By Kate Doyle
Posted 57m ago57 minutes ago, updated 9m ago9 minutes ago
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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.
Impacts along the NSW coast are expected to linger until Thursday.
Back up in northern NSW and south-east Queensland it looks like showers and storms could return as soon as Wednesday afternoon.
The BOM is saying there is the potential for severe thunderstorms, heavy rainfall, damaging winds and large hail.
With catchments already on the edge, it is a worrying time for low lying areas
Longer term, summer may be over but there are still two months left of the tropical wet season.
The autumn outlook suggests wetter-than-average conditions are likely to remain across much of the country.
There is still plenty of time for more tropical moisture to make its way south, bringing more heavy rain with it.
With the catchments so sodden, it won’t take much now to trigger more flooding.
If you are in a potential risk zone and have not yet thought about what you would do in a flood situation, this would be the time.
5. Where can I find the latest information?
ABC Emergency is the go-to place for up-to-date local emergency warnings.
The website also has a number of resources on how to prepare for and protect yourself from different disasters and emergencies.
ABC emergency: https://www.abc.net.au/emergency/
Radio frequency: https://reception.abc.net.au
The Bureau of Meteorology releases all its weather warnings on this website: http://www.bom.gov.au/australia/warnings/index.shtml
It also release near daily severe weather updates during big events on its YouTube page.
If you would like the latest updates on rainfall numbers and where all the river levels are sitting across the country that can also be accessed through the BOM website here: http://www.bom.gov.au/australia/flood/?ref=ftr
Flood maps for your local area should be available on your local council website.
Posted 57m ago57 minutes ago, updated 9m ago