Weekend forecast for rain across the Clarence Valley

THE Queen's Birthday long weekend is set to be a wet one, with the Bureau of Meteorology forecasting heavy rain for the Clarence Valley on Saturday and Sunday, which could continue into next week.

Bureau of Meteorology forecaster Rosemary Barr said the wet weather was being caused by a low pressure trough sitting off the coast of northern NSW and slowly moving north.

"The phenomenon is being driven by a system in the upper atmosphere which is called an upper trough," she said.

"Essentially it's an area where the wind field is changing sharply in direction or speed, and in this case it's both. This creates a wave in the upper atmosphere and this wave has an impact on the lower atmosphere which causes pressure disturbances.

"The combination of this with more moisture in the atmosphere along the coastal fringe will bring with it heavy showers, particularly along the coast.

"This feature tends to be linked to topography and small-scale pressure features, which can see sharply varying rainfall in small areas, so there can be quite a large variation in total rainfall between locations."

The bureau's forecast predicts Yamba could cop up to 70mm today and Grafton could receive 45mm, with a very high chance of showers throughout the day.

Ms Barr said there was a risk of heavy rainfall associated with the low pressure system on Monday and Tuesday.

"Our models don't expect an easing of the conditions until later next week," she said.

NSW State Emergency Service Commissioner Mark Smethurst said it was imperative people prepared themselves now for the severe weather this long weekend.

"People need to consider their travel plans if they're going to be travelling through areas that may be affected by heavy rain and flash flooding," Commissioner Smethurst said.

"If the roads are closed due to flooding, they should delay their travel or find another way to their destination.

"People who deliberately enter floodwater are not only risking their life but the lives of their passengers and our volunteers who have to perform the rescue.

"If residents need emergency help as a result of this severe weather, they should call the NSW SES on 132500, but if the situation is life-threatening they should dial 000."