This week's Perth storms are weather events the BOM finds almost impossible to predict

Updated

Perth residents have been left bewildered this week as ferocious storms have appeared seemingly out of nowhere, causing widespread destruction in a matter of minutes.

Key points: Severe storms are extremely hard to pinpoint until they appear on the radar

They need humidity, an unstable atmosphere and a trigger such as heat

Their hit-and-miss nature also means rainfall totals are difficult to predict



Many people were caught off guard by a particularly powerful and fast moving cell that barrelled in from the north on Tuesday, despite the risk of thunderstorms featuring on the forecast for Perth all week.

What forecasts by the Bureau of Meteorology (BOM) did not include was exactly where and when the storms would occur.

That is because it is almost impossible to predict the precise location and movement of a severe storm, particularly in summer, until it forms and appears on the radar, leaving very little time to warn the public.

"We do have an indication of the likelihood of a severe thunderstorm occurring in a broad area, but what we don't know is where that severe thunderstorm is going to occur until it pops up," BOM spokesman Neil Bennett said.

It is a difficulty faced by meteorologists all around the world due to the isolated nature of thunderstorms and the wide margin for error.

The smallest variation in temperature, wind or moisture can mean the difference between a large thunderstorm event and just a blip on the radar.

The three ingredients to a thunderstorm

Three ingredients are required for a thunderstorm to form:

Humidity

An unstable atmosphere

A trigger, such as a cold front or, as is the case in summer, heat or a trough of low pressure influencing the wind

"The tricky part is the trigger," Mr Bennett said.

"On a broad scale it's quite easy to spot, but on a local scale sometimes it can be something as simple as the strength of the sea breeze or the timing of the sea breeze.

"Sometimes you miss it by one degree in temperature. For example … if it's going to get to 33 instead of 32 on a particular day then the atmosphere will explode."

Tuesday's storm was detected on the radar just before 4:00pm, allowing little lead time to warn the public that wild weather was imminent.

It was reminiscent of Perth's March 2010 hailstorm, which also wreaked havoc during the peak hour commute, although the event this week was not nearly as severe.

But the timing was not a coincidence — both were fuelled by the afternoon heat and formed after the temperature had peaked.

"To the north of Perth the skies cleared for a period of time [on Tuesday], generating heat, and then there were little local [wind] convergences going on and that gave it a super kick," Mr Bennett said.

"And then once it fired up, it set off really rapidly because the atmosphere was very unstable.

"The biggest impact probably only lasted 10 or 15 minutes and that's fairly typical for a thunderstorm. It moves through and then it's like nothing happened half an hour later."

Storm deluges hit 'very defined area'

The hit-and-miss nature of summer thunderstorms also means rainfall totals are difficult to predict.

"You can very often see a clearly defined rain shaft coming out from the centre of the cloud — that's where the bulk of the rain is," Mr Bennett said.

"That can only be maybe a kilometre or two wide. If you're even 300 metres away from that rain shaft, you're going to get very little.

"It's a very defined area. Whereas with a cold front, which can cover hundred of kilometres, the rainfall is pretty widespread."

Perth usually has about two days of thunderstorm activity in February, but the city has now entered its fourth day of stormy weather and more is forecast for Friday.

A welcome start for farmers

Farmers are all too familiar with the sporadic nature of thunderstorms, but this week's downpours have given many parts of the state's grain-growing region an early boost to the season, drenching parched soils and filling up dams.

Daniel Spencer, who owns a small farm at Bencubbin in the Wheatbelt, has received just over 160 millimetres since Saturday, with 64mm of that falling in just 15 minutes.

"It's quite good, we're going to have a fair bit of subsoil moisture," he said.

"We got just over 200mm for the [entire] growing period last year — I think I had one thunderstorm just after Christmas, a couple of good rainfall events in June but no September rains to finish the crops.

"It's quite exciting times, we've had most of last year's rain in four or five days."

Rain has filled dams in the area, some of which locals have never seen full.

Mr Spencer said while the stormy weather was welcome, it had also added a few jobs to his to-do list.

"I don't think I've had a paddock on the farm where I haven't lost a fence or at least the majority of a fence," he said.

"I've only got one paddock now that I can put sheep in until everything dries out enough to tidy things up."

Topics: weather, storm-event, perth-6000, bencubbin-6477, wa

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