Uttar Pradesh Rain: Interaction of Two Weather Systems

30 Sep 2026

Tags: Geography   Physical Geography   Climatology

Source: The Indian Express

Context: Uttar Pradesh experienced an intense three-day wet spell, causing flood-like conditions across several districts.

• The State received 66.5 mm rainfall against the normal 7.6 mm during the period; 56 of 75 districts recorded excess rainfall.
• At least 62 people died, 46 were injured and over 1,000 houses were damaged.
• According to the IMD, the intense rainfall resulted from the interaction of a Deep Depression and a Western Disturbance (WD), even as the southwest monsoon continued withdrawing across northern and northwestern India.

Deep Depression: Role in the Rainfall

• A Deep Depression that formed over the Bay of Bengal crossed the North Andhra Pradesh–South Odisha coast before weakening over northeastern Madhya Pradesh and southeastern Uttar Pradesh.
• A depression is an organised low-pressure weather system in which atmospheric pressure is lower than surrounding areas, causing winds to converge towards its centre.
• Warm, moist air moves towards the low-pressure centre, rises, cools and condenses into clouds, producing rainfall.
• A deep depression generally produces strong winds and heavy rainfall; as it weakens into a well-marked low-pressure area, wind speeds and rainfall intensity decline.
• The system subsequently moved north-northwestwards across Uttar Pradesh, continuing to influence rainfall.

Western Disturbance: Role in the Event

• A Western Disturbance is an eastward-moving, rain-bearing weather system that generally originates in the Mediterranean and adjoining West Asian region and travels towards northern India.
• These systems are typically extratropical cyclonic systems, associated with interactions between polar and tropical air masses.
• The subtropical westerly jet stream helps transport Western Disturbances towards the Himalayan and Tibetan region.
• When moisture-laden air encounters the Himalayas, it rises, cools and condenses, generating clouds, rainfall and thunderstorms, particularly over northern and northwestern India.
• During the event, the IMD noted that a WD was positioned high in the atmosphere as a trough, i.e. an elongated zone of relatively low atmospheric pressure over northwestern India.

How the Two Systems Interacted

• The Deep Depression supplied abundant moisture and instability from the east, while the Western Disturbance provided an additional dynamical trigger from the west.
• Their interaction enhanced atmospheric convergence, uplift and condensation, producing intense rainfall over parts of Uttar Pradesh.
• Such interaction can generate rainfall that is substantially more intense than what would be expected from either system operating independently.
• The event demonstrates that weather systems need not remain geographically isolated; interactions between tropical and extratropical systems can amplify extreme-weather outcomes.

Key Concepts

• Depression: A low-pressure system stronger than a low-pressure area but weaker than a deep depression/cyclonic storm in the IMD classification.
• Deep Depression: A more intense stage of a tropical low-pressure system, capable of producing strong winds and heavy rainfall.
• Western Disturbance: An extratropical weather system travelling eastwards towards South Asia, important for winter precipitation over northwestern India.
• Subtropical Westerly Jet: A high-altitude, fast-moving west-to-east air current that influences the movement of Western Disturbances towards South Asia.
• Trough: An elongated region of relatively low atmospheric pressure, generally associated with rising air and cloud formation.
• Monsoon withdrawal: The southwest monsoon gradually retreats from northwest India and subsequently other parts of the country; its interaction with other weather systems can influence late-season rainfall.