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BOM explains why Brisbane and south-east Queensland has had storms with hail in May

By Hilary Whiteman, Nibir Khan and staff, Thursday May 13, 2021 - 05:50 EST
ABC image
Energex recorded 180,000 lightning strikes in the region in one afternoon. - ABC

May weather in south-east Queensland is supposed to be fairly unremarkable.


It is generally considered a transition month, Bureau of Meteorology (BOM) forecaster Shane Kennedy said, with a few bursts of rain not out of the ordinary.


But this year, for two weeks in a row, people in Brisbane, the Southern and Darling Downs, Gold Coast and Sunshine Coast regions have been told to brace for stormy weather.


And the storms certainly came.


Last week, unstable weather generated plenty of lightning and rain in the state's south-east corner and for three nights on the trot this week, thousands of households were woken up, lost power or got caught up in peak-hour traffic when heavy rain fell and hail pelted down.


Less than two weeks into the month, some areas have received up to four times as much rain as the average monthly total, and hailstones as big as 5 centimetres have been reported in some regions.


Mr Kennedy said the quick succession of recent storms was unusual.


BOM meteorologist Pieter Claassen says the storms are more common in the period between October to April, so we're "really right at the tail end of that severe thunderstorm season".


"So unusual to see three days of activity in a row like that as well," Mr Claasson said.


?We've had more than an average [number of thunderstorms], certainly," he said.


In Mr Claasen's view, the storms themselves weren't the only odd feature of the weather system.


The Esk region received "giant"-sized hailstones, which are classified as being more than 5 centimetres in diameter.


"It is unusual to see ? hail of that size," he said.


La Niña not directly linked to the wild weather


La Niña is not causing the higher rainfall. That weather system ended in late March after creating the wettest summer Australia had seen in half a decade.


Instead, the current weather is being caused by the remnants of La Niña ? warmer oceans off the coast ? combining with cooler troughs moving up from the south.


"Even though La Niña is over, we still have residual heat in the ocean," Mr Kennedy said.


"We've had quite a few upper troughs in the atmosphere and that's led to unstable conditions.


Mr Claasen says the atypical nature of the storms is driven by weather and not climate.


"It's all weather driven rather than climate driven, it appears at this stage," he said.


Flooding and downed trees


The combination of weather events ? hail, lightning, and flooding ? has surprised some people used to milder weather at this time of year.


Farmer Douglas Wunsch's sorghum crop at Warra, near Dalby on Queensland's western Darling Downs, suffered hail damage on Tuesday night.


"I've never seen storms of this capacity ?with hail ? in May ever," he said.


"You might get something through June that might thunder and carry on but that's only pea-sized, soft hail.


"This was fair dinkum October storm hail."


AgForce Grains president Brendan Taylor said the unusual storms had delivered welcome rain and a boost for winter crops in some areas.


"We haven't had rain at this time of year for a number of years," Mr Taylor said.


"The rain's very beneficial but it's unfortunate that some of these storms have been quite vicious."


The State Emergency Service (SES), which had more than a dozen call-outs on Tuesday for flooding in the Toowoomba region and the Lockyer Valley, answered more calls for help on Wednesday.


"The amount of water that fell in such a short period of time [meant] there was a lot of runoff," SES regional manager Bob Bundy said.


He said the weather was unusual for May.


"We thought the storm season had pretty much finished and we were looking at other things," he said.


"It's never over. You have to be prepared all the time.


"You never know where it's going to fall."


Energex spokesman Danny Donald told ABC Radio Brisbane said there were more than 500,000 lightning strikes generated by storms in the past three days.


He said the energy provider tracks lightning strikes using a series of aerials around the network that identified each unique lightning strike.


"On the AM radio, during a storm, you can hear a bit of a crackle ? each one of those crackles is a signature, and it's like a fingerprint," Mr Donald said.


"Every single lightning strike is different slightly, so each time one of those lightning strikes happens, that crackle is counted by our system and that's another lightning strike.


"You can even tell if it hits the ground or goes cloud to cloud as well, so it's pretty impressive."


Storm clouds loaded with hail


Mr Kennedy said cooler weather moving north from southern Australia was creating the right conditions for hail to "survive the journey" from the storm clouds to the ground.


He said the storms had "front-loaded" May with much higher than average rainfall totals.


For example, in the 24 hours to 9:00am on Wednesday, Forestdale received 126mm, more than four times the monthly average of just 29mm.


Caloundra has received 225mm so far in May, twice as much as the long-term average of 105mm.


Redcliffe's rainfall total is nearly double expectations, with 115mm of rain versus the May average of 64mm.


With 65mm of rain so far this May, Brisbane has already surpassed its average monthly total of just 60mm, but it is a long way short of its most recent May record of 269mm set in 2016.


Mr Kennedy said it was too soon to say if May would break any rainfall records, especially because calmer conditions were forecast for next week.







- ABC

© ABC 2021

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