SUMMER storms like the ones that tore across the Northern Rivers on Sunday night are likely to continue until March.
Bureau of Meteorology forecaster Ewan Mitchell said the storms were caused by hot and humid conditions on the ground and cold air in the atmosphere.
In Casino, the temperature reached 35.3 degrees, humidity was 37 per cent, while 500kms above the ground the temperature dropped to minus nine degrees.
While the storms brought a dramatic lightning display, strong winds and isolated heavy rainfall to the Northern Rivers, with the State Emergency Service attending one call-out to a Kyogle house with roof damage, its full force was felt in Grafton.
Grafton police and emergency service volunteers were called into action after the storm left a trail of destruction at 4pm.
Over a 15-minute period wind gusts of up to 90km/h were recorded at the South Grafton airport, while heavy rain, thunder and lightning pounded the district.
Seen throughout the Grafton district were powerlines brought down, trees ripped from the ground, roofs torn off buildings and flash flooding.
As the winds calmed, emergency services were bombarded with calls from people reporting damage across the district.
Grafton police received more than 70 calls, with reports of hailstones the size of cricket balls.
There were also reports of hail in Casino and Kyogle.
At least two road accidents were reported, including a car hitting a tree at South Grafton.
Clarence Valley Council deputy general manager Des Schroder said the clean-up bill for council would be about $100,000.
The highest cost would be replacing the stable roof at the Grafton Pony Club, which was destroyed.
During the storm, 5500 houses in the Grafton area lost power.
Mike Hely, from Country Energy, said at one stage there were problems at the South Grafton substation with debris blowing into the lines.
Today there is a chance of an early shower, then fine weather. On Sunday it is expected south-east winds may develop and bring with them more rainfall.