A boy rides a cycle on a Kochi beach as clouds gather above on Tuesday. (PTI)

New Delhi, June 5: The monsoon advanced over Kerala today but its weak current is initially likely to deliver only light rainfall rather than heavy downpours, India Meteorological Department (IMD) scientists said.

They added that it was still too early to predict when the monsoon might arrive in Gangetic Bengal and Calcutta.

Based on numerical criteria for rain and wind conditions, the IMD said the monsoon had advanced over the whole of Kerala and parts of coastal Karnataka, southern Tamil Nadu and southern and east-central Bay of Bengal.

According to the IMD, the weather appears favourable for further advance of the monsoon over more parts of the Bay of Bengal, parts of the Northeast and the west coast up to southern Maharashtra in the next 48 hours.

“But the flow (monsoon current) seems weak,” a senior weather scientist said.

He said the current was not strong enough yet to deliver heavy rainfall over the southern peninsular region, most of which should have been drenched by today under normal onset conditions.

Scientists believe that the western Pacific typhoon, named Mawar, has contributed to both the slight delay in onset and the weak onset current by sucking moisture and wind from the Indian Ocean region towards itself.

“The typhoon took away some of the energy from the region; so it might take a few days for the current to strengthen,” said D.S. Pai, a senior scientist at the IMD, Pune, and director of the agency’s long-range forecasts division.

The IMD has predicted heavy rainfall at one or two places along coastal Karnataka and Goa and in parts of north Bengal and the Northeast.

A senior scientist said last week that the severe heat wave over parts of Gangetic Bengal, Odisha and coastal Andhra Pradesh was likely to wane only through thunderstorms or monsoon activity. Yesterday, Panagarh in Bengal recorded India’s highest temperature at 46.5°C.

The IMD said the weather conditions in Kerala had fulfilled the criteria for the monsoon’s onset — at least 60 per cent of the weather stations should report 2.5mm rainfall or more for two consecutive days, along with certain westerly wind conditions.

Nine of the 14 stations (64 per cent) fulfilled the onset criteria yesterday and 12 (86 per cent) today. But scientists said the trace rainfall that Thiruvananthapuram received yesterday and the 0.5mm it witnessed today reflected the weakness of the current.

However, IMD stations further north along the Kerala coast and Mangalore received more rainfall: 58mm today in Kozhikode, 38mm in Kannur, 47mm in Kudulu and 43mm in Mangalore.

The forecasters expect the atmospheric conditions over both the Arabian Sea and the Bay of Bengal to turn favourable for copious rain over peninsular India in the coming weeks.

Under its normal dates of advance, the monsoon covers the entire country by July 1.

The IMD’s long-range forecasting division has predicted a 47 per cent probability that the monsoon rainfall will be normal, and a 24 per cent probability that it will be slightly below normal — that is, within 90 per cent of the long-period average.

In 2011, the monsoon had hit Kerala on May 29. Thiruvananthapuram Met office director K. Santosh played down the delayed arrival this year, saying: “The usual window is from May 25 to June 8.”

He added: “On May 15, we had predicted that the monsoon would touch down within four days of June 1, plus or minus. The margin of error of four days was set after taking into account a whole lot of factors that influence the onset of the rains.”

The IMD’s weather forecast warned of “rainfall exceeding 6cm at isolated places in Kerala till June 6 morning and exceeding 7cm in Lakshadweep”.