Sydneysiders will need to keep an umbrella on hand for at least a week - the remainder of the Vivid Festival - as the rain settles in, bringing the city's wettest stretch in months.

The wet weather will hang over Sydney on Saturday, with the chance of a storm threatening to drench the city further after 20-30 millimetres fell across the city from 9am on Friday.

Lighting up the night: more of the wet weather ahead. Credit:Kate Geraghty

The mid north coast and the Northern Rivers region will continue to endure the brunt of the bad weather, after being lashed by heavy rain and thunderstorms on Friday.

A severe weather warning, including flash flooding, have been issued for both areas, amid forecasts of rainfall totals in excess of 200 millimetres over the weekend.