No Consensus Between Models For Monsoon Onset Over Andaman Sea
Representational Image: AI-Skymet
Key Takeaways:
- Southwest monsoon may reach the Andaman Sea around 20th May, but timelines can vary significantly.
- Weather models are divided over the formation and movement of low-pressure systems influencing monsoon onset.
- Bay of Bengal activity may favour an early onset, while Arabian Sea development could trigger strong pre-monsoon showers over Kerala.
- A clearer picture regarding monsoon onset is likely within the next 2–3 days.
Historically, the southwest monsoon normally hits the South Andaman Sea around 20th May. It advances rapidly to reach Port Blair and the North Andaman Sea by 22nd May. Thereafter, it reaches Kerala around 1st June, leaving a gap of 10–12 days. However, these are only tentative timelines and are not sacrosanct. There have been rare instances when the onset of monsoon over the Andaman Sea and Kerala coast occurred almost simultaneously or within a very short interval.
The advance of monsoon is generally much faster in the Bay of Bengal branch (towards the Andaman Sea) than in the Arabian Sea branch (towards Kerala). A simultaneous onset requires a massive and abrupt strengthening of cross-equatorial flow over both the Bay of Bengal and the Arabian Sea at the same time, rather than a gradual progression.
The southwest monsoon had arrived over Kerala as early as 18th May in 2004. The onset took place on 19th May in 1990, whereas it was delayed until 18th June in 1972. However, since the year 2000, the monsoon arrival has not been delayed beyond 8th June. Last year, the monsoon arrived over South Andaman on 13th May and reached Kerala on 24th May, both nearly a week ahead of schedule.
There is some dichotomy among various numerical weather models regarding the advance of the monsoon this season. The NCEP GFS analysis suggests the formation of a low-pressure area over the Southeast Equatorial Indian Ocean around 12th May 2026. The system is expected to move northward, intensify, and get positioned over the Central Bay of Bengal by 14th–15th May. The Skymet GEFS output is also aligned with a similar development over the Bay of Bengal. Such a situation favours an early onset of monsoon.
However, some other models differ completely and do not support any significant system over the Bay of Bengal as of now. Instead, these models indicate the formation of a low-pressure area in the equatorial region near Sri Lanka around 15th May 2026. The system may then move further across the Maldives and Comorin region before positioning itself over the extreme southeastern edge of the Arabian Sea. Such a scenario can trigger heavy pre-monsoon showers over Kerala as a precursor to the arrival of monsoon over the mainland.
Overall, it remains a wait-and-watch situation for another 2–3 days before a decisive verdict emerges regarding the onset timeline of the southwest monsoon.
