Tropical Cyclones Affecting Perth

Historical Events |Tracks | Notable TCs | TC Alby

Although an infrequent occurrence in the Perth region, tropical cyclones have arguably been the most significant weather hazard in Perth's history. Cyclone Alby in April 1978 resulted in five deaths and caused coastal erosion, destructive winds equivalent of the strongest of winter fronts and devastating conditions for the spread of wildfires.

The region off the Northwest Australian coast is a prolific breeding area for tropical cyclones, experiencing about four to five cyclones each year on average. The vast majority of these storms dissipate well to the north of Perth, however a small number maintain considerable intensity as they travel south despite moving over cooler water and into a generally less favourable environment.

Media and public interest usually diminishes when a cyclone begins to lose its tropical cyclone characteristics and the intensity is downgraded as it moves south. However, occasionally a decaying tropical cyclone interacts with a cold front and evolves into an intense, fast-moving system. These systems can produce a range of destructive phenomena from intense rainfall, storm surges and large waves - resulting in coastal erosion and inundation through to damaging winds and hot, dry conditions - conducive to the spread of bushfires. The sudden onset of these phenomena tends to worsen the impact on a population more accustomed to benign weather conditions through the warmer months. The infrequency of these cyclones, combined with the accompanying changes in structure and motion also make them a difficult forecasting problem.

Cyclones affecting the southwest can move at speeds greater than 70 km/h in contrast to the average 10-15 km/h speeds in the north. As they accelerate, the structure of the cyclone changes so that the regions of dense cloud and heavy rainfall are displaced towards the right quadrants of the system (when looking along the direction of the track) leaving the left quadrants largely free of significant cloud. As a result the heaviest rainfall for example, would occur when a cyclone crosses the coast near and to the north of Perth as in March 1934.

The strongest winds associated with these fast moving systems occur in the left quadrants where the clockwise rotating winds are augmented by the system's translational speed. The cloud-free squally winds from the north or northeast are a recipe for severe dust storms and an extremely dangerous environment in which fires can engulf the countryside with frightening speed. The wildfires associated with cyclones in February 1937 and April 1978 (Alby) typify this potentially devastating weather scenario.

The change in structure described above is known as extra-tropical transition. This process is observed in other tropical cyclone basins around the world and can result in a re-intensification of the system even though it loses tropical cyclone characteristics. These accelerating tropical lows have been shown to be associated with intensifying cold fronts that moved to the northeast towards the tropical low. This 'capture' process and the resulting weather is shown schematically in figure 1. This is also demonstrated in the description of cyclone Alby.

Figure 1. Schematic diagram showing features of a tropical cyclone undergoing extra-tropical transition

Historical Events

During the 95 year period from 1910 to 2004 there were a total of fourteen tropical cyclones that either caused gales or caused wind-related property damage in the Perth region. Decaying cyclones that caused heavy rain but not gales were not included. This equates to an annual frequency of occurrence of 0.15 equivalent to about one every six to seven years. Interestingly there has not been a cyclone impact between 1992 and 2004 and this is likely to have led to an increased level of complacency in the population regarding the risk of a cyclone impact. There is considerable variation in decadal occurrence within the 95 year data period but given the small number of events it will require a much longer data set before meaningful conclusions in trends of occurrence can be made.

Cyclones affecting the lower west coast have occurred between the months of January and May and tend to occur later in the cyclone season than those in the tropics, peaking in March. Indeed cyclones in March and April make up over seventy per cent of the total number of cyclones affecting Perth.

While it is difficult to compare the severity and impact of these events cyclone Alby in 1978 is arguably the strongest in terms of winds in Perth (see TC Alby feature).

One of the early accounts of strong winds in Perth and in Western Australia was believed to be the result of a tropical cyclone on 11 April 1843. This was the first cyclone event for the fledgling Perth colony, transcended all experience of previous weather during the warmer months as described by the following account:

'Perth was visited by a most terrific gale from the northwest, amounting to a full hurricane. Strong gales from this quarter have frequently occurred during the winter months; but this gale was unusually severe, although it lasted for such a short time. It set in about 6 p.m., and lasted until nearly 10 p.m., when it gradually lulled. The gale was so sudden and tremendous as to force the Success to drift ashore with two anchors down.
At Australind (120 miles south of Perth) on Tuesday evening there was a short but most severe storm, which threatened alarming consequences, laying prostrate some fine trees.

At Bunbury, the day had been lowering, with wind from the eastward. In the afternoon a squall of wind and rain brought the wind around to northeast. About 7 p. m. it began to blow in gusts with great violence, and between 8 and 10 p. m. it increased to a perfect hurricane, the fury of the gusts was irresistible. The tide rose upwards of 4 feet in perpendicular height, and then in the course of twenty minutes fell 2 feet The North America, a United States whaler, 400 tons, drove with two anchors and grounded on the bar, as did the Chance schooner.'

While further cyclones were to affect Perth in 1845, 1867 and 1871, possibly the most severe summer storm experienced to that time occurred on 10 March 1872 as indicated by the following account of the storm:
'In Perth a storm of unprecedented severity occurred on the 10th March. About 7 am heavy rain fell, and from then for about 5 hours the gale raged with unabated fury. At noon the barometer read 29.20 inches, and the wind blew from eastnortheast. The destruction of property within the city was considerable, chimneys having been blown down and trees levelled to the ground. Telegraph communication was cut off between Perth and Fremantle, also beyond Guildford and between Pinjarra and Bunbury. The town hall of Perth and the Colonists Hospital were greatly damaged, Several cottages were blown down but no lives were lost.

At Bunbury, the town was visited by a most violent storm of wind and rain. The barque Midas dragged her anchors and finally ran aground and became a total wreck. The storm came from the northeast and north without warning. From noon it blew with utmost fury until about 4 p. m., when it began to gradually abate.

At Fremantle the port was visited by the most extraordinary weather that had been witnessed for some considerable time at that period of the year. The wind was so terrific that huge stones were lifted a distance of several feet from the ground.'

Tracks

The majority of cyclones affecting Perth originate from north of the Pilbara coast and initially move to the southwest then turn to the south then southeast. Some of these have affected large parts of the west coast. The 1956 cyclone moved down the west coast causing considerable damage along the way. Although very rare cyclones have originated from further west in the Indian Ocean and affected the west coast such as Marcelle in 1973 (not shown). The tracks of cyclones prior to the introduction of weather satellites to monitor weather systems in the 1960s should be treated with caution. A common factor with all of these cyclones is accelerated movement as they move to the south. By the time they reach Perth they can be moving at speeds in excess of 70 km/h.

See also the Interactive Tropical Cyclone Plotting web page to access tracks of historical tropical cyclones.

Figure 2. Tracks of notable cyclones affecting Perth. Click on image to enlarge.

Some Notable Cyclones Impacting Perth