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An objective technique for forecasting clear-air turbulence (CAT) is described. An index is calculated based on the product of horizontal deformation and vertical wind shear derived from numerical model forecast winds aloft. The forecast technique has been evaluated and is now in operational use at two forecast centers with international aviation responsibilities: the National Meteorological Center (NMC) in Washington, D.C., and the Air Force Global Weather Central (AFGWC) in Omaha, Nebraska. The index is also an operational forecast tool at the Canadian Atmospheric Environment Service (AES), and the National Aviation Weather Advisory Unit (NAWAU) in Kansas City, Missouri, both responsible for domestic aviation forecasts. The AFGWC index also includes horizontal convergence in its calculation. Thresholds were selected empirically by comparing index values with the location and intensity of observed CAT. Verification indicates that the index is quite reliable. The probability of detection (POD) varied from 70%–84%. False-alarm ratios (FAR) ranged from a low of 22% for the NMC aviation model to more than 40% for the AFGWC global model. An average threat score of 0.17 was calculated for the aviation model 24-h forecast. The operational capabilities of the NMC and AFGWC indices are compared in two CAT episodes that differ in synoptic-scale conditions and times of the year.
Ellrod et al. (Sun,) studied this question.