Between 6th and 17th August the Standardized Rainfall Anomaly Index from IMD, a tool to monitor active and break phases of monsoon, was below -1.0 coinciding with the break in monsoon period. Subsequently only 19th August saw the Rainfall index cross 1.0 indicating though the break in monsoon period came to an end the monsoon could not push onto become active once again with Peninsular Coast continuing to see poor rainfall. As a matter of fact Kerala cumulatively has recorded only 40.8 mm so far in August. To give a perspective 1911, the year with the lowest recorded rains during August, had 178.6 mm. There is a very high probability Kerala could possibly end up recording sub 100 mm cumulative rainfall for the first time during August since 1901.
The river basin rainfall anomaly charts for Peninsular rivers show the story of how poor monsoon has been this year over South India. Most of the upper catchment areas of Godavari, Krishna, Cauvery, Bhima, Thungabhadra are all cumulatively deficient while the lower catchment areas of rivers like Krishna and Godavari are above average or excess on the back of heavy rains around Telangana, AP & adjoining areas of Chhattisgarh and Maharashtra. With a 36% deficit in the Cauvery Basin resulting in a cumulative inflow of just 93 TMC no wonder the inter state relationship between Karnataka and Tamil Nadu is getting increasingly strained. The major catchment areas like Kodagu, Hassan, Wayanad all are showing large deficits for the seasonal rainfall tally.
On the other side of Peninsular Coast most of the stations in the plains, except Pamban which has recorded 0.2 mm and an anomaly of -28.6 mm, have got near normal or in cases like Chennai observatories recording excess rains. With nearly 6 weeks of Southwest Monsoon still left there is a very high possibility the airport IMD observatory could cross 650 mm cumulatively this year. While bulk of the 529.9 mm recorded at Meenambakkam came from the extreme spell in mid June, July was the only month that was poor overall during this year’s monsoon period with thunderstorms regularly marking their attendance over Chennai and surrounding suburbs.
Southwest Monsoon was able to put an end to the break in monsoon period with help from the Monsoon low that formed over North Bay and subsequently moved along Central India. Currently it has all but weakened over North MP and surrounding region and is expected to fade away while continuing to move in a NW direction. As it fades away the Monsoon trough which was pulled south by the Monsoon low to bring an end to the break is expected to once again push back towards the foothills of Himalayas leading to subdued rainfall over Peninsular West coast and core monsoon areas.
This movement back towards the foothills by the monsoon trough will bring a window of active thunderstorms for the leeward areas of Tamil Nadu over the next few days. Long term ensemble weather models indicate positive rainfall anomaly closer to the foothills this week and a corresponding near normal rains for most parts of Tamil Nadu indicating weak monsoon conditions over the West Coast. Wind charts also indicate mid level winds could carry a bit of instability as the Westerlies weaken leading to slow moving thunderstorms bringing rains over the plains of Tamil Nadu. While coastal areas of Tamil Nadu could benefit from thunderstorms triggered by sea breeze front today, fairly widespread thunderstorms is likely tomorrow and possibly day after as a more pronounced wind instability is expected to prevail over Tamil Nadu and adjoining areas of South AP.
While it is futile to expect daily rains for Chennai and suburbs, it is fair to say there is a very high possibility of daily rains over the KTCC region this week with at least one day of widespread thunderstorm activity over the next 2 / 3 days.