A low-pressure system over the Bay of Bengal is expected to intensify into a deep depression on September 22, bringing heavy to very heavy rain to coastal Andhra Pradesh and Odisha and winds gusting to 70 kmph.

Terms like ‘depression’ and ‘deep depression’ sound informal, but in meteorology they are precise categories defined by wind speed, part of a ladder that runs from a low-pressure area up through depression, deep depression, cyclonic storm and beyond.

Each step up that ladder brings a jump in expected wind speed and, usually, in rainfall intensity, which is why the IMD tracks and names the stage rather than just saying ‘a storm is coming’.

That precision is also why forecasts for coastal Andhra Pradesh and Odisha, in the direct path of the system, look very different from the forecast for Delhi, which the same bulletin describes as mainly clear.

The practical habit for anyone in a coastal or inland state watching such a system is to track the stage the IMD assigns it, since that single word carries a lot of information about how seriously to prepare.

Jharkhand is expected to see heavy rain with thunderstorms and lightning on September 22.

West Bengal and Sikkim are forecast to see thunderstorms, lightning and squally conditions.

West Madhya Pradesh is under a thunderstorm and lightning warning.

The latest bulletin did not include warnings for Delhi-NCR, Uttar Pradesh, Punjab, Himachal Pradesh, Jammu and Kashmir, or Rajasthan for September 22.

Delhi is expected to remain mainly clear, with a maximum temperature of around 35 degrees Celsius and a minimum of about 24 degrees.

Heavier rain and thunderstorm activity is expected to reach Uttar Pradesh from around September 24-25.

Fishermen have been advised not to venture into the Bay of Bengal in areas affected by the strengthening system.

Forecasts for weather systems like this can change as the system develops, so people in the affected coastal areas are advised to follow updated bulletins.

Tropical cyclone over the Bay of Bengal (NASA, representative image), Wikimedia Commons, public domain