Overview

Over the last several years the FAA has funded NCAR and others to develop a turbulence nowcast and forecast system for mid–and upper–level turbulence over the continental U.S. The forecast system, named GTG for "Graphical Turbulence Guidance," provides contours of turbulence potential based on RUC (Rapid Update Cycle) model forecasts out to 12 hours lead time. The system was recently updated in Feb. 2010 and is part of the NCEP operational suite (available through the ADDS web site http://adds.aviationweather.gov). Examples as they currently appear on ADDS are shown below.

Example of GTG2 6–hour turbulence forecast as it appears on the Operational ADDS web site

The GTG procedure uses numerical weather prediction model forecasts to compute a number of turbulence diagnostics which are then weighted and combined. The relative weights for the combination are dynamically optimized for best agreement with the most recent available turbulence observations (in situ EDR data and pilot reports). This procedure allows the algorithm to minimize forecast errors due to uncertainties in individual diagnostic performance and thresholds. Intense statistical verification exercises have been performed in which probabilities of yes and no detections were determined by comparing turbulence forecasts to PIREPs. These statistics have made it possible to compare performance of the individual diagnostics, as well as test various diagnostic thresholding and weighting strategies. The overall forecast performance using the weighted diagnostics provides superior skill to the use of individual diagnostics.

The GTG is a constantly evolving product, with specific mountain wave turbulence and convectively–induced turbulence diagnostics (DCIT) and probabilistic forecasts as well as a global forecast product currently under development (see the Oceanic Weather section). GTG2 is RUC–based, but will transition to use the WRF Rapid Refresh (WRFRR) model sometime in the last quarter of CY10.