IMD update: Monsoon may start to exit North-West India in five days

Gujarat, Punjab, and parts of Indo-Gangetic plains may experience dry weather from Saturday ahead of start of withdrawal of monsoon, according to European Centre for Medium-Range Weather Forecasts. | Photo Credit: www.tropicaltidbits.com

The monsoon is beginning to loosen its grip on North-West India, with conditions turning favourable for withdrawal from parts of West Rajasthan in the next five days, around September 19, about two days later than the normal date.

The retreat is unlikely to begin with a clean break. A rain-bearing low-pressure area continued to linger over south-east Rajasthan and adjoining north Gujarat, keeping the atmosphere sufficiently moist and unstable to produce heavy rain in parts of the region.

India Meteorological Department (IMD) on Monday said heavy to very heavy rain lashed parts of Haryana, Chandigarh, Delhi and West Madhya Pradesh during the preceding 24 hours. These are among the areas that the withdrawal line would broadly move into once the monsoon begins its retreat from West Rajasthan.

The withdrawal marks a decisive change in the atmospheric regime. The moisture-laden monsoon easterlies will gradually give way to dry westerly to north-westerly winds, sweeping in from the arid interiors of Pakistan, Afghanistan and Iran and across the Arabian Gulf.

The IMD does not declare withdrawal merely because rainfall happens to weaken. It looks for a sustained change in the atmospheric pattern - typically a reduction in rainfall for about five consecutive days; establishment of a rain-suppressing anticyclone; a fall in moisture; cessation of rainfall; and clearer skies.

The withdrawal line also does not necessarily advance southward in a neat, day-by-day progression. It can stall for several days before resuming journey towards Central and eventually Peninsular India .That is why the current spell of rain does not necessarily contradict the first signs of withdrawal.

The broader monsoon picture remains uneven. India’s cumulative rainfall deficit stood at 15 per cent as of Sunday, with the South Peninsula registering the sharpest shortfall at 28 per cent. East and North-East India followed with a 25 per cent deficit, while North-West India and Central India recorded deficits of 10 per cent and 6 per cent, respectively.

The national figure, however, masks substantially larger deficits at the meteorological-subdivision level. The pronounced shortfall over the South Peninsula is particularly notable against the backdrop of the El Niño episode, which has tended to suppress monsoon rainfall unevenly across the country.

On Monday, the monsoon trough ran from Naliya through the centre of the low over south-east Rajasthan and adjoining north Gujarat; Rajgarh; Damoh; Pendra Road; Rourkela; and Digha before entering the north-west Bay of Bengal.

The trough’s continued extension into the Bay is significant: the basin may yet produce another late-season weather system. An upper-air cyclonic circulation lay in wait over the north-west Bay and adjoining coastal Odisha and West Bengal. A circulation over Coastal Andhra Pradesh had merged with this system, leaving the Bay under watch for the possible formation of another ‘low.’

Meanwhile, a secondary trough extended from the circulation associated with the Rajasthan low across Madhya Pradesh and north Chhattisgarh. A western disturbance, in the form of a trough, was running roughly along Sriganganagar in Rajasthan and Bathinda in Punjab. Farther south, another rain-bearing trough extended from South Interior Karnataka to the south-west Bay across Tamil Nadu, keeping the southern weather map active.