From low pressure area to very severe cyclonic storm: how IMD

A small dot marked on the screens of weathermen at the IMD signalled to them that a very severe cyclonic storm was building up in the South Andaman Sea, nine days ahead of the landfall of 'Vardah' that ravaged coastal Tamil Nadu and Andhra Pradesh.
The dot on the screen signified pressure movement over a particular area, which had the potential of snowballing into a cyclone.
Weathermen at the IMD's Regional Specialised Meteorological Centre (RSMC) here, who monitor any slight change in weather patterns around India especially in the cyclone-prone Bay of Bengal, realised on December 3 that a low pressure area was being formed near the South Andaman Sea.

The pressure belt had traversed from the South China Sea, flowing over Thailand and Vietnam which had reduced its speed considerably due to its contact with the land.
"So, as soon as it reached the South Andman Sea, it again got traction. The warm water in the Indian Ocean and the South Andaman Sea gave it momentum and then started the process of forming a low pressure area.
"We realised that there was an anti-clockwise pattern, the wind speed around it had increased more than other parts of the sea," said M Mohapatra, Additional Director General (Services) with the India Meteorological Department, who has been in the business of forecasting cyclones. He was also the head of Cyclone Warning Division during Phailin and Hudhud.

September to December is also the time when weathermen at the Cyclone Warning Division of the IMD avoid taking leaves. For them, this is cyclone season.

Realising that the low pressure area was developing into something stronger, the officials started gathering more data, pressing into service IMD's two doppler radars at Chennai and Machilipatnam, coastal automatic weather stations and manned observatories, besides its buoys in the sea.
Satellite images from INSAT-3D and Met services of Thailand and Malaysia were also used.
With constant hourly updates, the data gathered was then coalesced, a usual practise. After analysis and clearance from senior meteorologists, a small dot was marked to track its path.

With every passing day, the low pressure area became more intense. On December 6, it turned into a depression and graduated to deep depression the very next day.
Since then there was daily monitoring and alert reports being sent to the Andaman and Nicobar Administration, and state governments of Andhra Pradesh, Tamil Nadu, Odisha, Puducherry and Kerala.
On December 12, the day of the landfall, monitoring was
carried out on minute-to-minute basis.
With the PMO, the Cabinety Secretariat and the ministry monitoring the situation, scientists were extra cautious in giving out details.
The IMD DG K G Ramesh, along with Mohapatra, was stationed in the RCMS control room, to track minute details and to declare whether the cyclone had made landfall.

The IMD says Vardah was different from other cyclones that occurred in Bay of Bengal, but not just because of its intensity alone.
"It was the fourth consecutive cyclone with re-curving track after cyclones Roanu, Kyant and Nada during the year, as it changed its track from initial northwards movement to west-northwestwards and then west-southwestwards after landfall," the department said in its summary report that has been sent to ministries concerned, state governments and the Cabinet Secretariat.
This was the reason behind IMD changing the initial forecast of landfall from south Andhra Pradesh to north Tamil Nadu near Chennai.
The life period of Vardah was 159 hours (6.6 days) against the normal of 4.7 days over north Indian Ocean during post monsoon season for very severe cyclonic storm category.

The 12 hour average translational speed of the cyclone was 5.2 kmph against the normal of 13 kmph over the Bay of Bengal for post-monsoon season.
However, prior to landfall, the cyclone moved with a speed of about 15-20 kmph.
The peak maximum sustained wind speed of the cyclone was 130 kmph, gusting to 145 kmph over west-central Bay of Bengal.
However, the system weakened over west-central and adjoining south-west Bay of Bengal before landfall. Thus, the wind speed at the time of landfall was 100-110 kmph, gusting to 120 kmph.
Even before becoming a cyclone, Vardah had started wreaking havoc over the Andaman and Nicobar archipalego, stranding tourists there. The navy had to be called in for rescuing people.

When it made landfall on December 12, it claimed lives of nearly 10 people, covering a path of over 1,795 kms.
Coastal Tamil Nadu, especially Chennai, and Andhra Pradesh had been battered. The economic losses ran into several crores, with Tamil Nadu demanding Rs 1,000 crore as relief.