As we get to the last couple of days of the official summer season most eyes are on Monsoon onset over Kerala. IMD’s statistical models estimated SWM may make its entry over Indian mainland around 26th May. But as things stand there is still sometime before monsoon winds hit Kerala. After hitting parts of A&N Islands nearly a week before onset the progress since has been very slow. Yesterday Southwest Monsoon covered the remaining parts of Andaman Sea nearly 10 days after initially touching down over the region.
IMD has a fairly robust criteria for Monsoon onset over Kerala which covers broadly rainfall, OLR, wind speeds and westerly wind depth etc. Onset over rest of the regions are mostly subjective in nature broadly looking at winds and rainfall. Depth of Westerlies is a key component in declaring onset over Kerala. Up to 600 hPa (4.2 km ASL) winds should be from the west. Looking at the wind charts currently even 700 hPa (3 kms ASL) has not yet completely changed to Westerlies over Peninsular India.
Sub seasonal models indicate first week of June the winds may get more streamlined. Westerlies are also likely to establish well around 2nd or 3rd of June. This possibly opens the window for Monsoon onset over Kerala around 4th of June. Even precipitation charts indicate rains that Kerala receive until then may be driven by thunderstorms. There is a chance dual vortex is likely to develop during this period. As we have often seen dual circulations strengthen monsoon dynamics making it favourable for onset. The vortex off Peninsular West Coast may also move up along the coast triggering a classic onset behaviour.
Even if Monsoon makes onset around 4th June it will still be within the normal onset window. Additionally it is likely we may see a normal and fairly robust start to monsoon. Weather models indicate first half of June may be good. There could be more stutters ahead as monsoon tries to cover rest of the country. But Peninsular India may make a good start to bring some much needed rains. Also it may bring down the heat over the region as well.
Until then thunderstorms may continue over interior areas of Peninsular India. Until then heat will continue over North Coastal TN including Chennai. When monsoon makes onset next week and move up we can look forward to some much needed thunderstorm activity.