Study of thermodynamic indices in association with pre-monsoon thunderstorms/Nor’westers over Northeast India during 2014-2018

: The weather systems that predominantly affect the north-eastern parts of India during the pre-monsoon summer months (March-May) are severe thunderstorms, locally named as ‘Kalbaishakhi’ or ‘Nor’westers’. The storms are devastating in nature particularly due to strong (gusty) winds, heavy rains and hails associated with it. Although these storms are well known for its power of causing damages, studies on them are relatively few due to their small size and sparse network of observations over the region. In this paper an attempt has been made to analyse different stability indices like Showalter index, Lifted index , K index, Severe Weather Threat index, Total Totals index, Equivalent potential temperature at 850hPa level, Dew point temperature at 850hPa level, Relative humidity at 700hPa level, Humidity index and Deep Convective index in connection with occurrence of thunderstorm over three stations over North East India viz.,Agartala (23.88 0 N, 91.25 0 E), Dibrugarh(27.47° N, 94.91°E) and Guwahati (26.10 0 N, 91.58 0 E) for the year 2014, 2015, 2016 and 2017 during pre-monsoon summer months (March-May) using 0000 UTC radiosonde data to review and evaluate the atmospheric instabilities that may lead to thunderstorm development. Favourable ranges of these indices for occurrence of thunderstorms over the three selected stations of the region have been determined. Validation of the suggested favourable ranges of indices was also carried out on the days of thunderstorm activity for the year 2018.