Western Disturbance

The weather system popularly known as Western Disturbance (WD) affects
day to day weather of northwest India during the winter season. The approach of WD is usually associated with cloudy sky, rise in night temperatures and precipitation on some occasions. In the rear of the system, clear sky and cold wave conditions are the common weather phenomenon over the Indo-Gangetic plains. On some occasions, dense fog and cold day conditions is also a common phenomenon over this region. Such stable weather conditions sometimes continue till another WD affect the region.
This January, a WD caused light to moderate precipitation at many places
over northwest India mainly on 2nd and 3rd. Thereafter due to the favourable conditions of moisture, temperature and winds, dense fog is prevailing over most parts of Indo-Gangetic plains except on 6th and 7th when the density and duration of fog was lower due to strong surface winds. Cold day conditions prevailed over Punjab, Haryana, north Rajasthan, Uttar Pradesh and Bihar where the maximum temperatures fell below 16° C. Delhi also experienced dense fog and cold day
conditions during this period. The lowest maximum temperature of 12.1° C was recorded on 8th January 2010 which was 9.0° C below normal. However, such extreme cold weather conditions are not very uncommon during January. In the recent past, Delhi recorded the maximum temperature of 12.1° C on 4th January 1980, 11.6° C on 10th January 1999, 12.4° on 5th January 2001, 11.2° C on 1st January 2003, and 12.4° C on 1st January 2004.
FORECAST:
Fog/Shallow fog conditions are likely to prevail during next 1-2 days over
Indo-Gangetic plains.
The latest meteorological analysis suggests that a fresh western disturbance is likely to affect northwest India on 12th and 13th January. It will cause fairly widespread snowfall over Jammu & Kashmir, Himachal Pradesh and Uttarakhand.
Light rainfall at a few places would also occur over plains of Rajasthan, Punjab,Haryana, Delhi and Uttar Pradesh. In view of obscured sky conditions due to lifted fog/low clouds, cold day conditions may continue. Dense fog conditions may reappear over Indo-Gangetic plains after 14th provided wind remains favourable.
In addition, due to confluence/merger of winds, light to moderate rainfall is also likely over parts of central and east India during 11th to 13th January.
Courtsey :IMD & Mr. Anonymous