Weather Bee: Monsoon 2025 would have been wetter if intense rain wasn’t concentrated
The headline performance of the season can be seen in the average rain India has received so far in the season: 833mm rain up to September 18.
Only over a week remains of the 2025 monsoon season, which generally lasts for 122 days from June 1 to September 30. How has it rained in India’s this season this year? According to HT’s analysis of the gridded rainfall data of the India Meteorological Department (IMD), the 2025 season is among the wetter ones overall, but does not appear to be made for the record books. If this is contradictory to the reports of flooding and landslides, the reason is that disaster-causing intense rain was concentrated in parts of the country and in particular times of the season. In fact, despite an early withdrawal of the weather patterns associated with the season over parts of Rajasthan, intense rain continued to affect other parts of the country this week. If such rain continues through September 30 as forecasts suggest, even the national average will have a bigger surplus by the time the season ends.
The headline performance of the season can be seen in the average rain India has received so far in the season: 833 mm rain up to September 18. This is 4% more than the 1971-2020 average, which IMD currently considers as the long period average (LPA) for tracking rainfall performance. The surplus is, however, not very unusual: 833mm rain up to September 18 is the 51st maximum rainfall for this part of the season since 1901, which places the 2025 season in the rainiest half of the 125 monsoon seasons, but far from the top.
The reason why the country’s average rainfall looks benign for the monsoon is multifold. One is that the weather patterns associated with monsoon hit the Kerala coast eight days ahead of schedule. If the rain India received in the last week of May was shifted to the June-September period, the season would have had an 11% surplus and jump to 12th wettest.
To be sure, while monsoon being more than a week early in Kerala is rare, as HT pointed out here, it also hardly ever arrives exactly on June 1. It is far more usual for it to arrive a little earlier or a little later than June 1. This is because the June 1 schedule is simply based on the average of the dates on which the monsoon’s weather systems became active at Kerala coast in the past. Therefore, the season is almost always assessed on its expected June-September schedule.
The second reason why the season’s overall rain performance looks relatively benign is that it did have a dry patch in July. It is the only month in the season when the national average hit a rainfall deficit. July rainfall is also ranked 25th lowest since 1901. This is why, despite the disaster worthy rain of June, August, and September, the season’s overall is not ranked nearer to the top. The rain in all these three months is ranked within the highest 50.
Another reason behind the low country average is the concentration of disaster-causing rains in particular places in the country. A bird’s eye view of this can be taken by calculating the area that received a surplus of 20% or more, which the IMD considers as ‘excess’ or ‘large excess’. Such area was 40% of India in June, 27% in July, 37.5% in August, and 45% in September. Therefore, it is no surprise that months with the most surplus for India on average are (in order): September, June, August, and July. This does not mean that part of the country – 27% that got a large surplus – was not prone to flooding in July.
The trend highlighted in the area-wise statistics above can further be illustrated by tracking such area of large surplus within some states that have made news for rain-related disasters this season. For example, more than 50% of Himachal Pradesh and Uttarakhand has been affected by a significant surplus in all months but July, with such area almost completely covering these states in August. To be sure, the degree of surplus became a ‘large excess’ over a large area only from August onward, which is also the case with Punjab. On the other hand, Gujarat has seen a big surplus over a large area only in June and September. As the accompanying chart for other states shows, this asynchronous arrival of surplus rain over different states is partly responsible for the country average not reflecting the experience of this monsoon being disaster heavy.
To be sure, it is quite possible that even the country average shows a bigger surplus by the time the season ends officially on September 30. Despite the monsoon’s weather systems withdrawing from the most western parts of Rajasthan three days earlier than their September 17 schedule, September has the highest surplus of all monsoon months this year up to September 18. This has already helped boost monsoon 2025’s rank: from 63 on August 31 to 51 on September 18. And IMD forecasts suggest that most parts of India will receive surplus rain in the week ending October 2. If any of the surplus falls by September 30, this monsoon will not be ending with a whimper.
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