Sydney is set to be drenched by a rain event over the coming days, with the chance of very heavy falls and flash flooding. A low pressure trough off the NSW coast caused showers to develop in the Sydney area on Saturday. By 11am 13mm had fallen at the airport, with 10mm at both Marrickville and Cronulla. Showers are expected to become more frequent and widespread as the day progresses and the trough deepens, with showers increasing to rain on Sunday. There is also a chance of some thunderstorms, particularly near the coast, with a risk of small hail if storms do develop due to a cold upper air mass. The cloud will keep Sydney cool this weekend, with an expected top of 16 degrees on Saturday and 14 on Sunday, making it the coldest weekend since last winter. The real deluge is expected to arrive on Monday with the possibility of an East Coast Low developing, with the amount of rain dependent on where the low forms. If the system develops north of Sydney, there is potential for some parts to pick up well over 100mm by the end of Tuesday. Coastal areas are most likely to be affected. If the low forms further south, Sydney will only see showers on Monday. Either way, this day will see blustery winds, making for a very chilly day. A flood watch has been declared for a range of coastal river basins, including the Wyong, Nepean and Georges Rivers, as a precaution for those living in their vicinity. By Tuesday the rain will ease, with the chance of a few showers hanging around until mid-week.