Sydney is in for a soggy few days as a complex system steers moist air over the coast, with the possibility of heavy falls further north up the coast over the long weekend.

On Wednesday evening, a cloud mass and rain band were bringing rain and thunder over parts of the basin, slowing traffic and causing some localised build-up of water on some roads.

"There will be isolated storms into the evening and into the night," Katarina Kovacevic, a duty forecaster at the Bureau of Meteorology, said.

Rain will be a regular feature for a while yet, potentially disrupting outdoor plans for the Queen's Birthday weekend. "For the next several days and into the long weekend, it looks wet for Sydney and areas north," Ms Kovacevic said.

The bureau is forecasting 15-30 millimetres for Thursday, 10-25 mm for Friday and similar daily totals until Monday.

Given the blowy conditions, it's "better to have wet weather gear rather than just an umbrella", Ms Kovacevic said.

On Wednesday, the city had collected about 25 mm by 5pm to go with the 54.6 mm of rainfall overnight, making it the wettest two-day spell in nearly four months.

Weatherzone: Sydney radar

Most meteorologists were reluctant to dub the current event an east coast low.

"Winds and waves aren't really that significant from this system," Brett Dutschke, a senior meteorologist at Weatherzone, said. The complexity involved "a few small troughs within the broader system".

Wet weekend up north

Forecasters, though, are turning their attention to a  potentially more intense event that may form in coming days.

"At this stage, we are anticipating a coastal trough will develop over northern parts of the coasts," Ms Kovacevic said. "Depending on the evolution of that system we could see heavy falls during the long weekend."

There is a possibility for the system to generate an east coast low off the south-east Queensland coast, she said, adding that predictions are "really quite uncertain at this stage".

According to the bureau, regions from the Mid North Coast northwards are likely to cop the biggest drenching over the coming week.

The bureau's eight-day forecast map (see below) predicts falls as much as 200 mm along the coastal fringe, with even heavier falls offshore.

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