The Bureau of Meteorology has stated that Tropical Cyclone Jasper is expected to make landfall on the Queensland coast in less than eight hours, despite the fact that there is still a “cone of uncertainty” in this exact area.
Jasper is expected to hit Port Douglas as a category 2 storm, despite the Bureau of Meteorology (BOM) stating that it might land anywhere between Cooktown and Atherton, which are around 300 kilometres away.
So what controls a cyclone’s speed, direction, and intensity? And why is it more difficult to forecast the course of some storms than others?
“Least resistance path”
According to senior meteorologist Dean Narramore of the Bureau of Meteorology, high-pressure systems, mid- and upper-level troughs, and ambient winds can all push or pull cyclones in different directions.
He referred to these as “steering influences.”
According to Mr. Narramore, “a Tropical Cyclone Jasper will typically take the path of least resistance.”
“They’ll stay away from the big weather systems that can rip them apart.”
If a Tropical Cyclone Jasper has a single, strong steering impact, its path can be predicted with ease.
According to the BOM’s post-event study, TC Ilsa’s projections were “exceptionally accurate” when it made landfall in northern Western Australia in April.
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The three-day estimate has a pitiful margin of error of 61 kilometres.
However, Mr. Narramore did point out that achieving this level of accuracy was more challenging when there were numerous steering variables at play.
When a cyclone travelled over the Coral Sea, as Jasper had done, he said it was common.
“It’s like a pinball jumping around between all those kinds of steering influences until one finally takes over and drives it,” he explained.
“Uncertainty cone”
Cyclone track charts are always accompanied by a “cone of uncertainty.”.
According to Mr. Narramore, winds coming from a sizable area of high pressure to the southeast of Australia are pushing Jasper towards the Queensland coast.
He explained that depending on its strength, the high pressure system will move to either the northern or southern boundaries of the “cone of uncertainty” on the track map.
Compared to last week, when there was a lot of uncertainty about how the high pressure system would combine with a storm system over South Australia, Mr. Narramore said the models were now much clearer.
He said that over the last three days, TC Jasper had slowed down.
Even though Mr. Narramore stated that in this specific instance, the cyclone’s course remained quite clear, this can make a cyclone’s path more unpredictable.
“Often, when it’s slowing down, it means it’s transitioning between steering influences or that it has lost its steering influence,” explained Narramore.
“Luckily, at this stage, it just looks like the high is grabbing it and will continue to steer it over the coming days.”
incredibly potent “ingredients”
TC Jasper strengthened to a category 1 system on Monday and subsequently re-intensified to a category 2 storm on Tuesday, having developed to a severe category 4 system well out to sea.
Mr. Narramore asserts that depending on the “ingredients” in the air surrounding the storm, this is typical cyclone behaviour.
Warm sea surface temperatures, humid air, and comparatively little windshear—a term used to describe the change in wind direction or speed with height—are necessary for cyclone survival.
If any of these circumstances change, a cyclone has the potential to weaken or disperse, but it also has the ability to re-form if it returns to a “favourable” environment.
Mr. Narramore claims that Jasper is experiencing this.
“As we moved into the weekend, not only did some dry air get wrapped into the system, but the winds increased in the atmosphere as well,” he stated.
“But then, as we move into Tuesday, it looks like the environment does moisten a little bit and that shear does drop away a little bit, allowing for a possible re-intensification again before it hits the coast.”
Jasper’s track chart indicates that the storm will weaken by Thursday and continue until December 15.
However, the longer-term prognosis from the BOM indicates that Jasper may regenerate this weekend over the Gulf of Carpentaria, which could have an effect on the Northern Territory.
Various steering influences—which are now unknown—will determine if this occurs.
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