An intense three-day wet spell in Uttar Pradesh has left several districts grappling with a flood-like situation, disrupting normal life and damaging crops. Between Friday (September 25) and Sunday morning, 56 people died and 46 were injured in rain and storm-related incidents, according to data shared by the state’s Office of Relief Commissioner.

Official figures noted that 1,021 houses were damaged. Heavy rain lashed Kanpur, Varanasi, Prayagraj, Gorakhpur, Bareilly, and several other parts of the state. Strong winds reaching 30-40 kmph accompanied the downpour, uprooting trees and electric poles at several places and inundating roads, lanes, and villages.

The India Meteorological Department (IMD) had forecast heavy rainfall in parts of the state on Sunday. The state received 45.1 mm rainfall in the 24 hours ending Sunday, against the normal 1.8 mm, according to official data. As many as 56 of the state’s 75 districts recorded excessive rainfall.

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According to the IMD, the likely interaction of two different weather systems has caused the flood-like situation in parts of Uttar Pradesh over the past three days. We explain.

Deep depression

In its press note on Saturday afternoon (September 26), the IMD had noted that a deep depression, which had formed over the Bay of Bengal and crossed the North Andhra Pradesh–South Odisha coast before weakening, lay centred over parts of north-eastern Madhya Pradesh and adjoining south-eastern Uttar Pradesh.

A depression is an organised low-pressure weather system. A low-pressure system forms in an area where atmospheric pressure is lower than the surrounding areas. This lower pressure is caused by localised heating in an area owing to greater sunshine over the landmass, which causes the air there to undergo warming. As this warm air is less dense than in the surrounding areas, it rises upwards and lowers the atmospheric pressure.

At the same time, this warm, moist air flows inwards towards the centre of the low-pressure area before moving upwards. Thereafter, it cools and condenses into clouds and rain or snow.

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According to the IMD, a depression produces strong winds and heavy rain. As it degrades into a well-marked low-pressure area, the system loses strength and wind speeds drop, which reduces overall rain intensity.

On Sunday, the IMD said the system had moved north-northwestwards and weakened into a well-marked low pressure area at 5.30 am. It is likely to continue moving in the same direction across Uttar Pradesh and weaken gradually into a low-pressure area during the next 12 hours.

Western Disturbance

In its September 26 press note, the IMD also said that a “likely interaction” with a Western Disturbance (WD) could result in the state experiencing strong gusty surface winds along with heavy to very heavy rainfall, with isolated extremely heavy rainfall in the central Terai belt, and exceptionally heavy rainfall (>30 cm) at one or two places.

WDs refer to weather systems that usually travel into northern India from the west. These are east-moving rain-bearing wind systems that originate beyond Afghanistan and Iran, and pick up moisture from the Mediterranean Sea, Black Sea, Caspian Sea, and the Arabian Sea. These are typically extra-tropical cyclones resulting from low-pressure areas, formed by interactions between polar and tropical winds.

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The subtropical westerly jet stream — a high-altitude, fast-moving air current that flows from west to east in the Earth’s atmosphere — carries these WDs to the Himalayan and Tibetan highlands. As moisture-laden WDs hit the Himalayas, they rise and cool, bringing cloudy weather, rain, and thunderstorms, particularly over parts of north and northwest India.

On Sunday, the IMD in its weather forecast bulletin for Uttar Pradesh said that this WD is present high up in the atmosphere as a trough — an elongated zone of relatively lower pressure — over northwestern India.

The interaction of the WD with the deep depression, even as the southwest monsoon withdraws across north and northwestern India, is producing spells of intense rainfall over particular areas.