The arrival of a conventional western disturbance on Wednesday over North-West India followed by an ‘active’ western disturbance two days later is expected to offer respite to the hills of the region by lowering minimum (night) temperatures though associated clouds could bring widespread rainfall/snow.
Western disturbances pick up moisture along the way as they travel from the Mediterranean, Iran, Iraq, Afghanistan, and Pakistan. This moisture usually gets loaded in the upper atmosphere, unlike weather systems closer to the equator (tropical cyclones) which carry the moisture in the lower atmosphere.
Active western disturbance
Typically, a high-pressure area over Eastern Europe intensifies, sending cold air from the Arctic towards an adjacent area of relatively warmer air with high moisture. This creates instability and lines up weather systems (low-pressure areas, depressions) into a waiting eastward-bound western disturbance.
‘Active western disturbances’ dip down to sweep incremental moisture from the Arabian Sea before they enter North-West India. They usually set up offspring cyclonic circulations over Central Pakistan, North Rajasthan, or Haryana, and help a fan in the moisture into the hills and plains of North-West India.
Warm nights, cold days
While associated clouds can lower night temperatures by not allowing cold air in the upper atmosphere to sink in, by the same token, they set up ‘cold day’ conditions as they block the sun from beating down on land, keeping day temperatures below a maximum of 16℃.
Additionally, the moisture can precipitate in the form of rain over the plains and sow in the Himalayan hills.
Current weather in the region is being influenced by a slow-moving western disturbance parked on Sunday over Bathinda-Ludhiana in Punjab. India Met Department (IMD) has forecast cold wave to severe cold wave conditions for some/many parts of Rajasthan until Wednesday; Punjab, Haryana, and Chandigarh on Monday and Tuesday; and Delhi from Monday to Wednesday.
Severe cold for now
Cold wave conditions in isolated pockets are also very likely over Himachal Pradesh and Uttarakhand until Wednesday; Punjab, Haryana on Sunday and Wednesday; West Madhya Pradesh on Monday and Tuesday; West Uttar Pradesh on Tuesday and Wednesday; Saurashtra and Kutch on Sunday and Monday; and down South over Interior Karnataka on Sunday.
Dense to very dense is forecast over isolated/some parts of Punjab, Haryana, Chandigarh and Delhi, and Uttar Pradesh during the next five days. Dense fog is likely over parts of Bihar until Wednesday; Jammu & Kashmir, Himachal Pradesh, and Uttarakhand until Tuesday; and over Gangetic West Bengal, Odisha, North Madhya Pradesh, Assam, Meghalaya, and Tripura until Monday.
Cold-day conditions may descend on parts of Punjab, Haryana, and Chandigarh for the next five days; Uttarakhand and Rajasthan during the next two days; and Uttar Pradesh for three days from Monday.