With April almost coming to an end it is time to look for signs of seasonal shift in winds. “யானை வரும் பின்னே, மணியோசை வரும் முன்னே”. Southwest Monsoon onset over North Indian ocean normally happens around middle of May. But indications towards it start much earlier. The first set of indications are seasonal shift in winds. From middle of October just before the onset of NEM till middle of April the overall wind regime is from the East. For SWM process to kickstart the wind regime has to shift to Westerlies. This shift to Westerly regime brings about a direct effect on Chennai Summer.
When the winds are from East Chennai does not get very hot but remains humid. When the winds shift to West the character of Chennai Summer changes its character. Afternoons gradually move from Hot to Hotter. It eventually becomes Hottest when westerly winds are strong and monsoon is almost there. As westerly winds strengthen the effect of sea breeze starts to slowly fade.
Of course it goes without saying the peak of Chennai Summer is when there is a summer cyclone in North Bay. During this period sea breeze becomes practically nil. Winds form WNW / NW bring the interior heat to dump along the coastline of Chennai. Additionally the cyclone absorbs all moisture making it extremely dry over Peninsular India. This aggravates the heat quotient as well.
Both short term and sub seasonal weather forecasts indicate we are at the cusp of seasonal shift in winds. This means gradually over the next few days the following things are expected to happen.
- Summer will come to an end over most of Kerala, Coastal Karnataka, Kanyakumari and parts of Coimbatore district in Tamil Nadu
- In addition to interior areas the Summer heat will now shift to Peninsular east coast from West Coast.
- Wind discontinuity, the first step to seasonal shift in winds, will bring back summer thunderstorms over interior areas of Peninsular India. The transit of MJO over the Indian Ocean in the next week to 10 days may enhance thunderstorm activity. We need to see if it triggers a pre monsoon cyclone over Norht Indian Ocean.
- The hottest period of the year for places like Chennai in coastal TN is starting from now.
Weather models indicate suburbs of Chennai like Tiruvallur and slightly further west like Arakkonam is likely to hit 39 / 40°C regularly from now on. As off shore winds strengthen gradually afternoon temperatures will climb up in city areas too.