New Delhi, April 2 -- Thunderstorms and hailstorms battering northwest India this past week - unseasonal even by pre-summer standards - are the downstream effect of a disrupted polar jet stream and a shifting polar vortex that have driven an unusual surge in western disturbances, meteorologists said. March recorded eight western disturbances against a normal of five to six, and at least three more are expected through mid-April. The spike is made more striking by what preceded it: January and February saw far fewer WDs than usual, resulting in low snowfall across the Himalayas. The systems have intensified sharply since mid-March. Western disturbances are moisture-laden cyclonic systems that originate over the Mediterranean and travel eastw...