Cyclone Arnab began as a low pressure area over the east central Bay of Bengal on October 24, and four days later it was a very severe cyclonic storm crossing the Andhra Pradesh coast with winds of 125 to 135 kilometres per hour. That journey, from a broad area of low pressure to a structured storm with a defined eye and a dangerous surge, is the work of forecasters who track a depression as it deepens, a cyclonic storm as it organises, and a very severe storm as it approaches land. Understanding that journey helps explain why the warnings changed as they did, and why the landfall forecast, close to the south of Yanam between Bapatla and Visakhapatnam, proved accurate.
The stages are defined by wind speed, and each stage brings a different warning. Arnab deepened into a depression on October 25, intensified into a cyclonic storm by the morning of October 26, and reached very severe cyclonic storm status, the third highest of the India Meteorological Department’s seven level scale, corresponding to sustained winds of 64 to 89 knots, or 119 to 165 kilometres per hour. Forecasters track the storm’s centre, its movement and its interaction with land, because a storm that interacts with the coast early can weaken, stall or change track in ways that matter enormously to the towns in its path.
The cone and the track
The forecast cone, the widening area that shows where a storm’s centre is likely to travel, narrows as landfall approaches and as observations improve. For Arnab, the cone narrowed to a stretch of coast between Bapatla and Visakhapatnam, and the landfall process itself lasted from 0200 IST to 0800 IST on October 28, prolonged by the storm’s slow movement near the coastline. At one stage the storm was described as practically stationary, with its edge on the coast while its centre remained over the sea, a pattern that extends exposure to wind and rain even as the headline wind speed begins to fall.
Forecasters also warn about hazards beyond wind. For Arnab, those included a storm surge of 1.5 to 2.5 metres above normal tide, likely to inundate low lying areas over the six hours following landfall, and extremely heavy rainfall that prompted red alerts across Andhra Pradesh and several Odisha districts, with heavy rain warnings extended to Telangana and Jharkhand. By 1130 IST the storm had weakened to a severe cyclonic storm with winds of 100 to 110 kilometres per hour, and was expected to weaken further within hours, yet the rainfall risk continued inland, which is why forecasters distinguish between a storm weakening and a storm ending.
Why forecasts deserve trust, and scrutiny
Reports of the landfall recorded the wind speeds and the timing that forecasters had anticipated, and News Manship’s account of Arnab’s growth traced the same stages from depression to very severe storm. Forecasts are not perfect, and their cone is a statement of uncertainty, not a promise. But Arnab’s track shows what a forecast is for: giving authorities days, not hours, to move more than 100,000 people to safety, and giving residents time to understand not just that a storm is coming, but where, when and how dangerous it will be when it arrives.
