A dry argument for June to August rain

A dry argument for June to August rain

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WA winter mean maximum temperature was the highest on record.

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WINTER in Western Australia was warm and very dry, with Statewide rainfall from June to August the eighth driest on record and driest since 2006.

Seasonal mean maximum temperatures were well above average for most of WA and the Statewide winter mean maximum temperature was the highest on record.

Seasonal mean minimum temperatures were above average for the southern two-thirds of the State and overall rainfall in winter 2020 was below average for most of WA.

The dry conditions came as a shock to most, as in May the Bureau of Meteorology (BoM) predicted winter in WA would likely to be wetter than average across most areas.

BoM head of climate operations Andrew Watkins said the original outlooks from early May were wet for WA, and while they eased back at the end of May, they still were on the wetter side.

"What we think is going on with the long-term models is that they were all thinking we were going to get this negative Indian Ocean dipole which is where you get warmer water between Western Australia and Indonesia and cooler water off Africa," Dr Watkins said.

"That tends to favour rainfall towards Australia, so the Bureau's model, and indeed all the models we assessed from around the world, were saying that this pattern of warmer water in the eastern Indian Ocean was going to build up and lead to more cloud formation off WA and more rainfall coming in.

"What happened is that it appears that pattern stalled in its formation, so essentially the patterns which were expected to develop in the Indian Ocean didn't develop and the science that research can pull out so far is that it was simply random variability which caused it, rather than some big, predictable pattern which we should have picked up."

Statewide rainfall was 46 per cent below the seasonal average, however above average rainfall in August lifted winter rainfall totals close to average along the south coast between Albany and Eyre.

Most of the South West Land Division (SWLD) received below average winter rainfall and it was the 10th driest winter and driest since 2014 for the SWLD as a whole.

From August 3-6, a cold front and associated low pressure system produced moderate daily rainfall of more than 30 millimetres along the south coast, with heavier falls between 80 and 100mm in and around Albany.

Agronomist Michael Lamond said the Grain Industry Association of WA (GIWA) used the Department of Primary Industries and Regional Development's (DPIRD) analysis, which was a combination of BoM and other sources.

"This has been fairly good for the past three to four years," Mr Lamond said.

"The turn around this year from a good chance of above average rain to below in June to July was completely dominated by the development of a very strong southern annular mode where the high pressure systems are very "high" in both strength and latitude.

"These came on quickly and dominated the scene for most of the winter, with the result being that even though the fronts were coming, through they were forced to stand up along the coast resulting in very strong north west winds and little rain."

Mr Lamond said BoM didn't take this into account in its predictions as most of their efforts were based in east coast predictions.

It was also a record warm winter for WA, with the Statewide seasonal mean temperature 1.60 degrees above average, the highest on record.

Winter daytime temperatures were above average across the whole of WA, with the mean maximum temperature 2.38o above average, which was the warmest winter on record.

Winter overnight temperatures were above average for the southern areas and the Statewide winter mean minimum temperature was 0.81o above average.

The extreme heat expanded to the south and south-east covering the eastern SWLD, Goldfields and Southern Interiors in late August, with some locations exceeding their previous winter record on multiple days.

Dr Watkins said farmers needed to remember that the outlook was not a weather forecast and they were not saying there was a 100pc chance of something occurring.

"If we say there is an 80pc chance there will be above normal rainfall, then eight years out of 10 it will be, but in two years it won't," Dr Watkins said.

"It's better than guessing because it has skills and accuracy over the long-term, however it's still a probability and it's a risk, but managing risk is something most farmers are pretty damn good at.

"I know it's tough, the odds were looking good at one stage there and people would have been pretty keen on the season, unfortunately it ended up being one of those misses out of the 10 years."

On July 27, the passage of a cold front produced strong winds and some damage in the SWLD, with strong north-westerly winds ahead of the front causing dust to blow through eastern parts.

Both Gnowangerup and Manjimup recorded a 83 kilometres per hour wind gust and Cape Leeuwin recorded a gust of 96 km/h.

On August 9, a strong cold front produced severe winds gusts and a 50 to 70 centimetre storm surge along the west coast of WA, though impacts were minimal due to low tides.

The highest wind gust was 109 km/h at Cape Naturaliste, as raised dust moved through Gascoyne and the winds brought down trees and caused minor roof damage in an area from Kalbarri to Albany.

Dr Watkins said they were trying to get across the message that the outlooks were probabilistic and they have to be used over the long-term.

"There will be misses, but over the long-term they will come out ahead," he said.

"We certainly don't rest on our laurels, we've upgraded the model several times and we will upgrade it again next year, plus we're collaborating with the United Kingdom Meteorological Office to get better models and combine our research together.

"We understand that people want better and more accuracy, but we want that as much as anyone else, so we're always working on it."

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