Heat wave conditions over North-West, Central and adjoining East and Peninsular India, though delayed, is good news for the South-West monsoon, as it turns round the corner off the Somali coast in East Africa, and inches its way across the Arabian Sea towards Kerala.
A cyclonic circulation has been lying in wait over the South-East Arabian Sea shifting position between off the Kerala coast and over the Lakshadweep Islands to host the incoming flows and direct it towards Kerala, adjoining interior South Peninsula as well as along the West Coast.
Likely monsoon vortex
This circulation could thus go on to evolve as a monsoon onset vortex, which is any synoptic disturbance that forms just before or with the onset of the monsoon, and drags the associated current northwards along the coast and helps in the northward advance of the seasonal rains.
Onset vortices occur over the so-called Arabian Sea mini-warm pool, where the sea surface temperature (SSTs) peaks just before the onset of monsoon. A warm pool with SSTs in the 30-31 degrees Celsius range already exists over the South-East and adjoining East-Central Arabian Sea.
Low-pressure area
Threshold SSTs of above 27.5 degrees Celsius are known to support convection (evaporation and process of cloud building) and sets up and sustains low-pressure areas/depression. It is argued that high SSTs are a necessary but not sufficient condition to support deep convection.
Global weather models as well as wind profile projections by India Meteorological Department (IMD) seem to agree on the formation of a low-pressure area by the month-end or early June, but differ in their views with respect to the track of movement and scale of intensification.
Some model forecasts take the system towards Karachi or Gujarat, while others project a familiar west-north-west track towards Yemen. Yet others see the prospect of a twin storm forming over the South-West Arabian Sea off Somalia, and tracking towards Yemen.
Meanwhile, the IMD has maintained a forecast for winds to pick up over the next two days over the South-East Arabian Sea (between Maldives and Lakshadweep) to near-depression strength.
Fishermen have been advised to not to venture out into these sea areas.
Rainfall is forecast to escalate over Kerala and Coastal Karnataka, among other regions in the South Peninsula.
Sri Lanka issues alert
Sri Lanka too has taken cognisance of the ‘atmospheric disturbance in its vicinity’ and said that the showery conditions over the islands are expected to enhance from today. The monsoon arrives here normally around May 22, but the country’s national forecaster has not declared it yet.
Showers or thundershowers will occur at times in the western, Sabaragamuwa, central, north-western, southern and Uva provinces and Ampara and Batticaloa districts. Heavy rainfall above 10 cm is likely over Kegalle, Rathnapura, Kaluthara, Nuwara-eliya, Galle and Matara districts.
Wind will be south-westerly, which are monsoon-friendly, and speeds can scale up to 40 km/hr at times over the island. Temporary localised strong winds (up to 70-80 km/hr) can be expected during thundershowers, the Sri Lanka Meteorological Department said.