At the same time, hailstorms were observed at isolated places over Himachal Pradesh, east Rajasthan, Madhya Pradesh, Vidarbha, Chhattisgarh, Madhya Maharashtra and Marathwada on Wednesday. On April 27, IMD said rainfall/thundershowers was recorded at Jammu & Kashmir, many places over Sub-Himalayan West Bengal, Sikkim, east Madhya Pradesh, Vidarbha region in Maharashtra, Kerala, Mahe, as well as at a few places over Himachal Pradesh, West Madhya Pradesh, Arunachal Pradesh, Assam, Meghalaya and Gangetic West Bengal. There is a hailstorm/ thundershower warning for nearly all regions of the country for Friday.

This is likely to be a result of intense heating in April over some parts of the country in combination with other simultaneous weather systems that are currently at play. Earlier this month, the heat was concentrated over eastern parts of the country particularly Bihar, West Bengal, Sikkim, Odisha and northeastern states. Gangetic West Bengal recorded heat wave conditions for nine days at a stretch starting April 11.

Barely two months ago, the country experienced its hottest February since 1901, the earliest year for which data is available. The maximum temperatures were several notches above normal over most parts of the country, especially in the west coast, touching 29.54 degrees Celsius as the monthly national average. The average maximum or day temperature was 1.73 degrees Celsius above normal all across the country during February and the average minimum temperature, 0.81 degree C above normal.

Interestingly, similar thunderstorm activity was recorded in mid-March also over several parts of the country which impacted the wheat crop in key producer states. Early heat in February and March led to very warm land surface temperatures and onset of widespread pre-monsoon activity in March, meteorologists had said.

mAt present, maximum temperatures are normal or below normal over most parts of the country except over Tamil Nadu, Puducherry, Karaikal, Kerala and Mahe where these are above normal by 2-3 degrees C. Maximum temperatures are very likely to be below normal to near normal during the next five days, the IMD said on Thursday.

Multiple weather systems are at play, said M Mohapatra, director general, IMD.

“We had forecast such thunderstorm activity. There was heating in April which makes conditions conducive for triggering thunderstorms. There is an anti-cyclone [areas of sinking air which result in high pressure ] over Bay of Bengal, there is also a trough or circulation over Madhya Pradesh and over south Tamil Nadu. A western disturbance over the Western Himalayan region is also triggering rain and thundershowers. This is likely to continue for 2-3 days followed by another spell. So, multiple weather systems are at play, at present,” he said.

A western disturbance as a cyclonic circulation is lying over south Pakistan and the neighbourhood. A cyclonic circulation is also lying over central Pakistan in lower levels. A trough/wind discontinuity runs from west Vidarbha to north Tamil Nadu in lower levels according to IMD. All of these systems act together to bring thunderstorms and hail to different parts of the country.

“Instability is very high in the atmosphere. We are expecting rain over almost the entire country after two days including over the South Peninsula, Central India, West India including Gujarat and Rajasthan and MP. A western disturbance is expected to impact the Western Himalayan region again around April 26 which will bring light rain over Punjab and Haryana on April 27 and 28… a north-south trough is expected to develop from Tamil Nadu to East Rajasthan and these multiple weather systems are expected to bring widespread rain and thundershower activity almost across the country till May first week. This is classic pre-monsoon activity and is very similar to what we saw in March,” said Mahesh Palawat, Vice President, climate and meteorology at Skymet Weather.

“What is unusual is that thundershower activity is likely to be prolonged, and over a very vast region covering almost all parts of the country. Clearly, the heat prediction for April was wrong. It will not heat up anymore,” he added.

“Heat can cause atmospheric instability and high clouds needed for development of thunderstorm activity. Both December and February were very warm this year but we cannot immediately say that early heating is the main reason behind widespread thundershower activity,” Palawat said.

“In fact it’s the other way round. The location of the anticyclones and impact of planetary waves may have led to early heat this year. Every year is very different. Last year, there was almost no thunderstorm activity in March. In 2014 there was a lot of hailstorm activity over central India,” said Soma Sen Roy, senior scientist at IMD.

Anticyclones over the ocean are pumping in moisture over the land. All these conditions are helping in creating ideal conditions for pre-monsoon showers, M Rajeevan, former secretary, ministry of earth sciences, said.

“During this season, there are three features that impact weather. The location of anti-cyclones over Bay of Bengal and Arabian Sea; western disturbances and easterly waves. This year the troughs associated with the western disturbances are very deep. There are anticyclones over Bay of Bengal and Arabian Sea which are much stronger than normal and on the trough line between them is the region where thunderstorm activity is taking place. A lot of moisture from the ocean is pumped over this region. There are easterly waves which also influence thunderstorm activity. In a couple of days, we are going to see thunderstorm activity across the country till the first week of May,” Roy explained.