It was possibly Dr. Ramanan the retired Director Area Cyclone Warning Centre, Chennai who popularised the word Veppasalanam. Many may not know the RMC Chennai had published a comprehensive glossary of Tamil Weather Terms a few years back. While Veppasalanam in basic sense indicates convective development often the role of wind is downplayed.
As mentioned often best period for Veppasalanam thunderstorms over the leeward areas of TN is Break in Monsoon period. A few years back this post explained how the Monsoon thunderstorms are fascinating to watch for the complex nature. A milieu of factors that need to strike the right balance for the thunderstorms to develop and sustain. But the biggest facto in all this is the Break in Monsoon period when Westerlies slow down. When the monsoon is strong the high wind shear pretty much shaves off the convection preventing thunderstorms to form.
Over the past 48 hours we have gradually seen the westerly winds slow down over the southern Peninsular India. Though a classic break in monsoon period is not yet there due to the presence fo two circulations in the monsoon trough. The first land depression of the season has moved along the trough to reach UP/MP/Rajasthan border. It is expected to further weaken and drift westward gradually slowing down the monsoon dynamics. Once these two circulations move further west Monsoon may weaken further bringing a dull phase. This dull phase until middle of August is likely to be the active phase of thunderstorms for Tamil Nadu. Yesterday was the first day of widespread rains from Veppasalanam thunderstorms with many places recording moderate to heavy rains. Few places in Chennai recorded rainfall in excess of 9 cms as two separate set of thunderstorms lashed the city.
It is always dicey to put an update on a day when early morning thunderstorms happen due to clouding. But there is a good chance for widespread thunderstorms to happen today also over most parts of Tamil Nadu. In particular North TN is likely to benefit from the wind instability at Mid tropospheric levels. The mid tropospheric wind instability (5.5 kms altitude) was responsible for the early morning storms to come in from East over the Chennai coast today. This mid level wind instability may provide the right platform for the storms to sustain. Once the remnant early morning storms fade and the skies clear off day time heat will set the conditions for convective development.
All in all for the next couple of weeks promises pretty much daily thunderstorms for the plains of Tamil Nadu. Depending on wind pattern some places may end up getting intense rains as the storm steering will remain week during this period. Chennai as usual will depend on interior thunderstorms to move in. But the presence of sea breeze front is the Ace in the Pack for Chennai and suburbs enhancing any remnant storm that moves in.