The Hindu Business Line : Northeast monsoon seen setting in early

MODEL projections indicate the build-up of favourable conditions for the onset of the northeast monsoon over Tamil Nadu and adjoining States by October 10, at least five days ahead of the normal.

Speaking to Business Line, Dr Akhilesh Gupta of the National Centre for Medium Range Weather Forecasting (NCMRWF) said the withdrawal of the southwest monsoon from the northeast takes place more or less concurrently with the onset of northeast monsoon in the southern peninsula.

The ongoing wet session over the northeast is likely to dissipate in another 2-3 days, leaving the field open for northeasterlies blowing in to set up a helpful east-west wind shear zone (noted for turbulence and storm activity) in the lower tropospheric levels. Onset of the northeast monsoon, also called the retreating monsoon, is the logical conclusion to this scenario.

According to Dr Gupta, the onset of the northeast monsoon this year is unlikely to be spectacular. The monsoon flows might take time to consolidate and will peak later only in stages.

Unlike monsoon westerlies, the easterlies lack in depth. One event that could strengthen the easterly flows is the shifting of the resident anti-cyclone to lower latitudes. This is expected to happen in tandem with the rain-driving low-pressure area in the northeast weakening.

The monsoon easterlies will then get drawn in to the strong wind pattern typically observed to the south of anti-cyclone, gaining in speed and depth. Isolated rains have been forecast for the southern peninsula over the next three days. The rainfall activity over Tamil Nadu and south coastal Andhra Pradesh is likely to increase thereafter.

In its weather update on Wednesday, the NCMRWF said the southwest monsoon has further withdrawn from east Uttar Pradesh, parts of Bihar, Jharkhand, Chhattisgarh and east Madhya Pradesh. The withdrawal line passed through Raxaul, Motihari, Patna, Daltonganj, Pendra, Seoni, Khandwa and Surat.

During the 24 hours ending Wednesday morning, heavy to very heavy rainfall has occurred at a few places over sub-Himalayan West Bengal, Sikkim and west Assam. Scattered rainfall also occurred over Kerala, Tamil Nadu and south Interior Karnataka. Chief amounts of rainfall (cms) are: Dhubri and Cooch Behar-17 each; Jalpaiguri-12; Coimbatore-7; Chitradurga and Alappuzha- 5 each.

Tuesday's `low' over Gangetic West Bengal has become less marked. However, associated upper air cyclonic circulation extended up to 2.1 km above sea level. Under its influence, fairly widespread rainfall with heavy falls is likely over sub-Himalayan West Bengal, Sikkim and Assam during the next two days.

The upper air cyclonic circulation extending in the middle tropospheric levels over southeast Bay of Bengal has since shifted to southwest Bay of Bengal.

The western disturbance as an upper air system over Jammu and Kashmir and neighbourhood has moved away eastwards.

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