GTG is generated operationally at the National Weather Service's Aviation Weather Center, which is supported 24 hours a day, 7 days a week. Its use is unrestricted (meteorologists, dispatchers, GA and commercial pilots, ATC, etc.) as a supplementary source of weather information.
The Turbulence page contains depictions of the Graphical Turbulence Guidance product, version 2.5 (GTG-2.5), which forecasts mid-level and upper-level clear-air turbulence (CAT) from 10,000 feet to FL 450, with 0, 1, 2, 3, 6, 9, and 12 hour lead-times. It does not provide turbulence analyses or forecasts below 10,000 ft MSL. GTG-2.5 is a Supplementary Weather Product. A Supplementary Weather Product is an aviation weather product that may be used for enhanced situational awareness. If utilized, a supplementary weather product must only be used in conjunction with one or more primary weather products ( AIM 7-1-3).
The GTG-2.5 graphics are computer-generated four-dimensional forecasts of information related to the likelihood of encountering Clear-Air Turbulence (CAT) associated with upper-level fronts and jet streams, and is not intended to predict turbulence associated with convection and thunderstorm clouds or breaking mountain waves. The product provides forecasts for the 48 contiguous United States, much of Canada and Mexico, and their respective coastal waters at flight altitudes from 10,000 MSL to FL450 only, that is, it does not provide forecasts from the surface to 10,000 ft. Users should also be aware that turbulence is a highly dynamic phenomenon and in case of rapidly changing conditions the product may not accurately convey a significant hazard.
GTG-2.5 graphics may be used as a higher-resolution supplement to AIRMETs and SIGMETs, but not as a substitute for the turbulence information they provide. GTG-2.5 graphics are authorized for use as an unrestricted, supplementary weather product. The GTG-2.5 does not have the capability to be amended. See the definition of primary and supplementary weather products below.
The GTG-2.5 product consists of a 00, 01, 02, and 03 hour forecast, which are updated every hour, and a 06, 09, and 12 hour forecast, which are updated every three hours, starting at 00Z. GTG-2.5 graphics are "snapshot" graphics, intended to depict forecasted clear air turbulence conditions at the valid time (for example, at 1200Z), not for a valid time range (for example, from 1200Z to 1300Z). The GTG-2.5 graphics suite is automatically produced with no human modifications.
Information on the graphics is determined from observational data, turbulence pilot reports, and automated turbulence (EDR) reports, all of which are integrated with computer forecast model output.
Due to the use of computer model output, the GTG-2.5 product issuance times are reliant upon the timing and resolution of the released underlying weather model data. GTG-2.5 uses 00, 01, 02, 03, 06, 09, and 12 hour forecast fields from the NOAA "Rapid Refresh" or "RAP" forecast model with a horizontal grid spacing of 13.5 km and 50 vertical levels. The RAP updates hourly, and the total processing time is dependent upon the amount of data in that update cycle. Due to the processing time of the model data, the GTG-2.5 will normally be released about 50 minutes after the valid time of the RAP data set. The longest update cycles occur at 00Z and 12Z due to additional data ingest and processing. GTG-2.5 will normally be released 100 minutes after the valid time of the RAP data set at 00Z and 12Z. ADDS is configured so the composite graphic with the most relevant valid time should display when you initially select the GTG-2.5 product, but all graphics in the GTG-2.5 suite are available for viewing via the valid time and altitude selection interface.
NOTE: Users should pay close attention to the valid time of individual graphics due to the delay caused by RAP update cycles required for the GTG-2.5.