Cyclone Hudah

Cyclone Vance

1999 in review

Tessi: April 1-2, 2000
Vaughan: April 6-7, 2000

North Queensland, Australia: This tropical paradise was recently reminded just how vulnerable its coastline is, when cyclone Tessi roared out of the Coral sea so quickly that computer models failed to predict its development.

According to Jeff Callaghan, meteorologist with Brisbane's severe storm centre, it was back to the good old days of 'synoptic reasoning' if Townsville residents were going to get any warning at all. Forecasters trawled through their knowledge gained from in-depth studies of past cyclone events to beat the computer to the punch. Tessi came ashore on the night of April 1 and saturated Castle Peak, causing landslides and considerable damage to expensive homes at the mountain's foot.

One week later, computer models were again confounded when Vaughan, spinning towards Cairns, first intensified then mysteriously evaporated, leaving only the surface-level eye to spin harmlessly onto the coast.

Is the weather getting weird, 
or is this natural science?

Callaghan says Tessie and Vaughan were classified as midgets, meaning they were created in the easterly trade winds without the help of moist and energetic monsoonal westerlies. These westerly winds are the lifeblood of northern Australia's wet season, and had failed to appear during the month of March.

Midgets develop quickly and degenerate quickly too. As Callaghan says, there's no big spinning system to keep the storm going once inflow of moist tropical air vanishes.

Ada, which clobbered Queensland's Whitsunday islands in the 1970s and Tracy which devastated Darwin on Christmas day 1974 were both classified as midgets. 

Because they are small and localised, midgets fly under the radar of computer modeling systems. Silicon-chip forecasting tends to look at the bigger picture. Often the data for small events has yet to be entered into the program. This was the case with Tessi, which wasn't even predicted as a small low pressure cell, let alone a tropical cyclone. 

Tessie's intensity was graphically illustrated by its pressure gradient: It's eye measured about 985 hPa, but dropped away quickly to 1008 at nearby Ingham and Townsville, where airport records showed sustained 60 knot winds. Such a large pressure drop over that small distance highlighted Tessi's localised intensity.

Fortunately the eye itself, only 10km in diametre, crossed the coast at a fairly remote township known as Bambaroo, 80km north of Townsville, where one resident huddled under a mattress for protection. Flooding in Townsville was huge, however, with 450mm dumped on this city over 48 hours, causing water-soaked earth on Castle Hill to collapse. Four hundred people in 100 homes were evacuated and 50,000 homes region-wide were without power. Early estimates from the Queensland Government estimated the damage bill at more than $12 million.

Callaghan reminds us that Tracy, which tore Darwin to the ground in 1974 was so intense on a local scale that residents thought the world was coming to an end. But, as he put it: "You could have driven out of Darwin and played cards on the night." Tracy's central pressure was 950hPa but Callaghan says it was equivalent to something like 930hPa.

One week after Tessi, cyclone Vaughan again caught forecasters by surprise when on the afternoon of April 5 it intensified, with winds gusting to 170kph, and headed west towards Cairns. Overnight, however, the storm seemed to evaporate. As the system headed west, driven by low and mid-level easterly trade winds, it ran into swift-moving westerly winds at high elevations which tore the storm apart. The meteorologists call it 'vertical shearing', and the result was spectacular - the upper portions of the storm literally blew away leaving the lower level eye to spin to its death all on its own.

As Callaghan said: 'The wind was blowing one way below and the opposite at the top and it just ripped it apart'.

Both Tessi and Vaughan were products of a La Nina year, in which low level easterly trade winds dominate off the coast of Queensland. Normally, says Callaghan, cyclones develop where the monsoonal westerlies and easterly trades collide. Without the moist westerlies, all sorts of things can happen - including the birth of a midget, or two.

© weather-wise.com 2000

April 1: Tessi approaches the coast. Note how loosely defined the storm is here. For reference, Fraser Island is at the bottom of the image.

April 2: Tessi, 24 hours later, intensifies and closes on the coast near Townsville.

Whump! Six hours later Tessi has reached her maximum strength and crosses the coast with 60 knot winds and 
soaking rains.

This infra-red image, courtesy of the
NRL Monterey Tropical Cyclone Homepage  shows cloud-top temperature at the centre of Tessi to be less than minus 80 degrees Celsius. Cold cloud equals lots of precipitation and in this image shows the storm's localised intensity.

Vaughan:

April 5: For the last week Vaughan has had its up and downs and here lies off the coast of Cooktown. Note the eye at the centre of a very disorganised cloud mass.

Six hours later Vaughan is suddenly a force to be reckoned with. Forecasters upgrade the storm to a category 3, the most severe cyclone to approach the
Queensland coast
this year.

April 6: 24 hours after the first image and 18 hours after it was upgraded to a category 3, Vaughan has been destroyed by high level opposing winds. All that remains is the surface-level eye (identified by the arrow). Six hours after this image was taken the eye disappeared entirely. What remained of Vaughan was a tropical depression that continued to soak Cape York and vicinity.

Black and white images courtesy of the
Australian Bureau
 of Meteorology.

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