Monsoon has been delayed by a couple of days and is likely to arrive in Kerala by June 3 this year. According to the latest forecast, the southwesterly winds would gradually strengthen further from June 1 which could result in rainfall activity over Kerala. On the other hand, the Regional Centre of India Meteorological Department (IMD) in Bhubaneswar on Sunday, issued thunderstorm warning for as many as 15 districts of Odisha.