may follow the altimeter setting group. A pertinent remark indicating the reason for the rapid change may also be included, such as CLD FROPA for a cold frontal passage.

GRADU GGG_eG_e describes a change in the prevailing conditions that will take place during a period of time lasting more than 1/2 hour but less than 2 hours. The group is followed by a complete description of the prevailing conditions after the change occurs and must include the wind, visibility, weather, cloud and minimum altimeter setting groups, as well as any pertinent remarks.  

GGG_eG_e is the forecast period indicator. GG is the last whole hour UTC before any change is forecast to occur, and G_eG_e is the first whole hour UTC after the changes end (up to 2 hours later than GG). The period must cover the entire time during which the conditions will be changing. If the forecast changes will occur over more than a 2-hour period, two or more GRADU change groups should be used. The first change group may cover a 2-hour period and describe the conditions that will prevail at the end of the change period, and the second period should cover any change during the following period lasting up to 2 hours, etc. If we are forecasting a gradual change in conditions over a 3-hour period from 0430Z to 0730Z, the forecast should be formatted as follows:

TEMPO GGG_eG_e is the change group used to indicate temporary changes in a prevailing forecast condition. Each change from the predominant condition should last less than 1 hour, and if the change is to occur more than once, the total time of all the occurrences should not exceed one-half the total time covered by the forecast period indicated by GGG_eG_e When using this group, it is followed only by the description of the elements that are forecast to be different from the predominant conditions during the period. It should not include a QNH group, but may include any pertinent remarks.

GGG_eG_e is the forecast period indicator. GG is the last whole hour UTC before any change G_eG_e is forecast to occur, and is the first whole hour UTC after the changes end. The period must cover the entire time during which the conditions will be different from the forecast prevailing conditions.

INTER GGG_eG_e is the change group used to indicate intermittent changes from a pre-dominant forecast condition. Intermittent changes occur more frequently than temporary changes and last for shorter periods of time. All occurrences together must last less than half the total time covered by the forecast period. This group is followed by a description of only those conditions that will be different from the prevail-ing conditions, including ceiling designation and pertinent remarks.

At this point, you should be able to read a TAF coded report. An example of a TAF for NAS Pensacola (Sherman Field) is provided along with an explanation of the coded forecast. As you go through the coded forecast, determine what each code group actually means. Compare your interpretation with the explanation.

The corrected forecast is for NAS Pensacola, Florida (NPA), valid from 1500Z to 1500Z. The initial condition (1500Z to 2000Z) forecasts winds from 120 at 5 knots, visibility 2 1/2 miles (4,000 meters) in fog (mist), sky cover 3/8 stratus (fractus) at 500 feet, 7/8 stratus at 1,200 feet, minimum altimeter 30.01 inches, ceiling 500 feet due to the partial obscuration by fog of 2/8 of the sky (2BR/ / /) plus the 3/8 of stratus fractus at 500 feet.

During the period between 1500Z and 1700Z, the above conditions will deteriorate briefly to calm winds and 1/2 mile visibility in fog, with an obscured ceiling at 100 feet.

Between 1800Z and 2000Z, the conditions will gradually change and become, by 2000Z, winds from 200 at 15 knots, visibility 3 miles (4,800 meters) in moderate to heavy rain showers. The sky cover will be 3/8 cumulus at 1,500 feet, 3/8 of altocumulus forming a ceiling at 12,000 feet and 4/8 cirrostratus at 25,000 feet, with moderate rime icing (in cloud) from 12,000 feet to 18,000 feet. The maximum temperature will be 18C at 1900Z. The minimum altimeter setting will be 29.95 inches. Thunderstorms will be in the area, but not over the airfield except between 2100Z and 2300Z. During this period, thunderstorms will briefly affect the airfield, producing winds from 2400 at 15 knots, with a maximum gust expected from 330 at 45 knots. Visibility will lower to 1/2 mile (800 meters) in rain during the thunderstorm, and the ceiling will be at 1,000 feet, due to 2/8 cumulonimbus and 3/8 cumulus. Altocumulus will also form a temporary 8,000-foot overcast and cirrostratus will form an additional overcast layer at 25,000 feet.

Rapid changes will take place with a cold frontal passage at 2300Z or shortly thereafter, so that by 0000Z the wind will be from 340 at 25 knots, with unrestricted visibility and no significant weather with 3/8 of stratocumulus at 2,500 feet and 2/8 of cirrus at 25,000 feet.

Apparently, the cirrus will be thin, since there will not be a ceiling. The strong winds will cause light, occasional moderate low-level turbulence in the clear air from the surface to 2,000 feet, and Low Level Wind Shear is expected to be present until 0100Z. The minimum altimeter setting will be 30.00 inches. Also, up until 0100Z, there will be frequent, but brief light rain showers, lowering the visibility to 5 miles (8,000 meters) with associated 2/8 cumulus clouds at 2,000 feet causing a ceiling at 2,500 feet (when added to the 3/8 stratocumulus predominant layer).

Between 0100Z and 0300Z, the conditions will gradually improve. By 0300Z, the wind will be from 340 at 15 knots, with unrestricted visibility, no significant weather, no clouds, and a minimum altimeter setting of 30.06 inches. After 0500Z, the winds will decrease to 320 at 5 knots.

Thin fog will gradually form between 1000Z and 1100Z, lowering the visibility to 4 miles (6,000 meters), with the wind from 320 at 3 knots. The minimum temperature will be 8C near 1200Z. Between 1400Z and 1500Z, the fog will clear as the winds increase to 330 at 9 knots, resulting in unrestricted visibility and clear skies, with a minimum altimeter setting of 30.10 inches.

The TAF coded forecast provides information about your airfields expected weather conditions to worldwide users. When composing a TAF, you must insure that it completely and accurately describes the conditions you expect to occur for the next 24 hours. Each TAF contains a header that is used to identify the forecast location as well as the valid period of the forecast. Different groups are used within the TAF code to describe various weather elements. Remarks and change groups are used to modify the basic forecast.

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing