Predicting the weather is notoriously a difficult task, but getting the severe weather forecast right can be even harder, according to Bureau of Meteorology senior forecaster Neil Bennett.
Mr Bennett was part of the BOM team trying to keep northern NSW residents up-to-date as a severe weather system wreaked havoc across the region over the past 24 hours.
"It's been a tough, tough night with this one," he said.
Yesterday afternoon, emergency services anticipated the weather would ease, but flood levels in and around Lismore continued to rise, causing residents to evacuate for a second time in 24 hours.
Floodwaters have since breached the town's levee and several evacuation orders have been issued across the region.
Flash flooding has also caused havoc, with towns such as Byron Bay underwater.
How is a forecast formed?
To produce a forecast, meteorologists compiled weather observations from satellites, radars, vessels at sea, air, and on land.
It was then fed into a computer to form a model.
"It's like throwing a fishing net over the planet, and with the knots in the fishing net you apply equations at those knot points," Mr Bennett said.
"The smaller the distance [between knots] then hopefully the more accurate you can get a prediction.
The computer's outcome is then analysed by a forecaster, who then checks the formula and slightly adjusts the model in order to produce an array of predictions.
Trying to keep pace with conditions
The weather system which hit the Lismore area overnight was very small and changed rapidly.
"Then the model has a real problem in trying to predict that."
The weather system impacting the Lismore region experienced an enhanced amount of convection, which led to heavier bursts of rain and the BOM's subsequent issuing of a thunderstorm warning overnight.
Predicting exactly when the rain would fall, given the circulation around the small low pressure system, is no easy feat, but improvements to technology could help in the future.
"The next step for computer modelling is to be able to run models in what they call a 'rapid update mode'.
"Instead of running them every 12 hours, you may have to run them in a very small scale, down to potentially only a 15- to 20-kilometre resolution every hour."
More rain on the way
Rain continues to drench parts of the NSW Northern Rivers and Mid North coast, resulting in flash and riverine flooding.
The challenges faced by forecasters with predicting the weather patterns over the past 24 hours has been mirrored by SES NSW Acting Commissioner Daniel Austin who described the system as "highly dynamic".
"There is an expectation that we will see heavy rain and heavy flooding for a number of days yet to come."
Mr Austin also defended the decision to evacuate residents twice within 24 hours.
"If we had a crystal ball, you may make different decisions," he said.
"The information we had at the time was that the rivers were falling and that conditions we expected were no longer going to present.
Meteorologists, along with emergency services, will continue to decipher the latest information as the water continues to impact communities.
"The weather system … is evolving and changing rapidly," Minister for Emergency Services Stephanie Cooke said.
"Please listen to our emergency services, particularly the SES but also the BOM, in relation to the warnings they continue to issue as more information comes to light about what this weather system is doing."
