HEAVY showers and severe thunderstorms could lash the northern parts of NSW this afternoon and in the coming days, with the Bureau of Meteorology urging people to monitor warnings and updates over the coming days.

BOM meteorologist Gabrielle Woodhouse said an inland trough was generating widespread storm activity throughout NSW, which could see rainfall continue for the rest of the week.

"What we're expecting is for that inland trough to shift east, and we may see rainfall enhancing through parts of the Mid North Coast, the Hunter as well as Sydney and Illawarra overnight and through to Thursday morning," she said.

"At the moment there's a lot of moisture in the atmosphere and that means we can see a lot of rainfall with thunderstorms, as well as these troughs that can produce a decent amount of rainfall.

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"We are looking at continued showers and thunderstorms across eastern and southern parts of the state today, and we are looking at severe storms particularly through the North West Slopes, northern parts of Central Slopes and out towards the Northern Rivers.

"These storms could produce damaging winds, heavy bursts of rainfall and some hail as well.

"We are expecting storms to be more widespread and it's more likely that we will see a number of severe storms this afternoon.

"As we move into Friday we will begin to see another system enter the west of the state, and that's going to bring renewed thunderstorm activity."

Ms Woodhouse said the forecast was for thunderstorm activity to be mainly across the south of NSW on Friday before again spreading further across much of NSW.