BOM released the 2020 Winter Outlook on Thursday which showed parts of inland NSW and the ACT had a high chance of above average rainfall throughout winter.

BOM manager of long-range forecasting Dr Andrew Watkins said coastal parts of the state could expect warmer than average daytime temperatures and across NSW and ACT overnight temperatures would also be above average.

"At the moment we're seeing quite warm conditions out in the eastern Indian Ocean, between Australia and Indonesia," Dr Watkins said.

"That's likely to put a lot more moisture and heat into the atmosphere so we're expecting more cloud to form and move over the country."

"If that interacts with cold fronts and low-pressure systems down to the Tasman Sea, we should see more rainfall over NSW."

Dr Watkins said autumn had also been wetter than average throughout inland NSW, whereas it had been drier and warmer along the coast.

BOM duty forecaster Helen Kirkup said thick fog could be a more regular occurrence throughout winter due to recent wet weather moistening the soil.

"We might be seeing more instances of these fogs. If there's more soil moisture, there's more moisture in the air that creates fog."

She said a front moving through on Sunday would bring a burst of cold air on the first day of winter on Monday with rain expected across the western ride of the ranges.

Ski fields can expect at least half a metre of snow throughout Monday and Tuesday, in time for the traditional first big weekend on the slopes, although due to the pandemic the season has been pushed back to commence on June 22.

BOM will release its autumn summaries on Monday.