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Spring storms can hit without warning, so why are they so hard to forecast?

By Irena Ceranic, Thursday October 11, 2018 - 16:30 EDT
ABC image
Spring storm front moving across Perth. October 2018. - ABC

Perth experienced one of its wildest weather swings seen this year on Tuesday, when the first 30-degree day of spring gave way within the hour to heavy downpours and a spectacular lightning display.

The wild weather was hard even for weather experts to predict, with the Bureau of Meteorology still forecasting the chance of rain at just 20 per cent half an hour before the storm hit.

When it did develop, the late afternoon thunderstorm rolled in from the north and brought heavy isolated showers, hail, gusty winds and a spectacular lightning display with 8900 strikes recorded.

The rapidly changing weather forced the BOM to update Perth's forecast four times during the day instead of the usual two at 4:30am and 4:30pm.

The first change to the forecast was made in the early afternoon to reflect the likelihood of the temperature reaching close to 30 degrees Celsius instead of the predicted 28.

The second change was made at 5:00pm, just half an hour after the afternoon forecast was issued, to update the chance of rain from 20 to 70 per cent.

Within moments it was bucketing down across the city.

"As we were looking at the radar just after 4:30pm we started to see that the thunder cells, the shower cells, the convective cloud — they weren't breaking up the way that we thought they would have been doing just even half an hour earlier than that. They were actually holding themselves together a little bit longer than normal," BOM spokesman Neil Bennett said.

"The thunderstorms were pretty well forecast, it was just a matter of changing the actual percentage chance of rainfall."

So why are these spring storms tricky to predict?

Wild weather during winter is generally produced by cold fronts which come up from the Southern Ocean bringing showers and gusty winds to south-western parts of WA.

Tuesday's storms which barrelled into Perth from the north were caused by a trough of low pressure and are typical for this time of year.

But Mr Bennett said they were generally more difficult to predict.

"One of the major features that we have with this change in the weather pattern from the winter pattern of widespread frontal rain, moving into this more convective rainfall — where you have individual shower clouds and indeed thunderstorm clouds producing pockets of rain rather than widespread areas of rain — trying to predict exactly where that rain is going to fall is a little bit harder than the broader areas of the cold front.

"We're looking at individual clouds — on occasions only 5 or maybe even 10 kilometres wide — and to pinpoint exactly where they're going to form and where they're going to run to even 6 or 7 hours ahead of time is difficult, so really we have to wait until they've actually formed."

Stormy Saturday predicted

People are being advised to keep the umbrella handy for a few more days as showers are set to return to Perth on Friday evening, with a wet and stormy Saturday forecast.

The BOM is predicting a very high chance of showers and possible thunderstorms.

But unlike the balmy Tuesday storm, Saturday is expected to be a cooler 21C.

"It's a slightly different set up. Over the last day or so we've had a trough of low pressure and that has been the reason why we've seen the very warm weather, because the air is coming down from the north-east ahead of that trough," Mr Bennett said.

"Whereas on the Saturday we're going to see a combination of trough and frontal activity … but there's still the risk of thunderstorms."


© ABC 2018

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