The sudden intense spurt in rainfall across Kerala is widely seen by climatologists as an uncharacteristic late surge of a deficient southwest monsoon. After a day of fierce activity, the rainfall intensity has dropped, and Thiruvananthapuram, the worst-affected district, has been taken off the high alert warning. 

This 'late surge', however, has not helped Kerala to make up for the monsoon deficit; Kerala still has a southwest monsoon deficit of 14 per cent.  

The sudden incessant rainfall has been caused, like any other unanticipated surge in southwest monsoon rainfall, by an intense cyclonic depression over the Bay of Bengal, this time over the central parts of north and adjoining central Bay of Bengal (Maharashtra, Madhya Pradesh, Odisha and Andhra Pradesh). This low-pressure system has pulled vapour-heavy air from the Arabian Sea in the north-westerly direction, causing 'light' (0.1 to 64.4 mm) to 'very heavy rainfall' (115.6 to 204.4 mm) in most parts of Kerala.

The low-pressure formation, according to the bulletin of the India Meteorological Department, persisted over the same coastal areas on Friday, too. Nonetheless, the IMD suggests that the system could weaken in two days. 

The formation is "very likely" to move westwards and concentrate into a "depression" over the northwest and adjoining west-central Bay of Bengal off the South Odisha-North Andhra Pradesh coasts during the next 24 hours. It is very likely to cross the South Odisha-North Andhra Pradesh coasts around the morning of September 27. 

The bulletin said a trough runs from the well-marked low-pressure area over the northwest and adjoining the central Bay of Bengal to the south Maharashtra Coast across Telangana, and North Interior Karnataka.

The IMD bulletin said that widespread rainfall activity with Heavy rainfall (64.5 to 115.5 mm) to very heavy rainfall (115.6 to 204.4 mm) is very likely to occur at one or two places in Kerala on September 26 and 27, and Heavy rainfall (115.6 to 204.4 mm) is very likely to occur at one or two places in Kerala on September 28. 

It was southern Kerala that was battered more, particularly the Thiruvananthapuram district. The area around Thiruvananthapuram airport received 151 mm (considered 'very heavy') of rain on September 25. The capital city received 129.6 mm, again 'very heavy' in meteorological terms. Neyyatinkara, in the southern part of the capital district, received the highest rainfall: 168 mm.  

Idukki, which till now had a deficit of nearly 40 per cent of monsoon rains, saw intense rainfall that seemed to offset the poor seasonal performance. Peerumedu received 138 mm of rainfall. Kottayam's Kanjirappilly received 100 mm, 'heavy rainfall' in climate jargon. There is a 'flash flood' warning only for Idukki, a possibility triggered by the fragile hill slopes. 

However, Thiruvananthapuram's 'red alert' warning has been downgraded to 'orange alert', which is the warning just below the 'red alert' that asks authorities to take immediate action. Northern districts from Malappuram upwards, which have not been given any warning on Friday, will be put on 'orange alert' on September 27. 

At the fag end of the season, it is usual for the strength of the monsoon to taper off. Traditionally, the southwest loses its vigour from the second half of August. By September, the daily rainfall average drops to less than even 5 mm. However, on September 25, Kerala received more than 30 mm. 

The northeast monsoon season officially begins from October 1.