Images taken of flooding around Lismore on Sunday morning June 5 2016 after a deluge of rain across the Northern Rivers.
Images taken of flooding around Lismore on Sunday morning June 5 2016 after a deluge of rain across the Northern Rivers. Hamish Broome

NOT AGAIN: Weather forecasters predict second East Coast Low

THURSDAY 8am: WHILE the Northern Rivers faces the possibly of more wet weather from Sunday, today's weather forecast looks bright.

The Bureau of Meteorology is predicting sunny conditions across the region today, with a top temperature of 27 degrees Celsius.

Enjoy the sunshine while you can!

 

WEDNESDAY 6.30pm: MORE rain is due to hit the Northern Rivers on Sunday and early next week - just as the Wilsons River catchment is recovering from last weekend's drenching.

Local river systems are expected to rise more rapidly in the next downpour because the catchment is now soaked, and all excess water will immediately become runoff, increasing the chance of flooding.

At this stage there isn't too much to be concerned about, with Bureau of Meteorology predicting a high chance of showers up to 40mm across Sunday and Monday.

However, local sky watchers Northern Rivers Severe Weather and North Coast Storm Chasers have flagged the possibility another low pressure system forming which could see heavier falls of up to 100mm.

MORE RAIN: This picture could repeat itself again this month of long-term La Nina predictions of more downpours come to fruition.
MORE RAIN: This picture could repeat itself again this month of long-term La Nina predictions of more downpours come to fruition. Hamish Broome

Michael Bath from Northern Rivers Severe Weather said there was a 25% chance of an East Coast low forming.

"If we do get a decent rainfall dump on Monday or Tuesday, you are going to see river rises even with 100mm," Mr Bath said.

"The river would probably go up to minor (flood) levels in 100mm of rain, or need 150mm to go moderate again.

"Just about everything is going to run off now (whereas) the previous one a lot of the rain simply soaked in.

"That's a factor into why we didn't go into a major flood.

"A big factor in last weekend too was the lack of really intense falls over the western catchment. All the really extreme rain fell in the coastal strip, the eastern half of the Nightcap range… those intense falls didn't quite get over to the western half into the Leycester catchment.

"The falls really tapered off once you got west of the line of Nimbin and Lismore."

 

ALL RIVERS LEAD TO LISMORE: A map of the Richmond catchment shows why Lismore is prone to flooding. Positioned at the confluence of the extensive Wilsons and Leycester sub-catchments, big Lismore floods usually happen when the Leycester catchment cops a lot of rain, as it has a more dominant influence on water levels at Lismore.
ALL RIVERS LEAD TO LISMORE: A map of the Richmond catchment shows why Lismore is prone to flooding. Positioned at the confluence of the extensive Wilsons and Leycester sub-catchments, big Lismore floods usually happen when the Leycester catchment cops a lot of rain, as it has a more dominant influence on water levels at Lismore. Contributed

June often a wet month

Long-term average rainfall statistics show the wettest month of the year is March, followed closely by its neighbours, February and April.

But where May is often dry and sunny, June is known as the month which offers a second-round of potential flood events with a higher level of average rainfall than May or July.

Mr Bath said once we moved into mid-July, the probability of flood would substantially diminish - but with the caveat that history showed anything was possible in the Northern Rivers at almost any time of year.

Anatomy of a storm

Mr Bath said the region was unlikely to cop another battering like last weekend which was rare for a whole lot of reasons.

"That one was showing up on models up to a week ahead and it was always going to be a bit dramatic," he said.

A couple of environmental factors played into the strength of the system: warmer than usual sea surface temperatures - which helped moisture feed into the system from the associated humidity - and a sharp upper trough which moved across NSW and turned into a high level cut-off low and provided the instability needed for a big system.

But perhaps the crucial factor in the widespread dumping and destruction all the way down the East Coast into Tasmania was the "blocking high" perched over New Zealand which kept the rain system "anchored" perfectly over the East Coast.

"The fact that the whole system was blocked meant it couldn't pass through in a matter of hours, it ended up being anchored in the one area for several hours," Mr Bath said.

It also helped drive the north-east winds into the system, which helped create all that rain.

"We had a great moisture supply coming of the Coral Sea driven by the high off NZ, which fed into an upper level low pressure system, and that's where all the intense rainfall came from," Mr Bath explained.

For the Northern Rivers, it was the formation of a genuine East Coast Low off the Coffs-Clarence coast in the wee hours of Sunday morning which actually helped the system moved south and away from our part of the world.

Mr Bath said most of the extreme rainfall events in Northern NSW were not caused by an ECL, but stationary troughs which drew in moisture over several days from the ocean.


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