A TROPICAL cyclone that tracks as far south as the south-east coast of Queensland and makes an east-west crossing onto the mainland is a rare phenomenon.
A tropical cyclone that tracks past the south-east coast of Queensland, then turns back to make an east-west crossing onto the mainland, as was the case with Tropical Cyclone Alfred, is an even rarer phenomenon.
Amateur weather enthusiast Neil Pennell explains the extraordinary aspects of TC Alfred.
“To my knowledge, the earliest years for which the Bureau of Meteorology (BoM) has tracking data is the 1906-07 cyclone season.
“The first thing to note is it’s been a long time since a cyclone made an east-west crossing of the Queensland coast between Hervey Bay and Coolangatta.
“It last happened 51 years ago during the 1973-74 cyclone season.
“The second thing to note is how rarely this has happened – only nine times in the last 108 years.”
Mr Pennell says there are two key reasons why it is such a rarity.
“Cyclones are inextricably drawn south as they head away from the tropics.
“To quote a passage from the BoM website … ‘cyclones typically form in the deep tropics (but not too close to the equator) … and initially move towards the west-south-west. As they get further away from the equator, they encounter winds that cause them to re-curve and be steered first towards the south, and then towards the south-east’.”
The second key reason has to do with the geography of the Queensland coast.
“It is a challenge for cyclones to make an east-west crossing between Hervey Bay and Coolangatta, simply because of the north-south orientation of this bit of coast,” he said.
“It makes complete sense that something moving south or south-east is going to struggle to hit a coast that is running north to south.
“This issue of angles and directions is important because it explains why Townsville to Bowen (orientated north-west to south-east) is, by some margin, the most common stretch of our Queensland coastline to have a coastal crossing.
“It’s also the reason why we have had significant flooding on many occasions here in South East Queensland from ex-cyclones approaching us over land from the north – notably ex-cyclones Oswald and Debbie.”
Cyclones passing by in the ocean off South East Queensland are not a rarity.
“Plenty do, on the way to New Zealand’s North Island,” Mr Pennell said.
“Mostly they are weakening and mostly they miss.
“Occasionally, very strong cyclones pass by.
“Cyclone Pam from that epic season of 1973-74 was still a Category 4 until it was at a latitude south of Grafton but fortunately it was well away from the coast.
“Three of the nine cyclones which have made an east-west crossing to the mainland were coast scrapers – unnamed in March 1946, Daisy in February 1972 and Zoe in March 1974.
“The remaining six were proper east-west coastal crossings – the unnamed ones in February 1928, in January 1947, in February 1954; and Annie in 1963, Dora in February 1971 and Wanda in January 1974.
“With the exception of Annie, this small group of cyclones produced floods of pretty epic proportions which is why TC Alfred was of such concern to the meteorologists.”
Of particular note was the Gold Coast – Northern New South Wales unnamed cyclone of February 1954.
“It is the only one that crossed the coast as a Category 3.
“It caused immense damage.
“Cyclones Zoe and Annie were Category 2s.
“The others have all been Category 1.”
As it happened, TC Alfred was a Category 1 when it passed over Moreton Island, but in the days prior to that it had intensified and fluctuated between a Category 3 and Category 4 (February to March 4), then weakened to a Category 2 through to the day before it reached the barrier islands north of Brisbane.
There is only one other precedent for a tropical cyclone heading back into South East Queensland after passing further south than Brisbane.
“That precedent was Cyclone Dora in 1971,” Mr Pennell said.
“So, what we saw happen with TC Alfred was extraordinarily rare.”
