Wettest in 125 years: Early monsoon, warmer days feed May rain

To be sure, there is some deviation from the LPA in most years. However, the deviation this year is far more unusual.

This May is, thus far, the wettest in India in 125 years, the period for which India Meteorological Department (IMD) has published gridded data, which allows detailed analysis of rainfall. An HT analysis of this data shows that the month will end up as the third wettest May since 1901 even if there is no rain in the remaining four days. While the early arrival of the monsoon has aided this trend, frequent storms across the country even before monsoon’s

The southwest monsoon made its onset over Mumbai on Monday morning, announced the India Meteorological Department. This comes just a day after the monsoon arrived over Maharashtra. (Raju Shinde/HT Photo )

The rain this May is 88.5% more than the 1971-2020 average of rain for the May 1-27 period. IMD considers the average for the 1971-2020 period as the Long Period Average (LPA) currently for tracking rain’s performance.

The numbers above make it clear that this May is going to count as exceptionally rainy. Another way to look at this is by ranking May rain so far with rain for the full month of May in other years. This shows that May 2025 will be the third wettest even if India does not get any rain in the remaining four days. To rank as the wettest or second wettest May -- a record currently held by 2021 and 1990 -- India needs just 12.4mm or 7.6 mm rain, respectively. It is possible that India receives this amount of rain in the next four days. The reason for this is that the LPA for the last four days of May is 9.9mm with a standard deviation of 4.1mm. This means that May can end up as the wettest ever even with usual rain in the next four days. Then, IMD has already issued rain-related warnings for large parts of India for every day up to May 31.

What is the reason behind May being very wet this year? The early arrival of the monsoon – it arrived on the Kerala coast on May 24, eight days earlier than usual – has helped but is not the only reason. For example, this May was unusually rainy even before the monsoon’s arrival. Only five days recorded a deficit from the LPA up to May 23, and each day from May 17 to May 23 received rain more than double the LPA for those days.

Moreover, while the monsoon has only covered peninsular and the north-eastern parts of India so far, there is a large surplus of rain even in the rest of the country. Almost two-thirds (62.8%) of the country’s area has a surplus of 60% or more compared to the LPA, a threshold used by IMD to classify local rain as “large excess”; and another 10% of the country has a surplus of 20%-60%, classified as “excess” rain. Regions with deficit of 20% or more cover just 9% of the country’s area. They are concentrated in western Bihar, eastern Uttar Pradesh, northern parts of Rajasthan, Himachal Pradesh, Jammu and Kashmir, Ladakh, Tripura, Manipur, Arunachal Pradesh, and eastern parts of Assam. (See map)

Why has India been rainy despite monsoon some distance away for most of the country? Frequent storms of different kinds is the short answer, as HT explained on May 23. For example, parts of northern states received rain from frequent western disturbances, a storm originating west of India, usually in the Mediterranean region.

In addition, day time heating in northern India led to air becoming warmer, which created low-pressure areas (often more than one in the country at the same time) conducive for attracting moisture. Moisture has been in abundant supply with hot sea surface temperatures on either side of the Indian subcontinent. It is these kinds of storms that have led to deaths in the plains, in places such as Delhi and Saharanpur, Jhansi, and Sonbhadra, all in Uttar Pradesh.