Heavy rain and hailstorms in May were due to six WDs, may interact with monsoon weather systems
Moderate to intense convection seen over several parts of the country on May 30, 2023. Photo: IMD India has witnessed an unusually high number of western disturbances (WDs) in the last three months. There were six WDs in May, a surprisingly high number for the summer months.
April and March also saw three and seven WDs, respectively, according to data from the India Meteorological Department (IMD), collated by New Delhi-based non-profit Centre for Science and Environment and Down to Earth.
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The extratropical storms that travel primarily from the Mediterranean region to India kept long-lasting heat waves at bay for most of the three months for some parts of India. However, they also caused heavy rainfall and hailstorms in many regions, which destroyed crops and added to farmers’ woes.
The heavy rain and hailstorms are significant, as WDs are sparse and weaker in warmer months. They usually peak in winter.
The trend of decreasing activity of WDs during winter months and their increasing activity in the summer months is a possible result of human-induced climate change. It could indicate changing characteristics of the typical weather conditions during these seasons.
“This is definitely a very high count for May, considering that the frequency of western disturbances should peak during the winter months,” said Akshay Deoras, a research scientist at the National Centre for Atmospheric Science and Department of Meteorology, University of Reading, the United Kingdom.
Normally there are two-three WDs during May. “Their frequency decreases sharply between June and September when the subtropical westerly jet migrates northwards,” he added.
Another negative impact of increasing WDs during summer could be their possible interaction with monsoon weather systems, such as low-pressure areas and depressions in the Bay of Bengal and the Arabian Sea. These systems cause rainfall in southern and central India during the monsoon.
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The interaction can lead to extreme rainfall events and even cloud bursts in the hills and mountains, which can cause floods, flash floods and landslides.
The WDs could also possibly interact with the monsoon trough — a low-pressure region that extends from northwest India to northeast India during the monsoon months. This happened in June 2013, causing the massive Uttarakhand floods, which killed at least 5,000 people.
“I don’t think there will be any negative impact of WDs on the monsoon in June. The subtropical jet (that carries the WDs to India) is showing signs of migration. Specific WD-monsoon interaction events can’t be forecast with such a prolonged lead time”, said Deoras.
The continuous activity of WDs over India in the spring and summer months has meant ample rainfall in many regions. Data from the IMD shows that 52 per cent of districts in the country received large excess rainfall, while 12 per cent of districts received excess rainfall.
Gujarat received the highest excess rainfall at 831 per cent of the normal for the season. Sixteen states received large excess rainfall. These were spread out in northwest, central and southern India, while northeastern states remained deficient in rain.
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Manipur received 66 per cent deficient rains, the least in the percentage of average rainfall. There were large deficits in Tripura (61 per cent), Mizoram (48 per cent), Meghalaya (47 per cent), Arunachal Pradesh (41 per cent), Assam (38 per cent) and Nagaland (33 per cent).
Outside the northeast, only Goa (65 per cent), Kerala (34 per cent), Andaman and Nicobar Islands (65 per cent) and Lakshadweep Islands (49 per cent) had deficient rainfall for the pre-monsoon season, according to the IMD data.
The country-wide rainfall from March 1-May 31 was 12 per cent excess.
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