As flooding and evacuations continue across Far North Queensland, locals are wondering how a relatively low-level cyclone could wreak such havoc.
The state may be used to flooding and heavy rains, but emergency services say these floods have reached new levels.
More than a century of flooding records were broken in the rivers outside Cairns and the Daintree as former tropical cyclone Jasper dumped relentless rain across the region, despite flood warnings remaining in force.
According to Queensland Police Commissioner Katarina Carroll, these falls were also “unexpected”.
So what happened?
The stalled system opens the tap for Daintree and Cairns
Central Queensland University climate scientist Steve Turton said the flooding was ultimately due to the movement and pace of former Cyclone Jasper, which unexpectedly stalled.
He said models showed the remnants of the system moving into the Gulf of Carpentaria, “taking the rain with it.”
But instead it landed just inland from Kowanyama, establishing what he called a “stationary convergence zone” and opening the tap to the region.
“Because it stood still, it brought very moist air down its northern flank, and this encountered air rising from a ridge of high pressure in the Tasman Sea, bringing equally moist southeast trade winds,” he said.
“This convergence [of winds] was the main driving mechanism that pushed the air up and released this very saturated air in the form of rain.
Professor Turton said the region's mountains also provided an additional “buoyancy”.
“And it's like a tap, and it's going to work pretty well until the bottom eventually either weakens or moves away,” he said.
The result was two days of continuous rain and an emergency that quickly spread across the region.
BOM warned for several days of landfall by a cyclone crossing as a Category 2 system. (Source: Bureau of Meteorology)
Figures from the Bureau of Meteorology show over 2 meters of rain fell at some gauges in the Mossman Gorge region and 1.9 meters at Kuranda station.
The Bairds rain gauge showed 870 millimeters at 9am on Monday, the third highest Australian 24-hour rainfall record and the heaviest across Australia since 1958.
Mossman South and Whyanbeel Valley both had over 700mm, which were records for these locations.
Meanwhile, Cairns received over 600mm of rainfall over the course of the event.
It is this perseverance that has made the event so remarkable, says Professor Turton.
“It is due to the amount of rainfall and the fact that it fell over a period of at least two to three days,” he said.
“That’s what makes it extraordinary.”
Similar to the 2019 Townsville flood
Professor Turton said there had been similar events in Australia before, including the Townsville flood disaster in 2019 and the Kimberley flood disaster earlier this year.
“The flooding in Townsville was caused by a similar blocking high in the Tasman region and essentially by a stationary low pressure system near Julia Creek,” he said.
“And basically on this occasion the convergence zone was directly over Townsville.
“They didn't get as much rain as this event, but for them it rained as much as a year in about a week.”
The 2019 Townsville floods (pictured) are similar to those in Cairns. (AAP: Andrew Rankin)
In January this year, former Cyclone Ellie, a tropical depression, caused Western Australia's worst flooding event over the Kimberley region after circling the region for days.
He said what makes this event a little more unusual is that it did not form in the monsoon valley like the other two events, but rather occurred during an El Niño event.
Short lead time for forecasters
Floods cover a road in Cairns as the remnants of Cyclone Jasper inundate the region. (ABC News: Kirsty Sexton-McGrath)
Ahead of former Cyclone Jasper, the Bureau of Meteorology (BOM) had warned of the category two system near Port Douglas for several days before it made landfall.
As well as damaging winds, these warnings also included flooding and flooding – particularly for areas on the coast and in the mountains between Cape Flattery and Port Douglas.
However, BOM senior meteorologist Laura Boekel confirmed that the lead time for forecasting extreme and persistent rainfall was short.
She said BOM started issuing warnings on Saturday afternoon when they saw the situation changing and rains starting overnight from Saturday to Sunday.
“We talked about it [the cyclone] over a week, but with the widespread rainfall we've seen, that's quite unusual and very complex in these tropical systems, so it was one of those situations where the lead time was greatly shortened,” she said.
“It moved very quickly as we didn't experience much reprieve in the following 24 hours.
“We saw very heavy rainfall in some areas with no respite.”
The BOM says severe weather will no longer occur in northern Queensland, but several major flood warnings remain in force.