Thiruvananthapuram, May 17
The southwest monsoon has set in over the southeast rim of the Bay of Bengal at least one day ahead of the predicted timeline of Thursday/Friday.
The strong westerly flows, moisture feed and persistent cloudiness aided the build-up over the past two days precipitating the onset of the Bay of Bengal arm of monsoon ahead of predictions, said Dr Akhilesh Gupta of the National Centre for Medium Range Weather Forecasting (NCMRWF).
The onset phase saw the Southeast Bay, Nicobar Islands and the South Andaman Sea being covered. Favourable conditions are developing for its further advance into the remaining parts of the Southwest Bay, the Andaman Sea and some parts of East Central Bay within next two to three days.
Predictions clearly suggest increase in moisture incursion into the South Bay of Bengal supported by strengthening of lower level wind flow and persistent large-scale rainfall activity over the region.
Wind speeds have exceeded expectations both at the surface and higher levels partly in unison with the flows that fired Typhoon Chanchu, the fiercest May storm ever to hit Hong Kong and southern China. Reduced to Category 2 status from its destructive highs, Typhoon Chanchu was bracing for a Hong Kong landfall late on Wednesday.
Chanchu seems to have breathed life into the Inter-Tropical Convergence Zone (ITCZ) whose progression to the north of the Equator during this time of the year and subsequent levels of accentuation decide the strength with which successive monsoon pulses bear down on the mainland. The ITCZ is a band of low air pressure air centred on the Equator.
The NCMRWF has predicted that the lower level flows will pick up gradually to set off large-scale cloudiness and rainfall activity over South-Central Arabian Sea and adjoining areas beginning from Tuesday next.
This will have cleared the stage for the Somali Jet, the strong low-level winds and harbinger of monsoon on the southwest coast, to aim its way into the mainland.
The Somali Jet continuously intensifies throughout the month of June, while progressing northward steadily. The heavy rainfall over the southwest coast is best associated with a high mountain terrain and abundant moisture supply transported across the Equator (cross-equatorial flows) by the Somali Jet.
Heat wave conditions
Meanwhile, heat wave conditions prevailed over Northwest Rajasthan where day temperatures were above 45 degree Celsius on Wednesday. The day temperatures are two to three degree Celsius above normal over parts of South Peninsular India and the Northeast.