Ex-Cyclone Alfred has passed. However, residents of northern New South Wales and parts of Brisbane and the Sunshine Coast should be on high alert for bands of intense rainfall and possible flash flooding.
This is the sting in Alfred’s tail. These storms are drawing down very warm, moist air from the Coral Sea in the north. If you happen to be under one of these slow-moving thunderstorms, they are getting ready to dump a lot of rain.
The situation is very volatile, as the atmosphere is very unstable.
What’s happening now?
Thankfully, the winds have died down in many parts of eastern Australia affected by ex-Cyclone Alfred, but the main concern now is these intense thunderstorms popping up in a fast-changing atmosphere.
The Bureau of Meteorology issued a severe weather warning around 11am (NSW time) on Monday, predicting heavy rainfall over the Northern Rivers, Northern Tablelands, Mid North Coast and the North West Slopes and Plains.
A similar severe weather warning has been issued for a large part of southeast Queensland, including inland areas.
It also issued flood warnings for many parts of Queensland, including Brisbane and the Gold Coast.
Radar images showed large bands of heavy rain hovering over the east coast.
This is a significant weather event because these rainbands are stretching from as far north as Bundaberg in Queensland, right down to Inverell and further south in NSW. And if you’re under one of these rain bands, there’s a risk of flash flooding.
The steering winds are quite weak, so they are moving quite slowly and dumping huge amounts of rain as they go.
Why is this happening?
Winds in the atmosphere are colliding with each other, and when they do, air is forced to rise. If the air is saturated and warm, it contains huge amounts of water.
The air cools as it rises and eventually reaches the temperature at which saturated air condenses and turns into water. Then, down comes the rain.
All of this is now happening over an extraordinarily large area.
Some of these storms may last just a few hours, but may dump phenomenal amounts of rain. Our drainage infrastructure is just not set up for this. It’s quite rare to have this amount of rain falling in such a short amount of time.
In some places, the water will rise very quickly because the catchments are already saturated from days of rain.
The hope is that conditions improve in the evening. But right now, people need to be vigilant.
The below flood maps were updated by Brisbane City Council over the weekend and show the areas at risk.
Climate change is increasing the risks
This situation has been made worse by the fact sea-surface temperatures off the coast of Australia are warmer than average.
Due to rising greenhouse gases from human activities, the atmosphere is trapping more heat. About 90% of this excess energy is transferring into the world’s oceans.
There it is stored, ready to feed heat and moisture into the atmosphere through evaporation. Even a small amount of warming of the atmosphere means the air can hold much more water.
This warmer atmosphere is feeding flash floods.
In the months ahead, authorities must think hard about how to protect our cities and towns from this ever-growing risk.
Steve Turton has previously received funding from the federal government.
This article was originally published on The Conversation. Read the original article.
