MUMBAI, June 4 (Reuters) - Monsoon rains hit the coast of India's southernmost state of Kerala on Thursday, three ‌days later than usual, the weather office said, offering respite ‌from a gruelling heatwave that has driven power demand to a record high.

The June-September ​monsoon rains - critical for India's nearly $4 trillion economy, Asia's third-largest - usually begin to lash Kerala around June 1 before covering the entire country by mid-July.

The monsoon delivers almost 70% of the rainfall needed to water ‌farms producing rice, corn, cotton, ⁠soybeans and sugarcane and to replenish aquifers and reservoirs.

Last month, the India Meteorological Department forecast an El Nino-weakened ⁠monsoon in 2026 that will bring the lowest rainfall in 11 years, fuelling concerns over crops, food prices and growth.

The southwest monsoon has covered ​the ​entire state of Kerala and parts ​of neighbouring Tamil Nadu, the ‌IMD said.

Conditions are favourable for it to advance further into Goa, parts of Maharashtra and Andhra Pradesh, additional areas of Karnataka, and the remaining parts of Tamil Nadu over the next two to three days, it said.

Several Indian states are reeling under heatwave conditions with temperatures ‌soaring above 40 degrees Celsius (104 degrees Fahrenheit), ​levels that usually ease with the arrival ​of monsoon rains.

India is ​forecast to receive below-average rainfall in June, less than ‌92% of the long-period average, the ​IMD said last ​month.

Planting of summer-sown crops is unlikely to be affected if the monsoon covers the rest of the country on schedule, as ​it has done ‌in many years despite a delayed onset, said a Mumbai-based ​dealer with a global trade house.

(Reporting by Rajendra Jadhav; Editing ​by Christopher Cushing and Joe Bavier)