Chennai: City residents woke up to sudden spells of rain on Wednesday, interspersed with sunshine and cloudy skies. Rainfall intensity is likely to increase over the next two days, with IMD issuing a yellow alert for heavy rain (6-12cm in 24 hours) in 15 districts — including Kancheepuram and Chengalpet — on Thursday and 29 districts — including Chennai, Tiruvallur, Kancheepuram and Chengalpet — on Friday. An orange alert has been issued for heavy to very heavy rain (12-20cm) in districts along the Western Ghats on Thursday.Meteorologists expect intermittent rain and cloudy skies to persist over the next two weeks as conditions become favourable for the withdrawal of the southwest monsoon and the simultaneous onset of the northeast monsoon on Thursday.The rain is being triggered by an upper-air cyclonic circulation over the Comorin area and a trough extending from it to the northwest Bay of Bengal off the Odisha coast. The circulation draws moist air upwards, while the trough brings winds together, forcing air to rise, cool and form rain-bearing clouds.V R Durai, director, Area Cyclone Warning Centre, Regional Meteorological Centre, Chennai, said steady northeasterly winds up to 1.5km above the surface and rainfall along coastal areas indicated favourable conditions for the monsoon onset.“The rainfall won’t be continuous, and depends on the position of the trough. On Wednesday morning, it was off Tamil Nadu coast, bringing heavy rain over south coastal areas. By evening, it moved inland. Rain intensity over north Tamil Nadu may increase on Oct 16 and after that heavy rain activity may move to interior districts and neighbouring Kerala and south interior Karnataka,” he said.On Thursday, Chennai and its suburbs may receive moderate rain with thunderstorms and lightning in some areas. Maximum temperatures are likely to be 34°C and minimum temperatures 25°C. Daytime temperatures may be 2-3°C above normal at isolated pockets over interior Tamil Nadu and near normal elsewhere.Tamil Nadu received 261.3mm of rain against a normal 328.4mm during the June-Sept southwest monsoon, a 20% deficit. Chennai’s five key drinking-water reservoirs held about 33.7% of their combined capacity on Wednesday, making the northeast monsoon crucial for replenishing the city’s water supply.Tamil Nadu receives around 45cm of rain during Oct-Dec, nearly half its annual rainfall. Chennai gets about 87cm, around 60% of its annual total.On Wednesday, Nagapattinam got 91mm, Mayiladuthurai 26.5mm and Mailam 24.5mm. Nungambakkam and Meenambakkam recorded 1.5mm and 5.2mm, respectively, while West Tambaram received 30.5mm and Puzhal 26.5mm.“Sudden spells of rain, bright sunshine and sudden spells of rain again. The rainfall intensity may be slightly higher at nights and early morning. It is a typical northeast monsoon pattern, and it may continue for the next two weeks,” said weather blogger Pradeep John.
