BoM - Cool season storms

During the cooler months of the year the southwest of Western Australia experiences moist westerly winds that generally produce over eighty per cent of the region's annual rainfall. Cold fronts embedded in the westerly flow bring the rain and strong winds on an irregular cycle every few days. Each front is unique, varying with the strength of wind, and amount and distribution of rainfall.

Some of the more vigorous fronts produce severe wind gusts in excess of 90 km/h, or may spawn tornadoes. The level of threat to life and property may also be described in terms of wind strength. Damaging winds refer to wind gusts between 90 and 125 km/h, and destructive winds refer to wind gusts in excess of 125 km/h. These events are not necessarily accompanied by lightning, but they are still referred to as 'storms'.

It is possible to distinguish two general types of storm threats that cause severe winds during the cooler months:
1. Strong fronts and intense lows that cause sustained gale-force winds and severe gusts over a widespread area, and
2. Fronts that cause localised severe winds, including tornadoes.

Widespread severe winds

Although fronts frequently move over southwest Western Australia in the cooler months, only a few times each year do very strong cold fronts associated with deep low-pressure systems affect the region. These can cause gale-force winds and heavy rain over the southwest, including Perth. The more significant events also produce an increase in tidal levels known as a storm surge, and also large waves that can result in coastal erosion (see storm surge). Wind damage, such as fallen power lines, typically occurs over a wide area, with the potential to cause significant power outages and traffic disruption. Often fallen trees and branches cause much of the property damage. More severe damage, likely to be caused by destructive winds exceeding 125 km/h, is usually localised (see localised severe winds).

The wind event is more significant if the strong westerlies remain for extended periods. Usually this occurs when a low or series of deep lows moves slowly south of the state and generates multiple fronts maintaining a strong westerly airstream over the southwest. On 7-8 June 1981, for example, winds at Fremantle (recorded at a height of 60 m) exceeded 55 km/h for 36 hours.

A characteristic of major westerly events is the strong winds associated with downdrafts accompanying showers and thunderstorms. These winds are transported from higher levels to the surface, causing severe squalls of at least 90 km/h, but usually less than 125 km/h. Gusts may be quite localised, becoming more widespread if associated with showers on a front or pre-frontal line.

An example of a deep low and associated strong pressure gradient over the southwest is shown in the mean sea level pressure analysis of 14 September 2002 in Figure 1. The accompanying satellite image in Figure 2 shows the distinctive cloud band along the frontal boundary followed by the speckled cloud indicative of the cold airmass to the west.


Figure 1. Mean Sea Level Pressure analysis showing a strong front extending through Perth to a deep low south of Albany, 14 September 2002. Click on image to enlarge.	Figure 2. Satellite image of a strong front near Perth, 14 September 2002 (Courtesy of the Japan Meteorological Agency). Click on image to enlarge.

Historical events
Winter storms have long been recognised as a major hazard of the area. The indigenous Nyungar people in the Perth region tended to move inland during the cooler months, partly to avoid the westerly gales. In June 1801, the French explorer Nicholas Baudin felt the full force of a winter gale as he rounded Cape Leeuwin en route to Rottnest Island. The early settlers were quickly made aware of the dangers of winter gales. The loss of the Marquess of Anglesea and damage to other vessels during a gale in 1830 threatened the existence of the colony, being described at the time as almost a 'death blow to the infant colony' (Bureau of Meteorology, 1929, p. 164). Winter gales posed a frequent threat to shipping, not only disrupting services but occasionally sending ships aground and sometimes wrecking ships completely. A list of some of the major winter storms of the last fifty years and their impact is shown in Table 1.

Table 1. Notable deep winter lows causing gales during the cooler months.

Date	Impact Description
14 August 1955	Several westerly gale events caused massive coastal erosion on Perth's beaches. At Cottesloe not a vestige of sand was left anywhere along the seaward side of the main 120 m promenade, leaving exposed the underlying limestone.
19-20 August 1963	A strong front caused a wind gust of 156 km/h in Perth, the highest ever recorded in the Perth area. A suspected tornado demolished a factory in Scarborough just before midnight and the trail of damage extended through Doubleview and Innaloo.
8 June 1981	Strong, squally winds, accompanied by heavy rain, produced widespread damage about the southwest coast. Perth metropolitan area was littered with fallen trees and power lines and the debris from damaged buildings. Damage extended north to Geraldton, inland to Northam, and south to Harvey.
28-29 June 1983	This storm was responsible for widespread damage in the southwest, including the loss of two lives. Bad weather was blamed for a road crash fatality and an American sailor was crushed behind a ship's door in high winds. The storm downed trees and power poles, with blackouts lasting up to three days after the event. Roads were cut due to heavy rain. The damage bill for this storm was estimated to be greater than $1 million (1983 dollars).
22 September 1988	The storm caused extensive damage from Perth to Albany. Hundreds of roofs were damaged and several ripped off entirely, trees were downed, and power was lost to over 100,000 homes in the metropolitan area. Damage was sustained to about twenty boats in the Perth area after breaking their moorings. Severe damage was also done to Perth market gardens and vineyards. The cost was estimated (in 1991 dollars), to be $8 million (McCready & Hanstrum, 1995).
23-24 May 1994	The windstorm was one of the most destructive weather events to affect Perth, with a total damage bill of $37 million (1998 dollars). Several houses were completely unroofed but the majority of property damage was minor, most claims being for fence damage. Downed powerlines, mostly due to fallen trees and branches, caused widespread blackouts, leaving up to one-third of Perth without power at the height of the storm. Two people on a yacht off the coast from Jurien Bay were lost at sea. Huge seas and above normal tides caused significant erosion to beaches. Parts of the Perth river foreshore were also inundated. Swanbourne recorded a maximum gust of 143 km/h, while winds at Fremantle averaged 107 km/h over a 30 minute period.
16 May 2003	A deep low moved over the coast near Cape Naturaliste where the pressure fell to 982 hPa, one of the lowest pressures ever recorded in southern Western Australia. Fremantle recorded a storm surge of 0.8 m causing the highest tide ever recorded at Fremantle, about 0.5 m above the highest astronomical tide. The resulting flooding in low-lying parts disrupted traffic and undermined the foundations of a canal property near Mandurah. The high tides and heavy surf caused widespread beach erosion. Wind damage was general, although mostly minor. Rottnest Island recorded a gust of 117 km/h and Ocean Reef 115 km/h.

In more recent times, the event of 23 May 1994 ranks as one of the most significant wind storms experienced in Perth, causing an estimated $37 million damage (1998 dollars). Apart from two houses that were unroofed, most of the damage was of a minor nature such as damage to fences, with the average insurance claim only $700 (McCready and Hanstrum, 1995). The Australian (25 May, 1994) reported that at least 600 homes in Perth had sustained some form of damage. Power outages were so widespread with more than 250,000 buildings affected, that it took almost a week to fully restore power to Perth homes. In addition to affecting businesses and homes, the loss of power affected pumping stations resulting in minor spills of raw sewage into the Swan and Canning rivers from 29 pumping stations (The West Australian, 25 May, 1994). Two people on board a yacht were lost at sea off Jurien, north of Perth. Huge seas and above normal tides caused significant erosion to beaches, while parts of the Perth river foreshore were inundated. Fremantle recorded 25 wind gusts of at least 129 km/h, more than three times the number recorded for any other event since the 1960s.

Frequency
A simple indicator of the intensity of fronts affecting the lower west coast is the pressure gradient from Geraldton to Albany. A 'storm event' can be defined when the pressure gradient exceeds 15 hPa. Although this does not always reflect the intensity of every system, it does provide an objective comparison between events since 1965, when three hourly pressure data was available from both Geraldton and Albany. This threshold generally correlates to gale-force winds on the coast and wind gusts exceeding 90 km/h. There have been exactly 100 such events over a 39-year period from 1965 to 2003, equating to about 2.6 per year. There have been eight major events when the pressure gradient has reached at least 20 hPa such as in May 1994. On average this equates to one major event every five years. The monthly frequency of storm events is shown in Figure 3. Seventy percent of events are relatively evenly distributed between the months of June and September, though storms events have been recorded as early as April and as late as November.

Figure 3. Monthly frequency of major winter storm events, 1965 - 2003. An event is defined by the occurrence of the Geraldton to Albany pressure gradient exceeding 15 hPa. Click on image to enlarge.

Severe local wind storms

The most severe winds that occur in Perth are very localised, typically causing a narrow swathe of damage just tens of metres in width. Severe local wind storms (SLWS) can be divided into tornadoes and downbursts. It is often difficult to determine the exact cause of damage, particularly for weaker storm events. Where significant damage has occurred, indicating destructive winds in excess of 125 km/h, it can be easier to attribute the cause of damage to a tornado. In the past a tornado has colloquially been called a 'cock-eyed bob' in Western Australia.

Apart from the strong fronts, more moderate systems can also produce a SLWS, making these events particularly difficult to forecast. They typically occur with deep convective clouds on the frontal boundary, but may also occur on convergent lines ahead of the front, or in the cold air following the front.

SLWS are typically short-lived and move at speeds of up to 80 km/h with the associated front. Their narrow width and short path lengths, typically less than 5 km, means that they affect only small areas. They are relatively weak, usually only rating F0 (62-117 km/h) or F1 (118-178 km/h) on the Fujita tornado scale, based on extent and severity of damage. Damage reports and eyewitness accounts of these events indicate that damage varies considerably along the track. Some houses may be significantly damaged, while others of a similar construction nearby can escape any impact at all. Some reach F2 category (179-250 km/h), such as the Collie tornado in 1960.

Damage is often discontinuous along the track, suggesting that severe winds do not move evenly over the ground, especially when the terrain varies. Exposed elevated areas are more vulnerable to severe winds than sheltered parts on the leeward side of hills. Trees are often affected at different heights. Larger trees snapped off near the base indicate a much stronger wind than those with higher branches only broken off. A farmer described a tornado at Williams in 1959 passing over a flock of sheep without disturbing them. Fronts may spawn more than one tornado at a time. On 17 October 2002, a front of what appeared to be moderate intensity caused widespread localised damage across the South West Land Division with four tornadoes in the metropolitan area and several other confirmed tornadoes. It is likely that many more tornadoes also went unreported.

Cool season tornadoes are very different from those associated with severe thunderstorms in the warmer months, including those on the Great Plains in the USA. It is quite rare to have a report of a funnel cloud extending from the base of a cloud. Visibility is typically very low as it is usually raining heavily and they move at typical speeds of 60 to 80 km/h. Many people have reported hearing a sound like an approaching freight train or similar, which is a common description of tornadoes elsewhere.

Historical events
The earliest record of a suspected tornado occurred during the early morning of 18 June 1842 at Australind, north of Bunbury. The Perth Enquirer reported a 'wind storm' which moved across the narrow strip of land between the sea and the Leschenault Estuary, creating a lane through the forest 300 to 400 yards wide and extending from the northwest to southeast across the Collie and Preston Rivers. In the centre of the path nothing was left standing but bare trunks of trees. The storm was accompanied by rain, hail and lightning and the wind turned from northerly on the 17th to the west southwest on the 18th.

The first observation of a probable tornado in the Perth area was reported by a Captain Wray on 4 June 1856. He described a 'whirlwind' which moved over Fremantle prison from northwest to southeast, accompanied by a loud hissing noise. The whole north boundary prison wall was laid flat, 'turning over its foundations like a hinge'.

One of the most intense tornadoes to affect southwest Western Australia swept through dense Jarrah forest at Lyall's Mill near Collie on 6 April 1960. The tornado cut a path averaging 240 m wide along a continuous track of 30 km. Forestry officers estimated that the volume of felled timber was enough to provide a year's work at a local mill. On 11 July 1964, a teenager was killed and several others injured when a tornado pushed a moving car off the road at Busselton (The Sunday Times, 12 July 1964).

Closer to Perth, a severe wind squall likely to have been a tornado destroyed 40 caravans and extensively damaged 24 cottages at Naval Base, south of Fremantle on 8 June 1968. The damage trail is shown in Plate 2.1. Plate 2.2 shows one of eight homes in Mandurah destroyed by storms which damaged more than 100 dwellings on 22 September 1993 (Hanstrum, 1994). The damage to an apartment building in Fremantle on 13 August 1999 is shown in Plate 2.3. The remains of a house after a reported tornado at Rockingham on 13 June 2001 is shown in Plate 2.4. The experience of a man who narrowly escaped death from the 12 August 1987 tornado in Mandurah is described in The West Australian (13 August, 1987):

He heard the terrifying scream of ripping steel. He looked up and saw guttering, tiles and branches flying across the street. At one stage, a shed flew about 20 m over a house on the other side of the street. A flash of white rebounded off the side of one house and on to the roof of another. It was an aluminium boat ... A 135 kg waste disposal bin from next door was picked up by the wind and smashed into the side of their house. It took several men to lift it back into place. The Bairds' water tank was carried about 70 m, smashing through fences and a sign before coming to rest in bushland.

Frequency
The occurrence of severe localised winds including tornadoes is much greater than commonly thought. While most events occur in coastal parts between Perth and Albany, they have been recorded across the South West Land Division from Geraldton, Southern Cross and near Esperance. It is not coincidental that the highest number of reports comes from the most heavily populated parts of the coastal strip between Perth and Busselton. The highly localised nature of these events mean that those in remote bushland are likely to go unreported, making it very difficult to give meaningful statistics on the frequency especially in an area of low population density.

Coastal suburbs are more likely to experience a SWLS than those further inland, probably due to frictional discontinuity at the coast. A particularly high proportion of reports are from the Mandurah and Rockingham areas south of Perth. It has been conjectured that the change in coastline orientation make these locations more susceptible to SLWS events than other parts of the Perth metropolitan area. The extra lifting and frictional effects of the Darling Escarpment suggest that the frequency of events should be higher near the escarpment than on the adjacent coastal plain but the extent of bushland near the escarpment makes this claim difficult to confirm.

Severe local wind storms have been reported between April and October. The earliest in any year that a storm has occurred has been on the 6 April (1960) and the latest on 31 October (2002). Severe local wind storms occur most commonly in June. They can occur at any time of the day and there appears to be no higher concentration of occurrence at any particular time of the day.

Foley and Hanstrum (19890) analysed SLWS during the cooler months in the southwest of the State from 1958 to 1988. They identified 51 cases on 47 days in the 31-year period using newspaper clippings, averaging 1.6 per year. This included 33 cases in the Perth metropolitan area. However, they concluded the frequency of events along the Lower West coastal strip to be higher at 5.6 per year, after accounting for variations in reports due to population density. From 1993 to 2003 there has been an average of 5.5 reports per year in the region, a considerable increase in reports over the 1958 to 1988 study. In Perth there were 31 suspected tornado cases on 26 days in the eleven years to 2003, almost the same number as in the 31 years in the previous study. However, this increase is likely to be more attributable to an improved detection rate rather than an actual increase in frequency. Additionally there were many more SLWS cases when wind gusts exceeded 90 km/h but have not caused any known damage.

An alternative method to assess the frequency of SLWS is to identify the atmospheric environmental features in which they occurred. In a study of 20 tornadoes in Western Australia and South Australia from 1994-96, Hanstrum et al (1998) identified the critical environmental features for tornado development. This study has not only improved the ability to forecast such events but also assists in more objectively assessing the frequency of occurrence.

An unpublished study of observational records from 1967 to 2003 showed the frequency of potential tornado days to be about nine per year for Perth. Experience suggests that this is probably an overestimate of actual tornado days, especially when considering the incidence in just the Perth metropolitan area. Based on reports from 1993 to 2003, the average number of tornadoes in the Perth area seems to be in the order of three per year. However, the theoretical number of nine potential tornado days may more closely match the number of frontal events that cause severe wind gusts in Perth. An unpublished study of cold fronts from 1995 to 2003 showed that there were an average 9.5 events per year causing severe wind gusts in Perth. Of these, two to three were considered major fronts accounting for widespread minor damage, four caused local damage, and three events had no known damage although severe wind gusts were recorded.

A description of some significant severe local wind squalls in Perth and the Southwest region is given in Table 2.

Rainfall

Although the aforementioned events causing severe winds can also cause periods of heavy rain, they do not represent the strongest rainfall producing events. Instead, cold fronts that generate slow moving cloudbands extending into the tropics typically cause the heaviest cumulative rainfall. These are discussed in more detail in Heavy Rainfall and Flooding.

Storm Surge and Coastal Erosion

One of the consequences of sustained gale-force winds about the lower west coast is storm surge. Storm surge is a rise in the normal water level caused by strong onshore winds and reduced air pressure. The impact is greatest if the storm surge peaks near the time of high tide. This can result in the inundation of the river foreshore such as Riverside Drive near the Perth city centre and on the freeway just south of the Narrows Bridge. Low-lying coastal areas such as at Mandurah, Bunbury and Busselton are particularly vulnerable to inundation. A storm surge also exacerbates flooding on the coastal plain by not allowing the river water to escape out to sea. A worst case scenario therefore is to have the peak in the gale-force winds coincide with a high tide, along with floodwaters coming down the Swan River.

During the 23 May 1994 storm, the tidal elevation measured at Fremantle showed a storm surge of 0.98 m. Fortunately the potential for more serious and extensive flooding was not realised as the peak surge occurred near the time of predicted low tide. A strong westerly gale on 20 July 1910 caused damage along the west coast to as far north as Geraldton. The Fremantle North Mole was damaged while on the Swan River all the surrounding low-lying lands and many of the jetties were submerged. More recently, gales associated with a low passing near Busselton on 16 May 2003 caused a storm surge of 0.8 m at Fremantle at close to the time of high tide as shown in Figure 6. The actual tide was 0.5 m above the highest astronomical tide and significant coastal erosion occurred. Figure 7 shows water overflowing onto Riverside Drive near the city centre. Fortunately existing river levels were low owing to dry conditions in the previous months, otherwise flooding would have been significantly worse.

Figure 6. Actual tide above the predicted tide at Fremantle, 15-16 May, 2003, showing the impact of a storm surge during a cool season westerly storm (Data courtesy of the Department of Planning and Infrastructure, Western Australia). Click on image to enlarge.

Figure 7. The effects of the storm surge on Riverside drive, 16 May 2003 (Courtesy of Lee Evelegh).

Coastal erosion results when the pounding of large waves generated by significant wind speeds combine with a storm surge. Every winter Perth's beaches are temporarily modified due to the scouring of beach sand from strong winter storms. A strong westerly gale on 20 July 1910 caused damage to the Fremantle North Mole and the foundation of the Jandakot railway line (Bureau of Meteorology, 1929). During the May 1994 event, significant coastal erosion still occurred despite the storm surge peaking near the time of the normal low tide because of wave action. The open water swell was estimated at eight to nine metres. Storms in August 1955 badly damaged many of the beaches between South Beach and Sorrento. South of the jetty at Cottesloe Beach, 150m of foreshore were eroded back about 25 m during the storms. An extract from The West Australian (18 August, 1955) illustrates some of the damage.

At Cottesloe not a vestige of sand is now left anywhere along the seaward side of the main 400 ft. promenade. The whole of the area is rugged dangerous limestone. The 12 ft. high wall of the promenade is firmly embedded in the limestone reef, but yesterday morning heavy seas were dashing high against it. A certain amount of undermining was going on. At the foot of the former jetty where the waves were heaviest a 4 ft. wide hole had been made in the promenade bitumen surface ... The main damage at North Cottesloe has been the undermining of an 50 ton wartime beach observation post of concrete and steel and the collapse of the 30 ft. steel shark alarm tower ... In Sunday's storm hundreds of tons of sand and big slabs of concrete were carried away from in front of the Swanbourne Nedlands Surf Club pavilion.

Hail

During the winter months hail can occur in the cold airmass following the passage of a front. However, the growth of hail is much less than in summer thunderstorm clouds and the size is generally on the order of one centimetre or less, and not large enough to cause damage. Nevertheless, a large amount of small hail can block drains that can lead to water inundation to properties. Larger size hail is more common in Spring and the warmer months, and is discussed in more detail in warm season storms.

Thunderstorms and Lightning

Despite cool season events being described as storms, only a minority of them actually cause thunderstorms and lightning. In Perth there are an average of two thunderstorms per month from May to July and one per month for the remainder of the year. One such lightning event cut all computers and telephones in the Royal Perth Hospital and subsequently put human life at risk (The West Australian, 1 July 1996). The electrification process is weaker in cool season thunderstorms than those in the warmer months, so the number of lightning strikes is considerably less from May to July than in the months of January to March.