The 'soaker trough' spraying rain into Tamil Nadu, Puducherry and coastal Andhra Pradesh will not get a move off its current perch over South-West Bay of Bengal until Saturday, global weather maps show.
India Met Department has maintained heavy to very heavy to extreme heavy rain over North Tamil Nadu coast for the rest of today.
The extreme heavy rain alert is valid for Chennai, Kanchipuram and Tiruvallur districts in North Tamil Nadu. Heavy rain has been forecast for the rest of the State, especially in the delta regions in the South.
Meanwhile, the trough is a only a primitive form of a low-pressure and represents an elongated (unlike circular in a 'low') area of lower pressure.
But it can do much more harm given the right ambient conditions, as is now being revealed.
The trough features a cyclonic circulation moving around Sri Lanka and which is responsible for anchoring the strong easterly North-East monsoon flows around, and by extension, across entire Tamil Nadu.
The flows are strongest over the North Tamil Nadu coast, which explains the targeted, unrelenting heavy to very rain over an area from south Coastal Andhra Pradesh to Chennai to Puducherry.
Cloud system
The flows have ensured that a semi-permanent and hyperactive cloud system hang over Nellore-Chennai-Puducherry half over land and half over sea to pour down its contents over the area.
This (Wednesday) morning, satellite pictures showed the heavy bank of cloud rather unmoved from the previous day's alignment covering south coastal Andhra Pradesh, and North and Central Tamil Nadu coast, and gradually probing areas to the South as well.
Compared to this, rain activity over Sri Lanka, around which the embedded cyclonic circulation is moving, is muted with only a couple of parcels of cloud over the North of the Island in Batticaloa and Jaffna.
The 'soaker trough' in the South-West Bay is forecast to push itself to the West of Sri Lanka and move into the South-East Arabian Sea, setting off rain and thunderstorms on the West Coast from next week.