Updated
It is now looking more likely that Cyclone Oma could approach Brisbane and the south Queensland coast — but a lot of uncertainty remains.
The system which brought such devastating, record-breaking rain to north Queensland a fortnight ago is refusing to go away.
It is now circling off New Caledonia as a category three cyclone and its impact is expected to be felt along the southern Queensland coast late this week.
An update from the Bureau of Meteorology (BOM) on Tuesday afternoon said there is “high confidence” that Cyclone Oma will move in a west-southwesterly direction towards Australia over the next few days.
From Thursday, however, there is a “big model spread” when it comes to predicting what the cyclone will do, according to Diana Eadie from the BOM’s extreme weather desk.
“As a result, there is significant uncertainty in terms of the movement of this system,” she said.
“There are two more likely scenarios, the first of which is that the system adopts a more south-easterly trajectory and moves towards New Zealand.
“Or, that it continues to move west and approaches the southern Queensland coast.”
But BOM maintained that regardless of where the system moves, significant impacts will be observed along the south Queensland coast, including abnormally high tides.
“We are seeing king tides at the moment and we are likely to see some inundation of low-lying areas before the system even really starts to approach the Queensland coast.”
As of 3:30pm (AEST) Tuesday, BOM issued warnings of hazardous surf for the Capricornia coast, Hervey Bay, Fraser Island coast, Sunshine Coast waters, Moreton Bay and Gold Coast waters.
“Amazingly, the low-level circulation that was associated with the low that brought all the rain in Queensland moved offshore, then all the way out towards Vanuatu,” said Dean Narramore, an extreme weather forecaster at BOM.
“It stalled there for a little while then redeveloped again into a tropical low and then strengthened further into a tropical cyclone.”
Cyclone Oma is in the Fijian cyclone monitoring zone, where it is expected to pass very close to the north of New Caledonia.
Why are cyclones so hard to predict?
BOM meteorologist Jackson Browne said some cyclones were well-behaved while others were not.
“Research has shown that in the Australian region, cyclones exhibit more erratic paths than cyclones in other cyclone basins,” he said.
Mr Browne said small changes in the initial environment surrounding a cyclone could make large differences in its eventual path, making it harder to predict its movement because it is difficult to work out exactly what the conditions are at sea.
“Compared to land areas the oceans don’t have a lot of observations,” he said.
“This data is valuable to numerical computer models and makes it harder for them to accurately predict paths.
“Accuracy of model predictions decrease the longer the forecast time, so the apparent spread in where a cyclone might travel to is also large.”
What controls the path of a cyclone?
Mr Browne said a tropical cyclone is like a cork in a stream as it is pushed and pulled along by the winds that surround it.
Fledgling cyclones are controlled by lower to mid-levels of the atmosphere, but mature cyclones, by virtue of being deeper and stronger systems, tend to be steered by winds over a greater depth, according to Mr Browne.
He said because cyclones are rotating storms on a rotating planet, they have an inherent and very small motion towards the poles and to the west, but they can also be influenced by low and high pressure systems as they move through.
“U-turns, wobbles, loops and near-stationary dwelling are all possible,” Mr Browne said.
“Slow-moving tropical cyclones tend to exhibit these erratic behaviours, while faster-moving cyclones are under a stronger steering pattern and move around closer to a straight line.
“Very rarely, tropical cyclones can interact with other tropical cyclones and become engaged in a spiral or orbit around one another, known as the Fujiwhara Effect.
“Usually the larger circulation will win out and absorb the smaller one.”
So what’s going on with Oma?
Cyclone Oma has already had an erratic path, completing a “loop-de-loop” west of Vanuatu late last week.
Mr Narramore said on Monday that Oma would be at the mercy of an upper trough and an upper ridge.
“I must stress a week is a long time in the tropics. We’ll definitely look at the next few days and see how it acts and then we’ll have a better idea of where it will head after that.
“Hopefully by mid-week we will get a better idea of the upper trough and also the upper ridge and the steering influences that will be affecting it by the weekend.”
Topics:
First posted
from Trend Gossip Now http://bit.ly/2Gu9pNW
0 Comments