Low-pressure area watch in South as fog and cold descend on North

Heavy rain forecast for South including Tamil Nadu, Puducherry

Classic pre-winter climes are emerging over the North and South of the country thanks to enhanced activity respectively of passing western disturbances and easterly waves, both seasonal low-pressure bands.

Passage of these waves to opposing direction would culminate in setting up fog to dense fog followed by cold weather in the North and a likely low-pressure area over the South-West Arabian Sea in the South.

The India Meteorological Department (IMD) has already put many areas of Tamil Nadu, Puducherry, Karaikal, Kerala, South Interior Karnataka, Coastal Andhra Pradesh and Lakshadweep under watch for heavy rain.

Prevailing system weakens

Western disturbances travel from as far North-West as the Mediterranean and move across the Iraq, the Middle-East, Iran, Afghanistan and Pakistan, and modulate the weather between cold, warm and rainy spells.

One such prevailing system has now weakened, and is now bringing cooler climes and the familiar fog over stretches of North-West, North, and East India in turns.

Western disturbances carry moisture and are therefore warm systems. Once they weaken or move out of the country, colder Arctic air fill in the trail, flowing all the way from farther North-West.

The prevailing one over North Pakistan and adjoining Jammu & Kashmir has weakened but the parent trough was still at large.

The intensity and depth of the western disturbance system had induced the formation of an offspring cyclonic circulation, which lay parked over North Rajasthan, India Meteorological Department (IMD) said on Friday.

The parent trough and the offspring circulation are combining to pipe in the moisture into North-West India which is condensing into moisture because of the cooler air.

Dense to very dense fog enveloped a few places over West Rajasthan, isolated places over Himachal Pradesh, Punjab, East Rajasthan, Jharkhand and Tripura during the 24 hours ending on Friday morning while moderate to dense fog was observed at isolated places in Bihar.

Dense to very dense fog

An outlook until Saturday said that dense fog may descend down over isolated pockets over North Rajasthan, West Haryana and South Punjab in the morning hours.

The IMD has forecast shallow to moderate fog at isolated pockets over parts of East and North-East India during the next three days as the warm head of the West-to-East-moving western disturbance steams in. A fresh feeble western disturbance is now forecast to affect the hills of North-West India tomorrow (Saturday).

The minimum (night) mercury level may dip further by 2-4 degree Celsius over North-West India from Saturday, a trend, which would extend to East India from Monday.

The western disturbance has already dumped its surplus moisture carry in the form of rain or thundershowers at many places over Jammu & Kashmir, Himachal Pradesh, Uttarakhand, West Uttar Pradesh, and at a few places over Haryana, Chandigarh and Delhi and isolated places over East Uttar Pradesh.

The winter rain or thundershowers brought to the region in this manner and during this season is crucial for standing Rabi crop over North-West India.

Easterly wave in South

Meanwhile, the easterly wave activity over the South Peninsula would bring fairly widespread rainfall with isolated heavy to very heavy falls likely over Tamil Nadu, Puducherry and Karaikal until tomorrow (Saturday) and heavy rainfall at a few places with very heavy falls at isolated places during the next two days.

The easterly wave activity in the South could reach a logical conclusion with the likely genesis of a low-pressure area some distance away from the Sri Lanka and South India coasts.

It would still be able to significantly help ramp up rainfall over both Tamil Nadu, Puducherry and Sri Lanka as the moisture-heavy easterly winds would be dragged over the region, snapping open the clouds along the way.

The 24 hours ending on Friday morning have already seen rain or thundershowers over Rayalaseema, Tamil Nadu, Puducherry and Karaikal; at a few places over and Andaman & Nicobar Islands and at isolated places over Lakshadweep.

Heavy rainfall is being forecast at isolated places over Kerala, South Interior Karnataka, Coastal Andhra Pradesh and Lakshadweep from Saturday to Monday.