By Tom Saunders, ABC Meteorologist
The weather bureau has issued multiple warnings for flooding rain, gale-force winds and damaging surf for the NSW coast. (ABC News: Ben Clifford ) Photo: ABC News: Ben Clifford
A "bomb cyclone" has formed in the western Tasman Sea, and its intensification shows signs of being even more explosive than initially forecast.
Along with its strength, the other key concern is the system's location - developing near the coast which will ensure significant impacts for New South Wales, including Sydney.
The low's coastal position will also nearly guarantee it becomes the first east coast low in three years, triggering the Bureau of Meteorology (BOM) to issue multiple weather warnings for flooding rain, gale-force winds and damaging surf.
When a low-pressure system transforms from non-existence to a formidable storm just a day later, meteorologists label it a "bomb cyclone", or a system that has experienced "bombogenesis".
The expression "bomb" is due to the explosive speed of development, however, its usage is restricted only to systems where the reduction in pressure exceeds a specific rate based on latitude.
Upgraded warnings for Tuesday now also warn of isolated destructive wind gusts above 125 kilometres per hour between Foster and Wollongong, including parts of Sydney's east.
Rapid deterioration through Tuesday
The pressure began falling on Monday afternoon off the NSW north coast as polar air over the south-east inland clashed with warm humid air off the Tasman Sea.
By the evening, a fully formed low had developed just east of Grafton and it deepened overnight with a central pressure at Port Macquarie falling at a rate of 5 hectopascals (hPa) per 6 hours.
Unsurprisingly rainfall was also increasing, and by 6am more than 50mm had fallen on parts of the Mid North Coast, Hunter and Illawarra.
The low will deepen further on Tuesday while moving slowly south, and is on track for a 24-hour drop in pressure of about 25hPA - well above the threshold for a bomb cyclone of 15hPa, at 32 degrees latitude.
The weather will continue to deteriorate through Tuesday as the low reaches a peak, including an increase in the intensity of rain, wind and surf.
Here is a breakdown of the full impacts during the next 48 hours.
By Monday 6pm, up to 50mm of rain was recorded near the coast. ( ABC News: Ben Clifford) Photo: ABC News: Ben Clifford
Flood watch for more than a dozen NSW-Vic rivers
The heaviest rain on Tuesday will gradually shift south down the coast, spreading from the Mid North Coast through the Hunter, Sydney, Illawarra and upper south coast.
Twenty-four-hour totals from midnight to midnight are likely to average about 50 to 100mm across these districts within 50 to 100km of the coast, although the BOM is now expecting localised torrential falls of up to 120mm in 6 hours along the coastal fringe.
At the higher end of this range, totals would exceed the July average in less than 24 hours - enough to trigger areas of flash flooding, including around Newcastle, Sydney and Wollongong.
River flooding is also possible, although the concentration of heavy falls near the coast is why the BOM's flood watch is only for minor flooding - since moderate and major flood events typically rely on heavy rain spreading further inland across river catchments to the Great Dividing Range.
The heaviest rain and flood risk will then shift to the south coast and Gippsland on Wednesday, and again totals should average up to about 100mm but with pockets above that mark.
It's possible heavy rain on Wednesday will track further inland, which would result in a greater flood threat for the more southern catchments.
Rain will also continue up the coast on Wednesday, including around Sydney, lifting event totals to above 100mm from about Taree to Gippsland, with the risk of isolated regions seeing well over 200mm.
All up there are 20 river catchments under threat of flooding from Wallis Lake near Forster, to Mitchell River east of Bairnsdale.
Well over 100mm of rain is ahead for much of the NSW coast during the next 72 hours. (ABC News) Photo: ABC News
Trees to topple from twin lows
The force of the wind will be fierce during this east coast low due to the very tight pressure gradient around the centre of the storm.
Peak wind gusts should hit close to 110kph on Tuesday along the central stretch of the coast, starting around the Mid North Coast and Hunter, then shifting to Sydney and the Illawarra in the afternoon.
Winds of this magnitude are strong enough to bring down trees and lead to power outages, and under a scenario where the centre of the low makes a close pass to the shoreline, minor property damage and more widespread toppling of trees could result.
A few thunderstorms could also form on Tuesday near the Hunter and lower Mid North Coast, which according to the BOM's storm forecast have the potential to produce destructive gusts over 125kph, and waterspouts.
Destructive gusts may also develop from the Mid North Coast to the Illawarra if the centre of the low makes a close enough pass to the coast.
Strong winds will continue through Wednesday, and possibly extend further north and south from about the Mid North Coast to the Victorian border.
Maximum waves to exceed 10 metres
A consequence of the Tasman gales will be a rapid increase in surf through Tuesday, with combined seas and swells reaching about 5m by Tuesday night.
Waves will continue rising into Wednesday when peaks between Seal Rocks and Batemans Bay could average up to 7m.
Maximum waves are normally double the average and may hit close to 15m.
Surf of this ferocity when combined with unusually high water levels from the low surface pressure is likely to cause significant beach erosion.
The areas most at risk are beaches facing the wind. So in this case, south-facing beaches may see the greatest damage.
While winds and rain along the coast should start to ease by Thursday, there is a lag for surf which will maintain huge waves until at least Friday.
- ABC
