A low pressure system will wreak havoc over Sydney on Monday with high rainfall and possible severe thunderstorms forecast for parts of New South Wales .
The east coast of Sydney is expected to receive between 20 and 30 millimetres of rain, although rain will be less heavy in the west, with about 15 to 25 millimetres forecast for Penrith.
Senior forecaster at the Bureau of Meteorology, Mohammed Nabi said it will be a wet morning for commuters in Sydney with rain increasing throughout the day.
"It becomes a bit more prevalent during the day, people get into work and it will just seem like it's raining even more as opposed to easing off," he said.
Weatherzone: Sydney radar
Thunderstorms are likely to form on the Blue Mountains and then move right across the city in the early afternoon and evening.
"There's a chance of thunderstorms with the low, it's going to create a lot of instability over the region," Weatherzone meteorologist, Craig McIntosh said.
Thunderstorms are likely to hit Sydney with the possibility they could be severe.
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"People will probably hear a bit of thunder in the afternoon and see some flashes of lightning," Mr Nabi said.
A southerly change in Sydney in the early evening will bring fairly windy conditions with showers continuing as commuters leave work on Monday.
"There will be strong winds throughout all of NSW coastal waters from later tomorrow but gale force winds will be confined to the Batemans coast, Illawarra coast, and Sydney and Hunter coasts," Mr Nabi said.
"The beaches will see some fairly heavy surf conditions from later tomorrow night, mainly driven by wind."
The south coast of NSW will be most heavily affected, anywhere between Nowra and Merimbula could see up to 100 millimetres of rain over the day.
Severe weather warnings for heavy rainfall and possible damaging winds are in place for south of Sydney.
The Bureau of Meteorology issued a warning on Monday morning, saying that severe thunderstorms in the Illawarra and Sutherland areas could lead to flash flooding, with areas near West Wyalong, Young, Yass and Goulburn also affected.
The Bureau also warned of damaging winds of up to 90km/h affecting the coastal areas between Wollongong and Eden, just north of the Victorian border.
"There is a possibility they could be severe with heavy rain, damaging wind and hail is a possibility as well," Mr McIntosh said.
"Keep an eye on the warnings throughout the morning as well, as potential storms develop."
Temperatures are forecast in the mid-20s for Monday and rainfall will ease moving into Tuesday as the low pressure system moves out towards New Zealand.
"This weather pattern more resembles Autumn leading into Winter than Spring leading into Summer," Mr McIntosh said.
The dreary day of rainfall comes in contrast to last Monday's searing temperatures in the mid-30s.
Weatherzone is owned by Fairfax Media, publisher of the Sydney Morning Herald