Better clarity has emerged with respect to the weather events lining up to mark the end of the year as well as ringing in the new.
It will take a deep western disturbance from across the Pakistan border and a brewing storm in the South to clear the dense to very dense fog in the North and bring some respite from the biting cold.
The deep western disturbance will dip low into the southern peninsula and scoop up the storm (a likely depression) in the southwest Bay of Bengal.
On Sunday, India Met Department has acknowledged the presence of a well-marked low-pressure area some 360 km east-northeast of Colombo, Sri Lanka.
It is seen prowling the waters for two more days before it ensures needed supply of moisture to intensify as a depression along the adjoining Tamil Nadu coast.
In doing so, it would dock itself into a ‘marginal’ environment for further intensification, according to model forecasts.
DENSE FOG, CHILL
The limiting factors are proximity to land (affects moisture availability) and moderate to strong vertical wind shear (change in wind direction and speed with height).
Meanwhile, stagnant air over the plains over northwest India over the past few days has perpetrated dense to very dense fog and a cold snap.
Absence of western disturbances and accompanying wind flows allowed an area of high-pressure area to settle over large tracts of the plains with its trademark biting cold.
This brought down mercury to below December-time minimum at many places and the presence of thick fog has not allowed the sun to reach the ground and warm it up.
The logjam would be broken only with the arrival of the deep western disturbance on Tuesday/Wednesday backed up from the South by a brewing depression.
WIND-DRIVEN
Both these systems are wind-driven and are expected to play a major role in clearing the air over the north by way of precipitation in the plains and heavy rain/snowfall over the hills of the region.
The western disturbance will pull the depression along the east coast through the Bay of Bengal. This will help replace the dense fog in eastern and northeast be replaced with heavy rain.
Ahead of this, Sri Lanka could witness flooding rains as the depression leaves it to the north and brings the moderate to heavy rain to prevail along the Tamil Nadu, Andhra Pradesh and Odisha coasts by turns.
The Met has issued warnings for fishermen along the Tamil Nadu coast against high winds. They have been advised to exercise caution while venturing out.
According to Met experts, Bay of Bengal has had a history of hosting cyclonic circulations (even cyclones) during December. Normal circulatory features do not support such an eventuality in the Arabian Sea.