Thundershowers line up along coasts, parts of peninsula

The monsoon trough continued to run close to the foothills of the Himalayas on Tuesday but the unsettled weather marked by widely separated thundershowers ruled the roost in the south peninsula. The monsoon has been vigorous in south interior Karnataka during the 24 hours ending Tuesday morning, according to an update.

TROUGH IN EAST

While the alignment of the monsoon trough indicated little or nil activity over northwest India, a weather-maker north-south trough ran down along the country's eastern corridor. The India Meteorological Department (IMD) update said that the latter trough cut a wedge from Bihar to south Tamil south coastal Tamil Nadu through Jharkhand, Orissa and coastal Andhra Pradesh. Moisture-laden easterlies empty their contents into this trough raising the moisture levels, which combine with the ambient temperature to set up thundershowers and cloudbursts in the region. This is being replicated along the west coast as well, resulting in localised heavy falls, the likes of which have been reported from Mumbai as well as parts of western Maharashtra. This is expected to continue till such time as the dry climes pushing in from the extreme western parts of the country bring the monsoon withdrawal line following not too far from behind. The IMD has not given any hint as to the advance of the `Big Dry' since the known meteorological events associated with the event have not materialised in their entirety.

WITHDRAWAL SIGNS

The withdrawal of the monsoon is more gradual than its onset and is characterised by the reduction in overall rainfall; decay of the anti-cyclonic circulation established over the Tibetan Plateau during monsoon; and the reappearance of the upper-level westerly jet stream south of the Himalayas. On ground, parts of Rajasthan and adjoining Gujarat are already witnessing elevated mercury levels even as thundershowers line up over the peninsula. The IMD said in its update that the subdued monsoon over northwest Rajasthan had driven up day temperatures appreciably to markedly above normal to reaching 40 deg C and above at many places. The US National Centres for Environmental Prediction (NCEP) indicates that the drying is obvious over these regions, which would gradually extend into adjoining Madhya Pradesh during the week ending September 22. Over the peninsula, activity emanating from southern Bay of Bengal will grow into adjoining southeast coast and the rest of the peninsula. Almost the entire west coast and adjoining regions will benefit, with showers tending to concentrate over Kerala and adjoining interior Tamil Nadu. The week from September 22 to October 1, conventionally the last active monsoon week over the mainland, would see a surge in thundershower activity over south Kerala and adjoining southern Tamil Nadu. In the east, Gangetic West Bengal is expected to come under thundershowers during this phase.

MAY BE DELAYED

This projected itinerary would mean that monsoon withdrawal would not be complete by the September-end and will get delayed until sometime into October. On Tuesday, the IMD traced the cyclonic circulation persisting from overnight to over east Uttar Pradesh. The presence of a trough in the westerlies would bring isolated thundershowers over the extreme north. The IMD has forecast scattered to fairly widespread rainfall with isolated heavy falls over south peninsular India during the next four days. Scattered to fairly widespread rainfall is also likely over the North-Eastern States during the next two days and increase thereafter. Satellite pictures on Tuesday showed convective clouds over south Andaman Sea, southeast Bay of Bengal, Assam, Meghalaya, Nagaland, Manipur, Mizoram and Tripura. A warning valid for the next two days said that isolated heavy rainfall is likely over Karnataka, coastal Tamil Nadu, Kerala and Andaman and Nicobar Islands. Fairly widespread rainfall activity with isolated heavy falls is likely over south peninsular India and the North-Eastern States.