The southwest monsoon reached the Andaman and Nicobar islands on Saturday, arriving four days ahead of schedule under the influence of a low-pressure area over the Bay of Bengal.

Thousands of kilometres away, Calcutta endured another sweltering day, as high humidity was the perfect ally to the heat, tormenting people. Despite a thunderstorm the night before, most of Saturday was very uncomfortable.

An active east-west trough from Bihar to Assam is triggering heavy rain in north Bengal. The Met office said thunderstorms are likely in south Bengal as well, but the showers will not be as widespread as in the hills.

“Southwest monsoon has advanced into some parts of the southeast Arabian Sea, the southwest and southeast Bay, many parts of the Andaman Sea, the Nicobar Islands and some parts of the Andaman Islands,” said a national bulletin issued by the IMD.

“The southwest monsoon is likely to set in over Kerala on May 26,” said the bulletin. The prediction comes with a margin of error of four days. The usual date of monsoon onset in Kerala and the Northeast is June 1.

The monsoon currents enter Bengal through the Northeast. The normal onset date of the monsoon in north Bengal is June 5, while south Bengal gets its first monsoon showers around June 10.

Monsoon arrival in Kerala and in Bengal is not linked.

A Met official told Metro: “Monsoon has reached the Bengal. The wind pattern is such that a bulk of the moisture from the Bay is moving towards sub-Himalayan Bengal. But some moisture is also reaching south Bengal. While north
Bengal is likely to get heavy rain, thundershowers are likely in south Bengal as well.”