The density and duration of seasonal fog over the Indo-Gangetic plains may recede from Wednesday as the region comes under the influence of a westerly trough and associated change in wind direction.
The colder north to northwesterlies would get replaced by warmer westerlies to southwesterlies as the system rolls in across the border on Wednesday.
It is likely that a secondary circulation may get formed over the north-west as the westerlies sweep north Arabian Sea and mop up available moisture.
This moisture is likely to emptied as rain over the plains even as India Meteorological Department (IMD) saw the prospects of confluence of westerly and easterly winds over central India. This could add to the instability of atmospheric conditions and associated weather.
The trough in the westerlies is seen affecting northwest India on Wednesday and Thursday, the IMD said. It saw fog to dense fog conditions prevailing in the morning over parts of Indo-Gangetic plains during the next 24 hours.
Cold day conditions (with lower maximum day temperatures) will occur over parts of Punjab, Haryana and Uttar Pradesh during the next two days.
The warm westerly system will cause minimum temperatures also to increase by 2 to 4 deg Celsius during this period, leading to abatement of cold wave conditions.
The impact of the westerly system would be the most over the windward side of the western Himalayan region since the air would be made to rise against the mountain heights, cool and be brought down as either rain or snow.
The IMD has forecast scattered to fairly widespread rain or snow over the western Himalayan region mainly on Tuesday and Wednesday. Scattered rainfall is likely over parts of plains of the northwest and adjoining central India during this phase.
On Sunday, the minimum temperatures were below normal by 2 to 4 deg Celsius over parts of Punjab, west Rajasthan, north Madhya Pradesh, east Uttar Pradesh and Bihar.
The lowest minimum temperature of 1.2 deg Celsius was recorded at Amritsar on Sunday night. Cold wave conditions have been prevailing over isolated pockets of west Rajasthan, north Madhya Pradesh, east Uttar Pradesh and Bihar.
The maximum temperatures were below normal by 7 to 11°C over most parts of Punjab, Haryana, Delhi and Uttar Pradesh and by 4 to 6 deg Celsius over many parts of north Rajasthan, Bihar and some parts of northwest Madhya Pradesh.
In this manner, cold day conditions prevailed over most parts of Punjab, Haryana, Delhi, north Rajasthan and Uttar Pradesh.
But the westerly system will keep moving to the east across the plains, bringing the ‘steaming head' marked by lower pressure and ascending air ahead to drop some rain over east and northeast India. In the process, it might also run into the easterly flows straying into east and central India.
Cold northwesterlies would promptly fill the region vacated by the westerly system and once again lower the mercury level progressively over north and northwest India.
The IMD has forecast a fall in minimum temperatures over northwest, central and east India until January 16 up to which forecasts were available.
Meanwhile, an easterly wave has been affecting south peninsular India. Rainfall has occurred at a few places over coastal Tamil Nadu and coastal Karnataka during the 24 hours ending Monday morning.
Isolated rainfall was reported from interior Tamil Nadu, Kerala, interior Karnataka and coastal Andhra Pradesh, an update from the Chennai Met Centre said.
Scattered rainfall may occur over parts of south peninsular India during the next three days, the IMD said, which is in agreement with the outlook of international models.
The Chennai Met Centre has forecast rain or thundershowers at a few places over coastal Tamil Nadu, Puducherry, and coastal Karnataka over the next two days. Isolated rain or thundershowers could occur over interior Tamil Nadu, Kerala, Andhra Pradesh, Lakshadweep and interior Karnataka.
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