No. IV-18.4 - (IMD), Pune

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iv)South Interior Mysore:— For South Interior Mysore, the southwest monsoon
season is the principal rainy season, the rainfall during October—December
being just 19% of the annual total; of this, a major portion falls in the
month of October. The rainfall in October is practically the same as in
September. By November, rainfall decreases considerably and in December
the sub-division is practically dry.
v) Kerala:— Similar to South Interior Mysore, in Kerala also the principal
rainy season is the southwest monsoon season, the northeast monsoon season
rainfall being only 18% of the annual total. October is the rainiest month
of the northeast monsoon season, the rainfall in October being more than ir.
September. Rainfall decreases in November though it is still significant
(35% of the season's total). In December, rainfall becomes much less,
being confined mainly to the southern districts.
3.3 There is some suggestion that in the northern districts of coastal
Tamil Nadu, the main period of rainfall begins a little earlier (by a week or
ten days or so) and also ends a little earlier, compared to the southern districts.
3.4 Thus, the northeast monsoon rain is confined to October and November
over the south Peninsula. It lingers on in Coastal Tamil Nadu and south Kerala
in December. Comparing the rainfall distribution with the mean monthly pressure
and lower tropospheric wind patterns, we notice that the rainfall in the
south Peninsula is mainly in the months when the trough overlies the south
Peninsula. Towards December, as the northeasterlies establish over the south
Bay and south Arabian Sea and the trough shifts close to the equator (in other
words, when the northeast monsoon is fully established) rainfall decreases considerably
in the south Peninsula. As the trough shifts south, a ridge extends
southwards over the Peninsula, leading to a decrease of weather over the interior
of the Peninsula, with rainfall confined to the coastal districts of
Tamil Nadu and south Kerala.