Perhaps it's no surprise that the massive storm that dumped the most rain on Sydney in a single day in more than 13 years, blew over huge trees and scoured beaches would in some parts of the world be defined as a cyclone.
As Rob Sharpe, a meteorologist at Weatherzone notes, wind speeds were gale force or stronger for parts of the city and up the coast since midnight Tuesday and even earlier.
"They have been consistently gale force. Gale force is the threshold for being Cat-1," Mr Sharpe said, referring to the lowest level of a cyclone. "We have seen that in Sydney and the Hunter."
In North America and Africa, east coast lows of the type endured by Sydney over the past couple of days are described as "east coast cyclones", according to the Bureau of Meteorology's website.
"From the wind statistics you can see that the Tropical Cyclone criteria for a sustained one-minute wind gust has been met," Agata Imielska, a senior climatologist at the bureau, said. "The more important point here is that the winds are damaging. Generally a threshold wind gust of 90 km/h or more is used to define damaging winds."
Both the bureau and Mr Sharpe are keen to distinguish the storm from tropical cyclones that originate far closer to the equator and typically draw their strength from the much warmer waters to the north – while being dubbed names such as "Larry", "Pam" or "Solo".
While east coast lows can develop from ex-tropical cyclones meandering southwards, their origin at this time of year is more usually within a pre-existing trough of low pressure.
For the current event, a relatively weak low-pressure trough on the coast was joined by "a really pronounced upper level trough" of cold air that had moved in from Victoria, Mr Sharpe said.
"Those two systems then combined and we saw that [cyclonic] rotation," Mr Sharp said. "That rotation was created with the warm moist air along the coast interacting with the really cold upper level air inland."
The strength of the current event was driven by the steep gradient between the warm over the Tasman and cool air at upper levels of the atmosphere, making for a classic east coast low set-up.
At this time of year, sea-surface temperatures off Sydney are about the warmest they get during the year, and this year they have been abnormally warm – right around Australia.
East coast lows "tend to peter out pretty quickly but that doesn't stop them being a very powerful system," Mr Sharpe said.
According to Manly Hydraulics Laboratory, wave heights off Port Kembla were literally off the charts, exceeding 8 metres:
According to Ed Couriel, the principal engineer at the Manly Hydraulics Laboratory, the highest wave recorded off Sydney was a whopping 13.6m, at about 11am on Tuesday.
The maximum significant wave height, a more general measure, was 7.3m, making it the second highest in records going back to 1987, Mr Couriel said..
Last night's high tide came in a 2.13m, implying a storm surge related to the event of about 0.25m, he said.
Rain and wind
Sydney's rainfall to 9am on Tuesday was 119.4mm, the most in any day since February 2002 and the most for an April day in 17 years, according to Weatherzone.
Wind speeds were also impressive with Wattamolla to Sydney's south recording at least 100 km/h winds every hour since just after midday on Monday. Just after 9am on Tuesday they were clocked at 135 km/h, or well within the range of a category 2 cyclone if sustained.
Norah Head and North Head to Sydney's north were also reporting strong gusts, with Norah Head also clocking 135 km/h before 5am on Tuesday.
That's the strongest winds for Norah Head since 2007, the same time as another powerful east coast low drove the commodity carrier, the Pasha Bulker, on to shore, according to Brett Dutschke, a senior forecaster at Weatherzone.
The highest significant wave height recorded off Sydney during the 2007 event was 6.9m, placing it now fourth in Manly Hydraulics Laboratory records, Mr Couriel said.
Relief in sight
The good news is that the deep low is likely to weaken and move away from the coast late on Tuesday - but not before dumping as much as 50mm more of rain on Sydney, Mr Sharpe said.
The heaviest falls so far have been at Dungog, north of Maitland, where the town copped a severe thunderstorm cell. That storm dumped about 145mm on the town in three hours, leaving a total of 312mm for the 24 hours to 9am, the highest in more than a century of records, according to Bureau of Meteorology figures.
Paterson's 242.6mm was also its highest daily rainfall for records going back to 1967, Ms Imielska said.
Williamtown recorded its wettest April and autumn day, with 155.6mm. The tally was the town's third-highest rainfall for any month in records going back to 1942, she said.
Aside from raincoats, woollens are also coming in handy. Monday's maximum of 17.2 degrees in Sydney made it the coolest April day in seven years.
Tuesday may struggle to reach even that, with a top so far of just 15.5 degrees at 8.48am.
Weatherzone is owned by Fairfax Media, publisher of this website.
