Little over a week after western disturbance whipped up violent storms in western Uttar Pradesh and eastern Rajasthan, northern India faced yet another western disturbance with wind speed reaching 109 km/h in some paces of the national capital. Several dozens were killed as thunderstorms accompanied with squall hit Uttar Pradesh, West Bengal, Andhra Pradesh and Delhi-NCR.  

According to IMD’s latest prediction, thunderstorm accompanied with squall (wind speed reaching 50­-70 km/h) very likely at isolated places over Jammu &Kashmir, Himachal Pradesh, East Bihar, West Bengal, Sikkim and Odisha. 

Western disturbance, which is not unusual during pre-monsoon days, happens when the cold and moisture-laden winds from the Mediterranean fill up the low pressure area created in north and northwestern India due to high temperatures. This year, western disturbances stand out because of their severity and spread. Weather scientists have attributed this to “a series of storms originating in the Mediterranean area and blending with local heat-induced storms,” reports Down To Earth.

A Western Disturbance is an extratropical storm originating in the Mediterranean region that brings sudden winter rain to the northwestern parts of the Indian subcontinent.[1][2] It is a non-monsoonal precipitation pattern driven by the westerlies. The moisture in these storms usually originates over the Mediterranean Sea and the Atlantic Ocean.[3] Extratropical storms are a global phenomena with moisture usually carried in the upper atmosphere, unlike their tropical counterparts where the moisture is carried in the lower atmosphere. In the case of the Indian subcontinent, moisture is sometimes shed as rain when the storm system encounters the Himalayas. 

Western Disturbances are important for the development of the Rabi crop, which includes the locally important staple wheat. Western disturbances originate in the Mediterranean region. A high-pressure area over Ukraine and neighbourhood consolidates, causing the intrusion of cold air from polar regions towards an area of relatively warmer air with high moisture. This generates favorable conditions for cyclogenesis in the upper atmosphere, which promotes the formation of an eastward-moving extratropical depression. 

Traveling at speeds up to 12 m/s (43 km/h; 27 mph), the disturbance moves towards the Indian subcontinent until the Himalayas inhibits its development, upon which the depression rapidly weakens, according to Wikipedia.