Warragamba Dam on Thursday. It is currently 97.7 per cent full. Credit:Edwina Pickles The shift out in the Tasman Sea could happen over the central or southern NSW coast "but there's almost no agreement between models or even between runs of the same model", she said. The bureau is still expecting the low to move away fairly quickly, certainly much faster than the huge event earlier this month that dumped the most daily rain on coastal NSW on record and led to severe beach erosions in area such as Collaroy-Narrabeen on Sydney's northern beaches. South or north of Sydney? With east coast lows, the strongest winds and heaviest rain typically hit areas to the south of the system, so it is possible that Sydney could get a soaking or only moderate falls.

On current forecasts, the city is expected to receive 15-35 mm of rain on Sunday, with Katoomba expected to collected 10-30 mm. Warragamba Dam currently needs net inflows of about 50 gigalitres of water to spill. Credit:Edwina Pickles The bureau has also pared back predicted falls for south coast town such as Bega, which may get 15-35 mm on Sunday and 8-25 on Monday, less than the earlier forecasts of more than 100 mm. How much rain will fall over Sydney's catchment area is "the hairiest question", Ms Kovacevic said. Warragamba Dam, Sydney's main catchment, last spilled over in August 2015 - and may again on Monday. Credit:Nick Moir

"If the low forms to the north of Sydney, the Warragamba catchment could see a lot of rainfall," she said. If it forms to the south, the catchment "could see probably not very much" rain. During the previous event, Sydney's catchment jumped 12 percentage points in a single week, and were at 97.4 per cent full as of Thursday. Warragamba, which is by far the largest reservoir, was at 97.7 per cent full, according to WaterNSW. Cataract, Avon and Nepean dams are all at about 100 per cent full and would be expected to spill during the coming week. (See bureau chart below for eight-day rainfall totals from Thursday.) For Warragamba, the dam spills when it reaches full capacity, a level that would be reached with net inflows of about 50 gigalitres of water. It last spilled in August last year, and also overflowed on five occasions during 2012-13.

Whether the spills affect communities depends in part on the water levels of downstream rivers. In the case of Warragamba, these include the Grose and Colo rivers and South Creek, all of which flow into the Hawkesbury River. WaterNSW said it is monitoring Warragamba dam in light of the bureau's forecasts for "a significant rain event in coming days". "It is possible heavy rain in the dam's catchment could result in sufficient inflows to cause the dam to exceed full supply level and spill," a spokesman said, adding the agency is working with bureau and the State Emergency Service as is usual for such events. Fairfax Media also sought comment from the SES and Hawkesbury Council. Tristan Meyers, a meteorologist with Weatherzone, said it may be difficult to predict where the east coast low will form "even on the day".

Still, the forecast continues to be that a low will deepen on Sunday and move off the central or southern coast of NSW, Mr Meyers said. The widespread rainfall will take in Sydney and it is possible some coastal pockets could receive as much as 100 mm of rain in a day, he said, adding, "The moisture's there". The swell and winds will get "a jump" as the low moves across the region, but their intensity is likely to be less and shorter-lived than the previous storm, Mr Meyers said. Coastal risks The bureau's Ms Kovacevic said that while tides will be high, they won't be as severe as two weeks earlier, with the maximum swell about 20-30 cm lower.

"This eases the potential for coastal erosion and inundation generally," Ms Kovacevic said, adding that there would still be a chance for localised impacts. Many parts of coastal NSW lost sand and had other protective barriers reduced or destroyed during the recent east coast lows. Further beach erosions is possible after the coming storm, particularly if the angle of the waves is different. East coast lows are intense low-pressure systems that are most frequent in June, and number about seven or eight in a typical year. They tend to intensify rapidly in strength when a pre-existing low-pressure trough meets favourable upper atmosphere conditions. Follow Peter Hannam on Twitter and Facebook. Weatherzone is owned by Fairfax Media, publisher of this website.