About 2000 homes in Sydney's northern suburbs are without power after a storm passed through Sydney's northern suburbs this afternoon.

The storm passed offshore about 4pm.

Sydney storm clouds gather again. Photo: Charles Foster

Electricity to 1200 homes near Hornsby and Asquith and 800 homes in Turramurra have been without power since about 3.30pm, when the storm struck. An Ausgrid spokesman said the homes affected were in an area that was also hit by storms on Sunday. 

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The storm brought 10 millimetres of rain to the Wahroonga monitoring station.

Anthony Duke, a meteorologist at the Fairfax-owned Weatherzone service, said a pocket of dry air had made the storm less severe than earlier projections had suggested.

Weatherzone: Sydney radar

Darwin-like conditions

Over the past week or so, a slow-moving inland low-pressure trough has directed moisture-laden winds from the north into NSW.

For Sydney, it has meant often sunny, humid mornings being disrupted later in the day by multiple waves of storms.

Temperatures - if not quite the humidity - have matched more tropical locations such as Darwin, with both cities reaching tops of 32 degrees on Monday.

"Because it's been moving so slowly and really stayed in the same area for a number of days, the conditions have remained very similar," Mr Haigh.

"The instability is from the warm moist air near the surface and cooler air in the upper atmosphere," he said.

With the low-pressure trough, winds are converging and have to go somewhere - up. As the moist air cools, clouds form and thunderstorms result.

A southerly change now approaching the Sydney region is likely to break up the week-long pattern, Mr Haigh said, with thunderstorms unlikely in Sydney for Tuesday and Wednesday.

"There's a bit more mobility in the weather systems today, which means tomorrow that the thunderstorms should be focused in the north-east of NSW," he said. "They're not on the forecast for Sydney."

The storms, though, may return as soon as Thursday, with the bureau predicting as much as 60mm of rain for the CBD area.