Withdrawal of the south-west monsoon stalled on Monday over Central and East India. Winds over the South Peninsula are now turning from south-westerly to easterly, signaling the north-east monsoon transition. While the region faced a rain deficit, heavy rain recently lashed parts of Tamil Nadu, Kerala, and Coastal Karnataka.
Withdrawal of south-west monsoon briefly stalled on Monday over Central and East India, with the retreating edge stretching in an arc through Supaul; Ranchi; Raipur; Brahmapuri; Parbhani; Baramati and Harnai, and the western end nestling in Maharashtra.
Farther south, the atmosphere is preparing for the next seasonal act. Winds over the South Peninsula are showing a growing inclination to turn from south-westerly to easterly, the defining signal of the transition to the north-east monsoon. Easterlies have established themselves over the Arabian Sea, and the adjoining South Peninsula is expected to follow suit shortly. The missing piece is the Bay of Bengal, where the wind regime must turn decisively before the north-east monsoon can properly take hold.
Reversal of winds
The normal window for this transition opens around October 15. Forecast models suggest the seasonal reversal could unfold broadly on schedule this year, with an easterly wave moving in from the South China Sea providing the initial trigger. It could set up a cyclonic circulation over the Andaman Sea before moving into the central Bay of Bengal, past the Andaman and Nicobar Islands, around October 14, and draw north-easterlies towards the South Coastal Andhra Pradesh and Tamil Nadu coasts, strengthening the seasonally reversing flows.
Helpful features
The atmospheric stage was already being set on Monday evening. An upper-air cyclonic circulation lay over North Interior Karnataka, with a trough extending from the Comorin area towards it across Tamil Nadu and South Interior Karnataka. Another trough extended from West Bengal to Madhya Maharashtra, passing across Jharkhand; interior Odisha; Chhattisgarh; Vidarbha; and Marathwada.
Together, these features are helping reorganise the piloting wind and rainfall pattern over the South Peninsula. The preceding south-west monsoon has left behind a significant rain deficit after being suppressed by the dominant El Niño over the equatorial Pacific. In this context, the South Peninsula is eagerly looking to a replenishing spell from the north-east monsoon. Reassuringly, heavy to very heavy rain lashed isolated locations in Tamil Nadu, Puducherry and Karaikal during the 24 hours ending Monday morning.
Heavy overnight rain
India Meteorological Department (IMD) also reported heavy rain over Coastal Karnataka, South Interior Karnataka, and Kerala and Mahe. Among the significant recorded rainfall amounts were in Kothagiri and Alakarai Estate in Nilgiris district (12 cm each) and Coonoor PTO (8 cm). Shakti Nagar recorded 10 cm, while Mangaluru Airport and Belthangadi (Dakshina Kannada) received 7 cm each. Udupi and Mangaluru recorded 5 cm each.
The IMD expects isolated heavy rain to continue over Tamil Nadu, Puducherry and Karaikal and Kerala and Mahe on Tuesday. Kerala and Mahe could see another heavy spell on Friday and Saturday, while South Interior Karnataka is likely to receive isolated heavy rain on Tuesday. Rainfall is expected to remain fairly widespread to widespread over Kerala, Mahe and Lakshadweep during the next six days.
Thunderstorms, lightning
Isolated to scattered rain is likely over Coastal Andhra Pradesh and Yanam, Rayalaseema, and Tamil Nadu, Puducherry, and Karaikal during the next six days. Coastal Karnataka could receive such rain for five days from Wednesday, while South Interior Karnataka is likely to remain wet for six days from Tuesday. North Interior Karnataka could see rain on Tuesday, Friday and Saturday, and Telangana could see scattered rain on Wednesday.
Thunderstorms, lightning and gusty winds will remain another feature of the transition. Tamil Nadu, Puducherry and Karaikal could see such activity for the next four days. Kerala, Mahe and Lakshadweep are likely to experience thunderstorms and lightning on Tuesday, with gusty winds persisting for three days from Wednesday. Coastal Andhra Pradesh, Yanam and Rayalaseema could see similar activity for three days from Tuesday.