The India Meteorological Department (IMD) has warned that a well-marked low-pressure area over the Bay of Bengal is likely to intensify into a cyclonic storm by the morning of October 26, raising the prospect of heavy rainfall along the coasts of Andhra Pradesh and Odisha later next week.
According to the weather department's latest bulletin, the system over the east-central and adjoining south-east Bay of Bengal is expected to intensify into a depression on October 24, followed by a deep depression on October 25. It is then very likely to develop into a cyclonic storm over the west-central Bay of Bengal by the morning of October 26.
The system is expected to move west-northwestwards and continue to intensify, reaching the west-central Bay of Bengal near the Andhra Pradesh coast around the morning of October 28.
The IMD has issued a pre-cyclone watch for Andhra Pradesh and the adjoining south Odisha coast. The Odisha government has also begun precautionary preparations in view of the developing weather system.
Heavy rain forecast for Andhra Pradesh and Odisha
The IMD has forecast extremely heavy to heavy rainfall over coastal Andhra Pradesh and Odisha between October 27 and 29 as the system moves towards the eastern coastline.
The developing weather system could also bring squally conditions and rainfall to parts of the eastern coast in the coming days. The IMD has advised fishermen to return to the coast and exercise caution as sea conditions are expected to deteriorate.
The system's precise track and intensity will become clearer as it develops. While the forecast points to a possible cyclonic storm by October 26, its formation and subsequent movement remain subject to changing weather conditions.
If the system intensifies into a cyclonic storm, it will be named Arnab. The name, which means "ocean" in Bengali, was contributed by Bangladesh to the regional tropical cyclone naming list maintained by the World Meteorological Organization (WMO) and the IMD.
Previous weather system crossed coast as deep depression
The latest forecast comes about a month after another weather system triggered a cyclone watch over the Bay of Bengal but did not intensify into a cyclonic storm.
That system developed into a deep depression over the west-central and adjoining north-west Bay of Bengal on September 23. It subsequently crossed the Andhra Pradesh-Odisha coast as a deep depression, bringing heavy rainfall to the region.
The latest system is being closely monitored as it is forecast to strengthen further while moving towards the north Andhra Pradesh and adjoining south Odisha coasts.
How the IMD classifies depressions and cyclonic storms
The IMD classifies low-pressure systems according to their intensity, taking into account pressure patterns and maximum sustained wind speeds over the sea. The progression from a low-pressure area to a depression, deep depression and cyclonic storm depends on the system meeting prescribed criteria.
A low-pressure area is identified by one closed isobar, while a depression has two and a deep depression has three. A system with four or more closed isobars is classified as a cyclonic storm.
Wind speed is another key factor in determining a system's classification. A system with maximum sustained three-minute surface winds of 17 to 27 knots, or 31 to 50 kmph, is classified as a depression. Wind speeds of 28 to 33 knots, or 52 to 61 kmph, indicate a deep depression.
Once maximum sustained winds reach 34 knots, or 63 kmph, or more, the system qualifies as a cyclonic storm.
A low-pressure area, therefore, does not automatically develop into a cyclone. It must intensify and meet the prescribed pressure and wind-speed thresholds before it can be classified as one.