123 Kolkata: A thunderstorm and a spell of rain on Saturday night and scattered showers in the morning pulled the mercury down by over four degrees on Sunday. The maximum temperature plunged to 31.9°C on Sunday, almost four degrees below normal, from 36.2°C on Saturday.With moisture flowing in from the sea with southerly winds, more thunderstorms and rain are likely over the next two days, said the Met office, which has also predicted a gradual rise in mercury over the course of the week."The mercury will start spiralling up from Monday and may touch 36°C again by the end of the week. It could touch 32°C-33°C over the next two days and climb up to the normal mark. But it is unlikely to cross 36°C during this spell. Meanwhile, spells of rain may pull the mercury down marginally yet again," said Regional Meteorological Centre weather scientist H R Biswas.Thunderstorm and rain have been predicted across the districts of Purulia, Bankura, West Midnapore, East and West Burdwan, Birbhum, Murshidabad, Nadia and most districts of south Bengal over the next three days.Kolkata's temperature has been on a roller-coaster ride since the beginning of May. While the month started on a relatively pleasant note, it turned dry and hot with northwesterly winds blowing into south Bengal. The hot winds flowed in at lower levels, pushing the mercury up. It touched 39.4°C last Sunday, making it the hottest day of this summer so far. A day earlier, the maximum temperature had been 39.1°C. On both days, real-feel temperature had climbed up to 46°C, according to a private weather forecasting service.The Met office, however, had predicted a few thunderstorms in south Bengal due to local heating. It has proved to be right with one hitting the city late on Saturday evening. On Sunday morning, the sky remained cloudy and it rained sporadically in pockets of the city. "The cloud cover helped pull the mercury down," added Biswas.