The northern coast and ranges of New South Wales can expect flooding rain on Thursday night and Friday with widespread falls of 100-to-200mm from Lismore to Newcastle.
The wettest area looks like being the coastal stretch from about Grafton to about Port Macquarie, where there is potential for more than 300mm. The most likely area for rain this heavy is in the Coffs Harbour-to-Kemspey area.
This sort of rainfall in such a short time is enough to cause rivers to rise rapidly and run fast, causing some to overflow their banks. It is also enough to flood properties and make driving hazardous due to the likelihood of a significant amount of water on the ground.
The low pressure system bringing this rain is the same one which brought some flooding to the far north coast of NSW and southeast Queensland a few days ago. The low had since moved offshore, causing rain to ease to showers in the region.
This intense low is heading in a southwest direction, towards the north coast of NSW, taking heavy rain and strong winds with it.
Heavy rain will spread west of the ranges and south along the coast as the low continues to move southwest over inland NSW. Between now and Saturday rain is likely to spread across the western slopes and plains with widespread falls of 10-to-30mm as far west as about Cobar and Lake Cargelligo. Along the coast 50-to-100mm is likely from newcastle to Narooma, including Sydney.
Wind will become strong enough to bring down trees on the north coast.
Once the low pressure system weakens on Saturday night, rain will become patchier and lighter, allowing flood waters to recede fairly quickly. Wind will also become lighter.
However, the atmosphere will stay fairly humid and unstable with northeasterly winds and low pressure troughs redeveloping.
As a result, showers and storms are likely to affect much of the state during the last few days of summer, mainly eastern parts.
For coastal areas this rain will push much of the region well above their summer average. Sydney is on target to have its wettest summer in five years.
Unfortunately, parts of the state's west will fail to reach their summer average. Albury, which is currently on 45mm this summer will struggle to achieve half of its seasonal average of 149mm.
The Albury area has not recorded any rain for 30 days, its longest dry spell in more than three years.
© Weatherzone 2013
12:38 EST Tropical Cyclone Jack has rapidly developed over the warm waters of the Indian Ocean in the past 24 hours, making it the 10th system to be named this season.