Briefing note: Depressions and Cyclones (13 May 2019)
Cox’s Bazar is at risk of cyclones forming out of depressions in the large Bay of Bengal between April and June, and September and December. Cyclonic storms form when large masses of air begin rotating at high speed around a low-pressure area. The Bay of Bengal stretches from the Bangladesh coast in the North to the latitude of southern Sri Lanka in the south, and from the Indian coast in the west to Myanmar and the Andaman and Nicobar Islands.
Cyclones are large systems affecting weather conditions well beyond its center before, during and after landfall. Even if a cyclone does not make landfall close to Cox’s Bazar or even within Bangladesh, the Rohingya response area of operation may still experience hazardous weather.
Hazards associated with depressions and cyclones include:
Strong winds – between 62 and 200 km/hour in the cyclone itself;
Heavy rainfall - with potential to trigger landslides and flash floods;
Storm surge – mounds of seawater driven towards the shore due to winds, wave and pressure.
It can be difficult to predict the development and path of a cyclone with a high level of certainty until it comes near to the coast. The strength and path of a cyclone is affected by atmospheric conditions, and have been known to turn in unexpected directions. Meteorologists run models that project how the cyclone can develop, including possible paths, intensity of wind, rain and storm surge, and when those hazards will affect human settlements. Those models are then compared to see if they tend towards the same projected development or if there are a lot of possible directions the cyclone can move and develop. Because of the chaotic nature of the earth’s atmosphere, certainty is difficult to achieve.
Because of the difficulty in predicting the strength, timing and path of a cyclone, cyclone warnings and advisories mainly come in stages and can change quickly. Meteorological departments issue advisories when they are confident that the information is reliable and actionable. Narrative bulletins from meteorological departments are issued on a running basis and will be dated and numbered. Visual cyclone path projections from meteorological departments are updated on a running basis and will usually show a “cone of uncertainty” around the projected path.