Southwest monsoon is expected to set over Kerala during the next two to three days, the Indian Meteorological Department said on Friday.
But in a departure from the script, the monsoon is expected to touch the northeast portion of India first.
Heavy rain is predicted in Arunachal Pradesh, Assam, Meghalaya, Nagaland, Manipur, Mizoram, Tripura, sub-Himalayan West Bengal and Sikkim during the next 48 hours, said Dr N Jayanthi, deputy director of meteorology (weather forecasting).
She said the onset of southwest monsoon over the mainland is first over Kerala and then it advances to northeast and rest of the country. But this time, she added, the monsoon is all set to arrive in the northeast first.
"Though such events are rare, there were earlier occasions also when monsoon advanced over northeast first and then to Kerala," she added.
About the unusual and prolonged heat wave conditions this year, she said it could be attributed to a variety of factors. This includes sweep of north Westerlies from higher latitude to east coastal areas, delayed and extended effect of the cyclone, which formed recently over the Bay of Bengal.
Elsewhere, Kolkata witnessed a short spell of pre-monsoon showers in the last 24 hours leading to a marginal dip in temperature.
The mercury came down to 36 degrees Celsius from Thursday's 37 degrees. The minimum temperature was recorded at 25.6 degrees Celsius.
Regional MET office sources said the heat wave was expected to gradually ebb in the coming weeks.
In the west, Nagpur experienced a little relief from the heat wave conditions with the temperature down to 44.6 degrees Celsius from Thursday's 45.2 degrees.
The mercury had soared to an all-time high of 47.7 degrees Celsius earlier this week
Temperatures cooled a bit in Vidarbha region, which had been facing torrid heat conditions in the last few days.
In the north, Delhi saw sporadic showers and high-velocity dust-storms, providing a welcome break from the stifling heat.
The maximum temperature in the national capital touched 43.1 degrees Celsius, two degrees above normal, and the minimum was 30 degrees, also two degrees above normal, the MET office said.
The MET office forecast, for next 24 hours, was a partly cloudy sky with dust storm and minor showers at some places.
Weather office said the heat wave conditions are showing signs of abating in south Rajasthan, Punjab, Haryana, Uttar Pradesh and Jharkhand.
Several parts of peninsular India also received pre-monsoon showers.