Thiruvananthapuram, May 20 The southwest monsoon is expected to make an onset over the south Andaman Sea and adjoining south Bay of Bengal within the next 24 hours, India Meteorological Department (IMD) said on Tuesday.
Importantly, it indicated the possibility of the onset over Kerala anytime after Friday (May 22) and its advance over the adjoining areas of south peninsula. It had given May 26 as the likely onset date earlier with a model error of four days.
PRESSURE GRADIENT Numerical weather prediction models indicate gradual increase in the monsoon-driving pressure gradient (from peninsular ‘high’ to upcountry ‘low’) over the Bay of Bengal and the Arabian Sea.
This would trigger stronger cross equatorial flows and increase in rainfall activity over south peninsula during the next four days.
Meanwhile, a low-pressure area will form over central Bay of Bengal around Saturday (May 23) and intensify thereafter.
International models saw a tropical cyclone spinning up out of the system later on.
CYCLONE LIKELY The Climate Prediction Centre (CPC) of the US National Weather Services said in its latest outlook that there is an increased chance for tropical cyclone formation in the Bay and further east across into the South China Sea during May 19 to 25.
This is being attributed to the anomalous westerly flows and areas of low vertical wind shear and above average sea surface temperatures (SSTs). While high vertical wind shear kills budding storms, raised SSTs aid storm building.
Numerical weather forecast model guidance also indicates the potential for cyclone formation.
On a scale of three (low, moderate and high), the CPC rated as ‘moderate’ the confidence level in the eventuality.
Interestingly, the CPC has extended a similar guidance into the following week (May 26-June 1) as well but restricted to within the Bay. It cited a similar set of enabling factors as in the preceding case with ‘moderate’ confidence in the eventuality.
MODEL CONVERGENCE Almost similar forecasts are available from the European Centre for Medium Range Weather Forecasts (ECMWF) and cyclone phase evolution modelling by the Canadian Meteorological Centre (CMC) and the US Navy’s Navy Operational Global Atmospheric Prediction System (NOGAPS).
The ECMWF sees the causative ‘low’ springing up over the central Bay by Friday and moving in a north-northeast track over the very warm waters (topping the 31-deg C mark) gathering strength in the process.
It will intensify into a cyclone around May 25/26 and hit the Kolkata coast for a landfall around May 27. Later, the remnant system is shown to move in a northwesterly direction into east India.
ARABIAN SEA ‘LOW’ According to the CMC, the ‘low’ will form as early as Wednesday, move north-northeast and possibly reach cyclonic strength within the three days.
But CMC sees the landfall over Bangladesh, subsequent to which the weakened system would head back into mainland India.
The NOGAPS sees the cyclonic storm being active within the Bay basin from Friday to Sunday after the causative ‘low’ takes birth in the central Bay.
As in the case of the CMC, it indicates the West Bengal coast as a probable landfall area.
Meanwhile, the ECMWF sees a ‘low’ brewing in the Lakshadweep area in southeast Arabian Sea immediately after the Bay storm blows over (around May 29), which is an indication the monsoon flows would be able to hold on and consolidate once again.