The Western Disturbance (WD) is a meteorological phenomenon defined as an extratropical storm system or low-pressure system that significantly influences the weather of the Indian subcontinent. It is a non-monsoonal precipitation pattern that brings sudden winter rain to the plains and snowfall to the mountainous regions of northwest India. The term was first coined by Indian meteorologists in the 1940s to describe this system.
The WD originates in the Mediterranean region, including the Mediterranean Sea, Black Sea, and Caspian Sea, where cold polar air interacts with warmer, moist subtropical air, leading to cyclogenesis. This system is carried eastward by the subtropical westerly jet stream (STJ), which acts as the steering mechanism. As the low-pressure system travels across West Asia, it gathers moisture and releases it upon reaching the Indian subcontinent, primarily during the winter months from December to February. The precipitation occurs when the moist air is forced upward by the Himalayas, causing it to cool and condense.
This winter precipitation is crucial for the Rabi crop season, particularly for staple crops like wheat, and the Himalayan snowfall is vital for replenishing river flows and water resources. The WD is intrinsically connected to the dynamics of the subtropical westerly jet stream and can also influence the timing and progression of the Indian Monsoon. Recently, due to climate change, the character of WDs has been changing; they have been observed to be less frequent and weaker in winter since 2020-21, leading to drier winters. Conversely, their increased frequency in summer can interact with the monsoon, a factor linked to catastrophic events like the Uttarakhand floods in June 2013.