Norwesters

GS1 – Geography

Context:

The India Meteorological Department (IMD), along with the Indian Institute of Tropical Meteorology (IITM) in Pune and the National Centre for Medium-Range Weather Forecasting (NCMRWF) in Delhi, is collaboratively working on India’s inaugural research testbed dedicated to the study of Norwesters.

This initiative focuses on the regions of West Bengal, Odisha, and Jharkhand to better understand how these thunderstorms form, evolve, and move. Advanced equipment including drones, mobile observational units, and precision instruments will be deployed to collect real-time data. This data will enhance short-term weather forecasting (nowcasting), allowing alerts within three hours to mitigate damage and safeguard lives.

What are Norwesters?

Norwesters are intense, short-lived thunderstorms predominantly occurring in eastern and northeastern parts of India. In West Bengal, they are locally termed Kalbaisakhi (meaning a disaster occurring in the Bengali month of Baisakh), while in Assam, they’re called Bordoisila. These storms usually appear during the pre-monsoon season, i.e., between March and May, and also affect regions like southern Nepal, Bhutan, and Bangladesh.

They are marked by violent winds, heavy rainfall, hailstorms, frequent lightning, and occasionally, tornadoes. Classified as mesoscale convective systems, Norwesters are driven by localized atmospheric instability and cover areas ranging from a few tens to hundreds of kilometers.

How Do Norwesters Form?

The origin of these storms lies in the intense heating of land surfaces during pre-monsoon months, especially over the Chhotanagpur Plateau in parts of Jharkhand and Bihar. This creates a localized low-pressure zone that attracts warm, humid air from the Bay of Bengal. At the same time, cooler, dry air from northwestern regions flows in at higher levels. The collision of these contrasting air masses sparks convective activity, leading to thunderstorms.

Over time, these storms often develop into derechos—large, rapidly moving thunderstorm clusters that generate strong, straight-line winds capable of causing significant destruction. As they move across the Gangetic plains, they draw additional moisture from the Ganges-Brahmaputra Delta, gaining further intensity.

Impact of Norwesters

Negative Impacts:

• Fatalities and Infrastructure Damage: The combination of lightning, squall-line winds exceeding 100 km/h, hailstorms, and occasional tornadoes can lead to loss of life and destruction of property. For instance, storms during April–May 2024 in eastern India resulted in at least five deaths.
• Agricultural Losses: Crops like jute, paddy, and fruits are particularly vulnerable to wind and hail damage.
• Service Disruptions: These storms can lead to power failures, transportation delays, and damage to structures.

Positive Impacts:

• Agricultural Benefits: Pre-monsoon showers are crucial for the initial growth of pre-Kharif crops such as paddy, jute, and tea, especially in eastern India and Bangladesh.
• Temperature Relief: The storms often bring sudden and significant drops in temperature, providing respite from the sweltering March–April heatwaves, where temperatures often soar above 35–40°C.