"In late October - early November, Kerala experiences a very unique weather regime called 'Tulavarsham' - the mornings are sunny and clear; by midday it gets very sultry and the afternoon sees a rapid and massive buildup of clouds followed by a thunderstorm that clears up in a couple of hours. The evenings are usually clear and humid."
And there are any number of other online pages which repeat: "During Tulavarsham, it commonly rains in afternoons with thunder and lightning".
That the Advancing (South West) Monsoon blows in from the Arabian sea due to the subcontinent getting hyper-heated in summer is no mystery. That the monsoon has to retreat once the temperature gradient levels off and then inverts in autumn is not hard to understand either. The Coriolis force explains why the monsoon advances and withdraws along a South-West to North-East axis rather than a North-South one - undergrad, if not high-school, stuff.
The Thulavarsham phenomenon is Kerala's manifestation of the Retreating (North-East) Monsoon; and there are many of its characteristics that I find very puzzling.
First up, why does it rain almost exclusively in the afternoons/evenings? Afternoon thunderstorms are quite the norm in equitorial rain forests (this again is school gyan). But there, what happens is local convection with no large-scale *horizontal* currents of air; Thulavarsham results from a large-scale drift of air masses over the entire Indian peninsula.
One could guess: the moisture laden monsoon (the moisture having been collected from the Bay of Bengal) blowing in from the north-east collides above Kerala with the afternoon sea-breeze from the Arabian Sea and this could cause the moist air to abruptly rise and condense into sharp thundershowers.
But there are other issues: Why does it not rain anywhere else on the western coast during Thulavarsham (rainfall is utterly scanty on the coast beyond north Kerala; Bombay, which gets much more rainfall from the main south-west monsoon than most places in Kerala, actually suffers a brief second summer called 'October heat'!)? Why don't the Western Ghats cause a rain-shadow effect over Kerala for Thulavarsham (just like they cast a severe rainshadow over western Tamil Nadu during the South West monsoon)?
The answer to the first question appears to be in the shape of the Indian peninsula - a triangle with apex at Kanyakumari. This shape, coupled to the North-Easterly direction of the withdrawal of the monsoon suggests that over say, Bombay or Goa, the withdrawing monsoon would have hardly any moisture since it would have passed almost entirely over land. To the south, the Bay of Bengal provides enough water to the withdrawing monsoon to drench the southern half of India's eastern coastline in pretty steady rain (the general behavior of NE monsoon over Chennai is rather similar to that of the SW monsoon over Cochin) - and even after traversing Tamil-Nadu, the NE monsoon winds could have enough moisture stocked up to give Kerala a spell of rains.
But one has to understand the role of the Western Ghats better. Why do the Western Ghats have such an asymmetric rain-shadow effect between Kerala and Tamil Nadu( in Maharashtra/Karnataka, the mountains drop sharply to the west coast and slope much more gently eastwards, but on both sides of the Kerala-TamilNadu border they are equally steep)? A possible explanation could lie in the coastal plain of Kerala being a very narrow strip of land; this could allow the sea-breeze to play a major role in causing the residual moisture in NE monsoon to get precipitated - on the other hand, the Bay of Bengal is hundreds of kilometers from the Western Ghats and a similar mechanism might not operate on that side during SW monsoon.
A legend from medieval Kerala relates: When he heard that the Portuguese had stealthily made off with some pepper wines, the Zamorin remarked: "Nothing will happen! They can't take away our 'Edavappathi' (the South-West Monsoon)"
I don't quite agree with Zam. the Edavappathi gives no *exclusive* benefits to Kerala - it gives good rains to Kerala all right but it gives much more copious precipitation to Coastal Karnataka, Konkan (including Goa where the Portuguese were well-entrenched) and so forth. But, the Thulavarsham is a different matter altogether; virtually ignoring the Indian west coast above Kasaragod, it extends the Keralan rainy season almost to December. And this is precisely why Kerala has, by far, the most persistently lush landscapes in the country. One sees drought-resistant scrub and cactus hedges in the countryside near Bombay but they are unheard of anywhere in Kerala except perhaps the farthest fringes. Even Cherrapunji suffers from a continuous rainless spell of 6 months, leading to near drought conditions every year; in Kerala the 'dry window' lasts just about a third of that!
Watching the rapid buildup and awesome approach of a Tulavarsham thunderstorm from the Ishanya (North East) corner of the October sky, one can't help recollect the inspired imagery of medieval Keralan poet Poonthanam's hymn addressed to Kali (Thirumandhankunnil Amma):
"Ghanasangham idayunna thanukanthi thozhunnen!"
("I behold and salute thy fearful form, a vision of dark, swirling rainclouds in thunderous collision!")