The Hindu Business Line : Westerly trough delays monsoon withdrawal

Thiruvananthapuram, Sept. 6 A trough in the inward bound westerlies seems to have set the ‘monsoon withdrawal clock’ back by at least five days with the western Himalayan region and adjoining plains in northwest India bracing to receive widespread to fairly widespread rainfall.

Normally, the monsoon prepares to withdraw around this time under the influence of the seasonal anticyclone (marked by high pressure, dry, sinking air and clear skies) ambling in from across the border.

But the presence of the rogue trough means that the anticyclone would need to look for a ‘much farther window’ for making its eastward-bound foray into northwest India. A prevailing weak monsoon phase has brought a seasonally migrating rain belt active over east and northeast India, while an offshore continues to empty moisture along southwest coast.

TROUGH DIPS INTO BAY

The eastern end of the monsoon trough has reverted back to probe the Bay of Bengal waters with its axis being routed through Ganganagar, Pilani, Allahabad, Daltonganj and Sagar Islands on Thursday. International weather models continue to suggest that the Bay would wake up to some activity by the middle of this month.

The prevailing weak monsoon will, as is its wont, unleash some meaningful rain along the southeast coast with south coastal Andhra Pradesh and north coastal Tamil Nadu being immediate beneficiaries. Satellite pictures showed associated clouds hanging over the region.

The offshore trough from Maharashtra coast to Kerala coast persisted. Wind chart forecast for the next 24 hours showed southeasterlies-to-southerlies becoming favourable southwesterlies along the extreme southwest coast. Rains are expected to sustain here through the week ending September 13, but will taper off going into the next.

CYCLONIC CIRCULATION

An India Meteorological Department update said on Thursday that the previous day’s upper air cyclonic circulation over Gangetic West Bengal and neighbourhood was located over Jharkhand and adjoining West Bengal and Bihar.

Under its influence, widespread rainfall with scattered heavy to very heavy falls is likely over sub-Himalayan West Bengal, Sikkim, the Northeastern States and Bihar during the next four days. Fairly widespread rainfall with isolated heavy to very heavy falls is also likely over East Uttar Pradesh, Jharkhand, Orissa and Gangetic West Bengal.

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