Why in news?

An intense three-day wet spell in Uttar Pradesh has left several districts facing a flood-like situation, disrupting normal life and damaging crops.

Between September 25 and September 27 morning, 56 people died and 46 were injured in rain and storm-related incidents, according to the state's Office of Relief Commissioner. As many as 1,021 houses were damaged.

What’s in Today’s Article?

• The Scale of the Impact
• The First System: A Deep Depression from the Bay of Bengal
• The Second System: A Western Disturbance
• What is a Western Disturbance (WD)?
• Why the Interaction Matters?
• Conclusion

The Scale of the Impact

• 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 and inundating roads, lanes and villages.
• The state received 45.1 mm of rainfall in the 24 hours ending September 27, against a normal of just 1.8 mm — a massive deviation.
• 56 of Uttar Pradesh's 75 districts recorded excessive rainfall.
• According to the India Meteorological Department (IMD), the flood-like situation stems from the likely interaction of two different weather systems.

The First System: A Deep Depression from the Bay of Bengal

• The IMD noted that a deep depression lay centred over parts of north-eastern Madhya Pradesh and adjoining south-eastern Uttar Pradesh.
• The depression had formed over the Bay of Bengal, crossed the North Andhra Pradesh–South Odisha coast, and then weakened.
• What is a Depression?
  • It is an organised low-pressure weather system.
  • A low-pressure area forms where atmospheric pressure is lower than surroundings, typically due to localised heating over a landmass from greater sunshine.
  • The heated air becomes less dense, rises, and lowers atmospheric pressure further.
  • Simultaneously, warm, moist air flows inward toward the low-pressure centre and moves upward, where it cools and condenses into clouds and rain (or snow).
• Effects and Trajectory
  • Depressions typically produce strong winds and heavy rain.
  • As a depression degrades into a well-marked low-pressure area, it loses strength — wind speeds drop, reducing rain intensity.
  • By 5:30 am on Sunday, the IMD noted the system had moved north-northwestwards and weakened into a well-marked low-pressure area.
  • It was expected to continue in the same direction across Uttar Pradesh, gradually weakening further into a low-pressure area over the following 12 hours.

The Second System: A Western Disturbance

• The IMD's note also flagged a "likely interaction" with a Western Disturbance (WD), which could bring:
  • Strong gusty surface winds.
  • Heavy to very heavy rainfall, with isolated extremely heavy rainfall in the central Terai belt.
  • Exceptionally heavy rainfall (>30 cm) at one or two places.

What is a Western Disturbance (WD)?

• WDs are weather systems that typically travel into northern India from the west.
• They are east-moving, rain-bearing wind systems originating beyond Afghanistan and Iran.
• They gather moisture from the Mediterranean Sea, Black Sea, Caspian Sea, and the Arabian Sea.
• WDs are essentially extra-tropical cyclones, formed through interactions between polar and tropical winds.
• How WDs reach India?
  • The subtropical westerly jet stream — a high-altitude, fast-moving west-to-east air current — carries WDs toward the Himalayan and Tibetan highlands.
  • As moisture-laden WDs strike the Himalayas, they rise and cool, producing cloudy weather, rain, and thunderstorms, especially over north and northwest India.
• Current Status
  • The IMD's weather bulletin for Uttar Pradesh noted this WD was present high up in the atmosphere as a trough — an elongated zone of relatively lower pressure — over northwestern India.

Why the Interaction Matters?

• The interaction between the Western Disturbance and the deep depression, occurring even as the southwest monsoon withdraws across north and northwestern India, is what has produced these intense spells of rainfall over specific areas of Uttar Pradesh.
• This timing is significant: two independent weather systems — one tropical (from the Bay of Bengal) and one extra-tropical (from west/central Asia) — converged over the same region during the monsoon withdrawal phase, a period that already tends to be meteorologically unstable.

Conclusion

Uttar Pradesh's floods illustrate how India's weather is shaped by systems from opposite directions — a Bay of Bengal depression from the east, and a Western Disturbance from the west — meeting over the same terrain.

As the monsoon's retreat becomes less predictable, such cross-system interactions may become a more frequent source of extreme, localised rainfall events.