While North & West Tamil Nadu has been getting on and off thunderstorms since the start of Southwest Monsoon many areas of South Tamil Nadu has been seeing practically dry weather particularly around Madurai, Virudhunagar, Thootukudi etc. Though it is pretty much a fact major parts of South Tamil Nadu fall under the rain shadow region for majority of the Southwest Monsoon season as of 20th July even by the minimum standards with districts like Thootukudi seeing extremely dry weather since June. As indicated by the table out of 9 deficient districts in Tamil Nadu as of 20th July 5 of them are from South Tamil Nadu.
Things are expected to become better for South Tamil Nadu today with numerical models predicting fairly good rains at many places over South Tamil Nadu particularly around Sivaganga and adjoining places of Madurai district. There is a possibility of one or two places in Western Ghats around Theni / Dindigul district to get fairly heavy rains around the Bodinaickanur / Periakulam belt.
While North Tamil Nadu has a potentially a day of high instability particularly around Tiruvallur / Vellore & Kanchipuram districts which indicate a day of possible rains in the region. But the factor that could go against the rains are unfavorable wind pattern that may not bring enough moisture to trigger the rains. While we don’t rule out possible thunderstorms in one or two places the chances look lesser as things stand in the morning. A possible clearer day in the region could change things around during the course of the day and bring better possibility of thunderstorms to places in and around Chennai.