A method for forecasting the onset of cloud-to-ground (CG) lightning flashes is examined. Reflectivity data and upper-air measurements were analysed for 49 convective storms observed in 36 different days over southern Romania during the convective season (May–September) from 2003 to 2005. The radar reflectivity data were associated with the CG lightning flash locations obtained from the Romanian National Lightning Detection Network. The CG lightning initiation signature is based on reflectivity thresholds (35 and 40 dBZ) at a given environmental temperature aloft (−10 and − 15 °C). We have found that the best predictor is the 35 dBZ radar reflectivity at the height of the − 10 °C isotherm, with a probability of detection of 95%, and mean lead time of 17 min before the first CG lightning. In previous studies, it was found that the onset of CG lightning flashes had greater threshold value for reflectivity (40 dBZ) and shorter lead time (13 min). The results presented in this study are currently used in the nowcasting operational activities at the Romanian National Meteorological Administration. Copyright © 2012 Royal Meteorological Society
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