During the Pre-Monsoon season, the weather activity over Chennai occurs due to three factors. One of them being the land and sea breeze which spark some light showers over the area. Secondly, the wind discontinuity over Peninsular India which is a semi-permanent feature during the Pre-Monsoon season keeps oscillating and is confined to the interiors. It also travels to the coast rarely resulting in some rainfall activity.
Thirdly, the Inter Tropical Convergence Zone (ITCZ) which is a huge band of convective clouds oriented east-west, extending up to 1000 km with a width of about a couple of 100 km. This system in the equatorial region of the Indian Ocean runs close to the southern parts of India and Sri Lanka.
Also, the ITCZ has many disturbances and is also the generator for Pre-Monsoon systems in the area. The storms which form during this season get triggered due to the feature and later strengthen due to favourable sea conditions.
Areas close to the equatorial region witness massive thunderstorms but due to the minimal presence of Coriolis force, the region is not conducive for generation of tropical storms. As and when the disturbances do originate in the Bay of Bengal which is a cyclogenic zone, favourable oceanic conditions sustain and enhance the growth of the weather systems.
Usually, when the weather systems form in Bay of Bengal, they usually track across the Bay and head towards Bangladesh. The rare ones that do target the Indian coastline head towards Andhra Pradesh and Odisha coast.
The current system which has now intensified into a cyclonic storm Roanu, kept on lingering for about two to three days and had a slow movement. When these storms come in contact with land, frictional forces reduce its strength.
The system kept on travelling from the Bay of Bengal and came in close proximity with Sri Lanka and Tamil Nadu and kept on moving east-west, which doesn’t normally happen. The system was also a slow moving one that lingered along the Sri Lanka and Tamil Nadu coast as a well-marked low-pressure area.
When a well-marked low pressure area forms over the region, the rain spread is more and the system is capable of giving heavy rain over nearby areas. Thus, heavy rains were witnessed in Chennai and its adjoining areas during the month of May when the city remains hot and mostly dry.
The storm resulted in heavy rain over Chennai which remained parched for four long months. The city recorded 117 mm of rain in 24 hours, which is the highest in a decade. A similar situation had occurred in the year 2010 when Cyclone Laila had formed in the Bay of Bengal.
Image Credit: dnaindia.com
Please Note: the information picked from here should be attributed to skymetweather.com