Southwest Monsoon since the start of July has been vigorous over many parts of West and Central India while Coastal Karnataka & Kerala that were also seeing subdued monsoon performance during the month of July saw much needed revival increasing the storage in reservoirs across various river basins. The most notable among those has to be the Godavari basin which saw flows around 30 lakh cusecs as the Meddigadda barrage, one of the largest irrigation projects of Telangana, part of the Kaleswaram project saw a peak outflow of nearly 29 lakh cusecs
While the catchment areas of Cauvery Basin did not see the extreme rains seen over the Godavari catchment areas the good pre-monsoon storage had meant the few days of heavy to very heavy rains were enough to trigger more than 1 lakh cusecs inflow into Mettur. With the dam expected to reach FRL today we could see a case of outflow from Mettur also increase in the coming days. Across both Karnataka & Tamil Nadu most Cauvery basin dams are near FRL levels.
The extreme rains since the start of July has brought the overall seasonal performance of Southwest Monsoon to 14% excess over normal for the first 45 days. As they say excess of anything is not a good thing so nature possibly has decided to give some rest to the Monsoon dynamics and bring thunderstorms to the leeward areas of Peninsular India like South AP and Tamil Nadu. The well marked low over Northeast Arabian sea and adjoining areas is expected to become a monsoon depression and move further west dragging monsoon moisture along with it thereby reducing the rains over Peninsular West Coast.
Monsoon trough which has been held to the south of its normal position by the two circulations on either side will gradually move up North as the influence of the dual circulations start to fade away, one by weakening and the other by moving west, the monsoon trough is expected to push up North which in effect will also weaken Westerlies over Peninsular India and bring the possibility of thunderstorms over the leeward areas like Tamil Nadu. Additionally the moving up of the monsoon trough will also mean some of the areas like Gangetic Plains in Uttar Pradesh which has been seeing poor monsoon so far could see much needed rains.
While today may seem a tad early for thunderstorms to increase over Tamil Nadu, one or two places over North Coastal TN may see moderate to heavy thunderstorms due to instabilities brought by the weakening westerlies and remnant moisture migration from the West. From tomorrow for the next few days until possibly middle of next week we may see increased thunderstorm activity which could bring one or two good spell of rains for Chennai and suburbs. It may not be a surprise if the IMD observatories at Chennai get closer to the seasonal SWM tally by the end of next week.