Graphical Turbulence Guidance (GTG) is a computer-generated forecast product that predicts the probability and intensity of clear-air turbulence (CAT) at specific altitudes. It does this by analyzing jet streams, wind shear, and other atmospheric patterns known to produce turbulence.
How It Works#
CAT is turbulence that occurs in clear sky, away from visible clouds or storms. Pilots cannot see it coming. GTG addresses this by running numerical weather model data through a set of diagnostic algorithms that flag atmospheric conditions favorable to turbulence.
The algorithms look for known turbulence triggers. These include strong jet streams, rapid changes in wind speed or direction over a short distance (wind shear), and areas where stable and unstable air masses meet. Each trigger produces a local turbulence index value.
GTG combines multiple diagnostic indices into a single, weighted probability output. The result is a color-coded map showing turbulence likelihood at a given flight level, updated every hour. Higher values indicate greater probability and potentially greater intensity.
The FAA's Aviation Weather Center (AWC) produces GTG forecasts for the contiguous United States. The current operational version, GTG3, covers altitudes from the surface up to FL450. A global version also exists for oceanic and international routes.
Example in Aviation#
A flight dispatcher is planning a transatlantic route at FL380. She pulls up the GTG product and sees a band of moderate-to-severe turbulence probability stretching across the North Atlantic at that altitude. The diagnostic output shows a strong jet stream core with significant wind shear above and below it.
She adjusts the cruise altitude to FL360, where GTG shows a lower probability and cleaner conditions. The crew notes the forecast in their preflight briefing and receives a SIGMET (Significant Meteorological Information) that confirms CAT potential along the original path.
Why It Matters#
GTG gives pilots and dispatchers a quantitative, spatially precise tool for turbulence avoidance. Pilot reports (PIREPs) remain essential, but they are point observations. GTG fills the gaps between those reports with continuous, gridded probability data.
Understanding GTG helps pilots evaluate preflight weather packages more critically. A high GTG value at your planned altitude should prompt a closer look at the wind data and potentially a route or altitude adjustment before departure.
Key Takeaways#
- GTG forecasts CAT probability by analyzing jet streams and wind shear at specific altitudes.
- It combines multiple atmospheric diagnostic indices into one color-coded output.
- GTG3 is the current FAA operational version, covering up to FL450.
- Updates run hourly, providing near-real-time turbulence probability maps.
- GTG complements PIREPs but does not replace pilot judgment or real-time reports.