The North Indian Ocean basin (Arabian Sea and Bay of Bengal) typically see two cyclone seasons every year. The first season happens when transition happens between Easterly to Westerly winds. April and May see cyclones often playing a role in these seasonal reversal of winds. The second season coincides with the Northeast Monsoon season. Once again often cyclones during the transition from Westerly to Easterly play a role in seasonal reversal of winds.
One may wonder why the intervening period between June and September does not see cyclones. During Southwest Monsoon Tropical Easterly Jet, a jet stream from east at the height of 9 – 12 kms, is active. The presence of this TEJ prevents the development of cyclones through wind shear though we see fairly strong monsoon depressions. Monsoon depressions additionally have the benefit of Monsoon trough as a safety net allowing the convection to thrive.
Over the past many days weather models showed a possibility of a tropical disturbance around the 3rd week of May. The location of the expected Low Pressure area ranged from Andaman Sea to Southeast Arabian Sea. As the locations changed with every run the rainfall impact also moved around along with the circulation locations. Models at times showed North Coastal TN to receive rains like Northeast Monsoon time. On many occasions models showed Kerala to receive rains like Southwest Monsoon period. This pretty much summed up the confusing state of atmospheric dynamics models were going through.
Now there is a little more clarity on this with models consistent about a LPA forming over Southwest Bay in the next 48 hours. The Low Pressure is expected to form out of the cyclonic circulation currently seen over parts of South TN. It is expected to drift across Sri Lanka under the influence of a strong westerly burst and reach Southwest Bay by tomorrow night. The warm ocean temperatures will ensure there is sufficient support for the circulation to strengthen into an LPA. With Bay abnormally warm there is a very high potential for this LPA to intensify into a cyclone. With MJO expected to increase in amplitude as well things could turn favourable for intensification.
It becomes extremely essential to take 48 hours at a time since the genesis of LPA is likely to play a very crucial role in how events pan out subsequently. With potential intensification we may see a more pronounced Northward track pushing the LPA further East than model estimates. For places like Chennai rains will depend a lot on how close the LPA forms to the South TN / Sri Lanka coast. A pronounced Eastward shift will reduce the rainfall chances for North Coastal TN.
What to expect for the next 48 hours. Kerala is expected to see widespread rains with westerlies strengthening. Along with Kerala parts of Kanyakumari, closer to the Ghats may also see heavy rains. Interior areas of Tamil Nadu will see thunderstorms continue due to wind discontinuities which will fade away gradually as the circulation strengthens. Places like Chennai in North Coastal TN may see light to moderate rains during the night / early morning hours.
Once there is clarity on the genesis and potential for intensification it may be prudent to look at post 48 hours window.