There is a lot to get your head around with the weather at the moment.
But here are the answers to five quick questions about the floods.
1. How unusual is this rain?
The rainfall totals from this event have been staggering.
From 9am Thursday to 9am Monday three stations recorded over a metre of rain:
– 1637mm at Mount Glorious, QLD – 1180mm at Pomona, QLD – 1094mm at Bracken Ridge
Brisbane has absolutely smashed its three-day rainfall record with 677mm, by recording over 200mm each day for three days in a row.
Before this week it had never even had two consecutive days over 200mm and had only ever recorded eight in total.
The mean annual rainfall for Brisbane is 1011.5mm and it recorded 741mm in just the four days from 9am Thursday and 9am Monday.
Speaking of records, Weatherzone is reporting Dunoon in NSW recorded the second-highest daily rainfall total in NSW when 775mm fell in just the 24 hours to 9am Monday.
If you are not sick of stats yet, Doon Doon in NSW picked up a whopping 1040mm of rain in just the 48 hours to 9am Tuesday. That is over a metre of rain in just two days.
But it is not just the big totals that have made this rainfall event unusual.
Bofu Yu of Griffith University’s School of Engineering and Built Environment and Australian Rivers Institute observed that while the rainfall amount over south-east Queensland from Thursday to Sunday was huge and widespread, the intensity of rain was moderate at around 50mm per hour.
“This is distinct from the 2011 event when rainfall was concentrated in the western part of the Brisbane River Basin with a much higher peak rainfall intensity,” Dr Yu said.
The result is the rainfall has been spread far more liberally around the catchment this time and more water is flowing down the small creeks and tributaries, which has a flow-on effect further downstream.
“The peak discharge may not be as high compared to the 2011 flood, but high flows will persist over a much longer period of time,” Dr Yu explained.
South-east Queensland and northern NSW are historically flood prone and have certainly flooded before but this event is definitely different from those we have seen in the past.
2. Is climate change involved?
Attributing any one event to climate change is tricky, especially in the case of rain, which has many contributing factors.
But there is a clear link between a warming atmosphere and its ability to hold more moisture and deliver that moisture in the form of heavy rain.
“With each degree increase in the atmospheric temperatures, air can hold roughly 7 per cent more water vapour that is eventually available to fall as rain,” as Nina Ridder, research associate in the UNSW Climate Change Research Centre, explained.
“This means that under future conditions which are likely to be higher than what we have seen in the past.
“Over the past decades we have already seen an increase in the number and intensity of extreme rainfall events and we are expecting this trend to continue into the future.”
Another major climatic factor at play at the moment is the La Niña, which the BOM declared last year. It has been busy enhancing the rainfall over Australia all summer.
When La Niña conditions are in place warm tropical waters in the north and strong trade winds from the east encourage moisture onto Australia.
So, when individual weather systems come through it gives them another moisture kick.
David Karoly, Honorary Professor in the University of Melbourne School of Geography, Earth and Atmospheric Sciences, concludes that both climate change and the occurrence of La Niña are likely to have contributed to the increased risk of heavy rainfall in south-east Queensland in the current event.
“The difficult part is to precisely quantify the increase in risk or the contribution to the amount of rainfall, both of which are uncertain,” he said.
3. What is a rain bomb?
This event has been commonly referred to as a “rain bomb” over the past few days.
But while it may have felt like the rain has been bombing down, a “rain bomb” is not a meteorological term.
There is a thing called “bomb cyclogenesis”, which is when a low pressure system develops unusually quickly, but that is not what happened this week.
Likewise there is another phenomena called a “wet microburst”, which is when a huge amount of rain drops suddenly from a storm, but that is generally over a small area.
What has been going on over the past week has been a surface trough, with upper atmosphere enhancement funnelling tropical moisture off the Coral Sea onto the coast which was blocked from moving off.
This created a large area of prolonged heavy rain.
4. What’s to come?
More extreme weather is forecast over the coming days as an east coast low develops off the NSW coast.
Severe thunderstorm warnings are in place for large parts of New South Wales this evening and flood watches are in place for parts of the NSW coast from Newcastle to Bega, pushing down into Victoria.
Wind gusts are forecast to be up around 90kph and could uproot trees and powerlines.
Where exactly the worst of the impacts will be felt in the coming days will largely depend on where the low moves to.
But heavy rainfall is expected on the southern side of the low, and Sydney residents have been urged to brace for flooding.
South East Queensland was the first part of Queensland to be settled and explored by Europeans. Settlements initially arose in the Brisbane and Ipswich areas with activity by European immigrants spreading in all directions from there. Various industries such as timber cutting and agriculture quickly developed at locations around the region from the 1840s onwards. Transport links have been shaped by the range of terrains found in South East Queensland.
The economy of South East Queensland supports and relies on a wide diversity of agricultural manufacturing industries, commerce and tourism. The region has an integrated public transport system, TransLink. The gross domestic product is $ 170 billion
South East Queensland, classified as an interim Australianbioregion, comprises 7,804,921 hectares (19,286,380 acres) and includes the Moreton Basin, South Burnett, and the Scenic Rim along with ten other biogeographic subregions. The term South East Queensland has no equivalent political representation. The area covers many lower house seats at the federal and state levels. As Queensland has no upper house, there are no Legislative Council provinces or regions to bear the name either.
South East Queensland was home to around 20,000 Aboriginals prior to British occupation. The local tribes of the area were the Yugarapul of the Central Brisbane area; the Yugambeh people whose traditional lands ranged from South of the Logan River, down to the Tweed River and west to the McPherson Ranges; the Quandamooka people whose traditional lands encompassed the Moreton Bay Islands to the mouth of the Brisbane River to Tingalpa and south to the Logan River; and the Gubbi Gubbi people whose traditional lands were known to exist north of the Pine River, to Burrum River in the north, and west to the Conondale ranges. According to history researchers the Aboriginal population declined to around 10,000 over the next 60 years.
An emergency alert has been issued for the Gympie area from the Gympie Regional Council regarding major flooding.
Council advises if you live at Southside and are in an impacted area, you need to evacuate now and seek shelter with friends or family on higher ground. If you live on the hospital side of the river and are in an impacted area, you need to evacuate now and seek shelter with friends or family on higher ground. Take essential medication and secure your property.