The Hindu Business Line : Monsoon hits Mumbai, but northern plains will have to wait

THE southwest monsoon hit Mumbai on Sunday 10 days behind schedule, but meteorologists find no supportive build-up to predict its progress into the west and northwest of the country in the next 3-4 days.

"It all boils down to the vertical wind shear (change of winds with height) over the `head' Bay of Bengal settling down to the optimum levels. At its lowest value, the wind shear will aid the formation of the much-awaited monsoon low," Dr Akhilesh Gupta of the National Centre for Medium Range Weather Forecasting (NCMRWF) told Business Line.

The first such low in the Bay can drastically alter weather conditions by bringing in cooler easterlies in the lower levels and, along with them, the monsoon in the north and west. But there is no of sign that happening in the next 3-4 days, Dr Gupta said. Planners in the Government and other stakeholders have been apprised of the situation at a meeting held in New Delhi. "The vertical wind shear over head Bay enjoys absolute veto power over conditions turning favourable any time soon. This is clear from today's satellite pictures. There's intense cloud formation over the Bay but to no avail. The wind shear can mar things, even while all other parameters are favourable."

Vertical shear often destroys all but the strongest storms by literally blowing the updraft away from its base. The shear anomaly is the main reason why the onset phase of the monsoon did not deliver. Higher values of the wind shear means winds are westerly in the lower levels and easterly in the upper levels.

"Such a prognosis simply doesn't allow for the formation of helpful disturbances over the Bay. Westerlies continue to hold strong in parts of Central India and this explains the severe heat wave," Dr Gupta said.

On the western flank though, the Arabia Sea arm of the monsoon is in good health, and is able to feed the Bay of Bengal sibling to some extent. The southwesterlies are strengthening and conditions are not too bad either for its further progress into the hinterland. The net result is that Bihar, Jharkhand, Orissa and Central India can hope to receive the first showers during the days ahead, but not Uttar Pradesh or plains of the North. Here is where a properly behaving Bay of Bengal arm can make all the difference.

Dr Gupta said the wind shear anomaly is not an unusual occurrence, and ruled out the possibility of any external factor, El Nino included, being involved. Even in the Pacific, which borders the Indian Ocean to the east, high vertical wind shear has interfered with formation of cyclones. It is quite natural for this to happen, he added.

In its update on Sunday, the NCMRWF said the southwest monsoon further advanced over some more parts of east central Arabian Sea, south madhya Maharashtra, parts of north interior Karnataka, entire Rayalaseema, parts of Telangana, coastal Andhra Pradesh and some more parts of west-central and north Bay of Bengal. The northern limit passed through Mumbai, Pune, Kurnool, Kakinada, and Gangtok.

Model prediction suggests that monsoon may progress further and cover entire Konkan and Goa, south Gujarat region, more parts of madhya Maharashtra, parts of Marathwada, some more areas of Telangana, entire coastal Andhra Pradesh, parts of Orissa, entire west Bengal, parts of east Bihar and Jharkhand during the next five days.

Severe heat wave conditions continued to prevail over many parts of Orissa, Jharkhand, Bihar, Chhattisgarh, Uttar Pradesh and parts of coastal Andhra Pradesh.

Heat wave conditions also prevailed over parts of Madhya Pradesh, west Rajasthan, north madhya Maharashtra and Vidarbha.

Improvement in severe heat wave condition is expected in Orissa, Coastal Andhra Pradesh and madhya Maharashtra during next 3-4 days with considerable fall in day temperatures over these areas. Day temperatures are also likely to fall over Bihar and Chhattisgarh.

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