The Hindu Business Line : Onset of N-E monsoon by Wednesday

Thiruvananthapuram, Oct. 23 International model forecasts on Friday indicated that the north-east monsoon may break over coastal Tamil Nadu and Kerala by Wednesday.

India Meteorological Department (IMD), too, said in its outlook that south Andhra Pradesh, Tamil Nadu and Kerala could witness an increase in rainfall activity from that day.

TROUGH SPREADS

An east-west trough of low pressure has spread itself out across the south peninsula with an embedded upper air cyclonic circulation over south-east and adjoining southwest Bay of Bengal on Friday.

In this manner, the stage has been set for the eventual onset of the northeast monsoon over the next three days, the Noida-based National Centre for Medium Range Weather Forecasting (NCMRWF) said in its outlook.

International models, however, did not give much mileage to the upper air circulation. The system may travel west-northwest into the central Bay but could run into north-south flows off the southeast coast and weaken in the process.

No system development is forecast anytime soon but a likely easterly wave is what models see as precipitating organised showers over the southern peninsula from Wednesday onwards.

IMD satellite imagery showed convective clouds over parts of south Bay of Bengal, Andaman Sea and southeast Arabian Sea on Friday.

Outlook until Monday spoke about the possibility of scattered rainfall over Andaman and Nicobar Islands but isolated over Kerala and Lakshadweep as stray northeasterly winds drop their moisture content.

BAY WARMING

The US Navy’s Coupled Ocean Data Assimilation charts revealed that the entire Bay of Bengal basin has warmed up ahead of the onset of north-east monsoon.

The sea-surface temperatures (SSTs) are above normal by two to three deg Celsius over most parts of the Bay, especially along the coast on Friday. But the warming anomaly is only one deg Celsius to the southeast.

The Fleet Numerical Meteorology and Oceanography Centre (FNMOC) of the US Navy saw organised showers take off along and off coastal Tamil Nadu and adjoining Kerala, Sri Lanka and adjoining Indian Ocean and southeast Arabian Sea from Wednesday.

The rains are forecast to accelerate the next day, with winds whipping up speeds of 15 to 20 knots (28 to 37 km/hr). Combined with the building SSTs, this could set up the platform for storm development.

The FNMOC sees initial signals of this over the southwest Bay, off the southeast coastal Tamil Nadu and northeast Sri Lanka, during this phase.

The US National Centres for Environmental Prediction (NCEP) forecasts, too, points to concentrated churn establishing over the southwest Bay around this time.

RAIN WAVES

Rain waves may push into the southeast coast from October 28, the NCEP said which is in agreement with peer model predictions. The week ending October 30 would see the entire southern peninsula slipping under a wet cover.

The rains are forecast to scale up during the next week (October 31 to November 8) when the Tamil Nadu coast, especially to central and southern parts, could likely get a battering.

Rain cover may extend to over Sri Lanka and adjoining southwest Bay; off and along coastal Tamil Nadu; Kerala; Andhra Pradesh, Orissa, Maharashtra, Konkan and Goa during the week. The wet phase of the Madden-Julian Oscillation (MJO) wave is also forecast to hold strong concurrently.

In line with what could be expected of a wet MJO phase, the Roundy-Albany model for cyclogenesis (birth of storm) anticipates build-up of some activity over the southwest Bay during the period.

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