New Delhi: The southwest monsoon today hit Kerala, three days before its scheduled arrival, the India Meteorological Department (IMD) said.

The conditions are favourable for further advance of the monsoon into some parts of central Arabian Sea, remaining parts of Kerala, parts of coastal and south interior Karnataka, parts of east-central and northeast Bay of Bengal, and some parts of northeastern states during the next 48 hours, it said.

The onset of monsoon over the southern state of Kerala marks the commencement of the four-month-long rainy season in the country.

"Today, the southwest monsoon has further advanced into remaining parts of southeast Arabian Sea, Comorin Maldives area, entire Lakshadweep, most parts of Kerala, some parts of Tamil Nadu and some more parts of southwest, central and northeast Bay of Bengal.  Thus, the southwest monsoon has set in over Kerala, today, the 29th May 2018, three days ahead of its normal date," the IMD said.

June 1 is the official onset date for arrival of monsoon in the country and it takes more than a month-and-half to cover the entire country. The usual date for the monsoon over Sri Ganganagar in Rajasthan, its last post, is July 15.

This is the second consecutive year when the monsoon has made an early arrival. Last year, the annual rainy season commenced on May 30.

Skymet, a private weather forecasting agency and a rival of the IMD, had, however, said that the monsoon made its arrival in Kerala yesterday.

Owing to the arrival of monsoon, isolated places over coastal and south interior Karnataka, Kerala, Lakshadweep, Andaman and Nicobar Islands, Nagaland, Manipur, Mizoram and Tripura are expected to receive "heavy to very heavy rain" tomorrow.

But there will be no respite from hot weather conditions in parts of north and central India.

Heat wave conditions are "very likely" to severely affect parts of west Rajasthan, one or two pockets over east Rajasthan and Madhya Pradesh tomorrow.

The IMD has also issued an alert of thunderstorm accompanied with squall at isolated places in Uttar Pradesh tomorrow.

"Thunderstorm accompanied with gusty winds very likely at isolated places over Himachal Pradesh, Uttarakhand, Punjab, Haryana, Chandigarh, Delhi, Bihar and sub-Himalayan West-Bengal and Sikkim," the weather body said.

According to the IMD, if after May 10, 60 per cent of the available 14 stations- Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudlu and Mangalore- report 2.5 mm or more rainfall for two consecutive days, the onset of monsoon over Kerala can be declared on the second day. This is one of the main parameters for declaring the arrival of monsoon.

Besides this factor, the westerly winds must be up to 15,000 feet above main sea level and outgoing long-wave radiation less than 200 wm-2 (watt per square metre) to declare the arrival of monsoon.

All the necessary parameters were met following which the onset of monsoon over Kerala was announced today, Mritunjay Mohapatra, Additional Director General, IMD said.

The southwest monsoon is responsible for about

70 per cent of the country's annual rainfall and is critical to the economy, that is still largely dependent on agriculture.

The IMD has made a forecast of "normal" rainfall this season. Last year, the MeT department recorded a "below-normal rainfall"