The harsh 'kathiri' period of summerset in over Tamil Nadu on Monday with the city and the rest of the state bracing for hot days as the Met office forecast mercury levels to be over 40 degrees Celsius next week.Also known as 'agni nakshathiram', the over three-week long hottest period of the summer season, is expected to last till May 29.The maximum temperature is likely to be in the range of 40 degrees C at isolated pockets in Dharmapuri, Salem, Karur, Tiruchirapalli and Madurai districts during the period, according to a forecast by the India Meteorological Centre.On Monday, Karur district recorded the maximum temperature of 40 degrees Celsius, the highest in the state, while Chennai City and Airport recorded 35.3 and 37.2 degrees Celsius respectively.The IMD has forecast temperatures could touch 40 degree Celsius in the city for a day or two but may generally be around 37 degree C during this month.However, the state could get a respite if a system which is forming over the Andaman Sea moves towards Tamil Nadu by the end of next week as that is likely to bring widespread rainfall in many areas including this city.Meanwhile, private weather forecaster S Ramachandran forecast early onset of southwest monsoon (June-September) from the second week of May."The temperature will change from second week of this month. Moderate pre-monsoon rainfall is forecast between the second week of May and the first week of June. The best rainfall benefits of southwest monsoon 2020 will be gained between first week of May and second week of August," he said.He also said 2020 was also likely to become the first year to experience more cyclones in a calendar year.