As the calendar ticked over into 2023, downpours in Western Australia's north were raising water levels in the Fitzroy River to astonishing heights.
It has now been described as WA's worst-ever flooding event.
And yet the type of weather system that caused such immense chaos – a tropical low – was not uncommon, nor did it break any significant rainfall records.
So how did it happen?
According to the Bureau of Meteorology's WA manager James Ashley, it came down to an unfortunate combination of three key factors: time, energy, and structure.
And he said the result was "exponential".
Days spent circling the Fitzroy
As ex-Tropical Cyclone Ellie circled the north, it poured heavy rainfall over the region.
Up to 800 millimetres of rain fell in some locations in the space of a week – more than Melbourne receives on average in a year.
But Mr Ashley said the rainfall figures alone were not unheard of.
"When you think of these record floods, with water so much higher than we've ever seen, you'd think there must have been more rainfall than we've ever seen," he said.
"And it was heavy, but we often see tropical lows and cyclones with that sort of rainfall."
Instead, he said it was the time spent over land that made the difference.
The system hit the coast as a low-category cyclone on December 22 and quickly weakened to a tropical low.
But unlike a typical system, which could spend about a week over land before breaking down, ex-TC Ellie spent close to three weeks meandering the country.
A large portion of that stay was spent running laps around Fitzroy Crossing and, more significantly, the entire Fitzroy River catchment, according to Mr Ashley.
The Fitzroy River catchment is 95,000 square kilometres — an area bigger than Tasmania.
Mr Ashley said usually a tropical low would move out of an area after two days, but in this case it remained in the region for five to seven days.
"It's the fact that it was over the catchment for so long, and the whole catchment got heavy rainfall."
The Fitzroy River, from Noonkanbah to Willare, remains at a minor flood level.
Monsoon helped fuel system
Mr Ashley said the system's stamina in maintaining its structure was also unusual.
"There wasn't anything to rip it apart," he said.
With its structure still intact, it was able to fuel itself for longer, drawing in moisture from the tropical north which it would dump over the landscape.
"It was just a really good environment for a tropical system to be in," he said.
"That vertical nature allows it to keep its structure, continue to suck in the moisture, and continue to dump the rain."
Volume of water 'amazing' and 'exponential'
Mr Ashley said the outcome was astonishing.
"To have it that much higher than the 2002 floods, it's exponential," he said.
"When we were forecasting the path of this system, noticing its slow movement, and maintaining that structure, we certainly were concerned about the flooding impacts.
"But if you had asked if it was going to be almost two metres higher than we've ever seen before in Fitzroy Crossing, I don't think anyone would have predicted that."
He said it was almost hard to comprehend the volumes of water that surged through the river.
"For every centimetre you add in height, the river needs to be so much wider," he said.
"So to add every 100 millimetres to the river height you're probably expanding the river by a kilometre."
At one stage, some points of the Fitzroy River were up to 40 kilometres wide.
Ex-TC Ellie has now dissipated, and water levels are subsiding.
Authorities have said it will be a lengthy and costly clean-up ahead.
