A pool of cold upper-level air interacting with tropical moisture will cause rain and storms to develop in north-western NSW late on Friday.

This wet and stormy weather will then spread from west to east across some other areas of the state during Saturday and Sunday.

Most inland districts should see at least some rain from this system, although falls will be hit and miss, meaning some areas will see decent rain and others could miss out altogether.

The heaviest rain is likely to fall over far western and southern inland areas of NSW, where widespread totals of 10 to 30 millimetres are a good chance and some places could pick up more than 50mm, particularly with thunderstorms.

There are signs that a passing low pressure trough could bring another round of rain and thunderstorms to NSW at the beginning of next week. At this stage, central and northern inland districts look most likely to see useful rain from this trough, although this may change in the coming days.

The coming week's wet weather comes one week after the US Climate Prediction Centre declared that La Nina was underway in the Pacific Ocean.

While the Bureau of Meteorology is yet to formally declare La Nina, they agree that there is a distinct La Nina-like pattern in the Pacific Ocean at the moment.

La Nina typically causes above average rain over inland NSW during spring and in some areas of the state during summer as well.

Most global climate models predict that La Nina conditions will persist in the Pacific Ocean until at least the end of the year and possibly into early next year.