BoM - Warm season thunderstorms

Compared to eastern and northern Australia, thunderstorms are infrequent in Perth during the warm season between October and April, occurring on average five to six times. In addition, only a small number of these reach a level of severity that poses a serious threat to the community. The number of warm season thunderstorm days varies considerably from year to year, ranging from 14 in 1991-92, to just one in the 1990-91 and 1993-94 seasons.

Most storms develop in the afternoon when surface heating is greatest. A typical feature of the weather pattern in the warmer months is a low pressure trough that extends from the Pilbara to southern parts of the state. Surface heating combined with convergence near this trough in the low levels of the atmosphere can trigger convection providing there is sufficient moisture. Figure 1 is a satellite image showing storms developing along the trough near the west coast.

Storms are more frequent to the north and east of Perth. Typically, a trough forms inland of the city, or moves inland during the day with the seabreeze prior to storms developing along it. On such days storm clouds can be clearly seen to the east of the city over the ranges. When the trough is near the coast they are usually more likely to form in the warmer air to the north and be steered to the southeast by mid-level winds. For these reasons the eastern hills areas such as Chittering and Gidgegannup have a higher incidence of warm season storms than other areas.

About thirty to forty per cent of Perth's warm season storms occur in the early morning. They usually result from uplift forced by strong easterly winds near the surface rather than from surface heating. These storms are more difficult to forecast. Such events are not usually associated with severe weather phenomena, but when combined with other forcing mechanisms or tropical moisture such as on 8 February 1992, strong winds, hail and heavy rain can be produced.

Figure 1. Satellite image showing thunderstorms developing near the west coast trough just to the north of Perth on 30 January 2000 (Courtesy of the Japan Meteorological Agency).

Most storm events are highly localised and do not usually affect large areas, in contrast to tropical cyclones and winter fronts. Individual thunderstorms typically extend over an area less than 10 km, move at typical speeds of 20-40 km/h, and last only a few hours. It is quite common for multiple thunderstorm cells to form in any given situation so the area affected may be considerably greater. Figure 2 shows radar composite tracks of thunderstorm cells over four hours on 27 March 1995. Storms developing over a widespread area usually require abundant moisture inland, possibly associated with a tropical low or cyclone to the north. The satellite image in Figure 3 shows widespread storms over the southwest associated with a low off the Pilbara coast during the 22 January 2000 rain event. Some types of severe storms can last for several hours. Figure 4 shows low level cloud indicating the approach of storms on 30 March 2003. On this day, 29 mm of rain was recorded in just 30 minutes at Perth Airport.

Although all thunderstorms pose a danger to life and property through lightning strikes, very few of Perth's thunderstorms reach 'severe' status. A severe thunderstorm is defined as producing one or more of the following:
(a) hailstones with a diameter of 2 cm or more;
(b) wind gusts of 90 km/h or greater;
(c) flash flooding; and/or
(d) tornadoes.

Table 1. Some notable warm season thunderstorms affecting the Perth region.

Date	Impact Description
1 January 1960	An afternoon storm that initially caused heavy rain and hail in the western suburbs produced a tornado at Byford that caused a swathe of damage through mostly bushland. Apart from one house that was unroofed; damage was minor.
20 February 1977	An afternoon storm caused severe winds, recorded to 113 km/h in the City. The winds created havoc on the Swan River, with one person drowning when a yacht capsized. In adjacent eastern and southern suburbs buildings were unroofed, caravans overturned and trees and power lines brought down.
2 January 1982	Golf-ball sized hail occurred in the northern suburbs.
7-8 November 1986	Overnight storms lasted for ten hours and caused some isolated strong gusts.
16 January 1991	A dry microburst, measured to 108 km/h moved a parked passenger jet at Perth Airport, resulting in structural damage in excess of $1 million.
8 February 1992	Slow-moving storms caused Perth's wettest day on record, with falls over 100 mm (Medina 231 mm). Strong winds and golf-ball sized hail damaged apartment blocks in Glendalough in the morning.
23 February 1995	An afternoon severe thunderstorm downburst caused damage amounting to over four million dollars in the eastern suburbs of Perth. A five-tonne demountable home was flipped onto the roof of another building and 30 one-tonne swimming pools were picked up by the winds and scattered about. Perth Airport recorded a gust of 115 km/h.
27 March 1995	A severe storm caused hail measured to 5 cm diameter at Woodbridge, north of Perth.
14 December 1998	Lightning killed a jockey at Ascot racecourse. Wind damage affected tens of properties in the southern suburbs while hail damaged vegetable crops.
22 January 2000	Slow-moving widespread storms caused heavy rain resulting in flooding. Perth registered 104 mm, its second highest daily fall.
18 November 2001	Overnight storms caused severe localised winds that damaged more than 25 houses in the northern suburb of Ridgewood (Wanneroo).
14 January 2002	A supercell formed north of Perth causing hail at Regans Ford, before moving offshore and striking Kwinana, shifting an oil tanker from its moorings, and causing millions of dollars damage to the refinery.
29-30 March 2003	Storms overnight on the 29th and during the 30th, caused power outages to 70,000 Perth homes at its peak and the SES received over 100 callouts for flood-related damage. Karragullen registered 40 mm in 45 minutes.

There are several different types of severe warm season thunderstorms that affect Perth. Although quite infrequent, the supercell type storm can maintain an intense state for several hours. It has an organised structure of co-existing updraught and downdraught and can move in different directions to normal thunderstorms. Of particular concern to Perth are those that form to the north of Perth and move to the right of the typical southeasterly track. Therefore, rather than moving inland they move to the south or even southwest near the coast as shown in the radar tracks in Figure 2. On 27 March 1995 and 14 January 2002, supercells caused large hail just north of Perth before moving offshore. These storms are capable of producing large hail, heavy rain severe wind gusts and even tornadoes. In January 1960, a tornado caused a swathe of damage in the Byford area. However, such events are rare in the metropolitan area.

radar tracks

Figure 2. Composite tracks of thunderstorm cells over four hours (4-8 pm 27 March 1995). Most cells move to the southeast but severe cells A and D split away and move to the south-southwest. Fortunately these were most severe after moving off the coast but cell D caused hail to 5 cm diameter near the coast.

satellite image showing storms

Figure 3. Satellite image showing thunderstorm area over the southwest during the 22 January 2000 rain event. A tropical depression off the Pilbara coast provided the moisture while an upper level low off the west coast combined with a surface trough near the coast to caused widespread heavy rain (Courtesy of the Japan Meteorological Agency).

Rainfall

While most thunderstorms are capable of producing heavy rain for a period during their mature phase, the amount of moisture in the lower levels of the atmosphere is usually a constraining factor for storms near the west coast. The continental airmass is usually significantly drier than that over the tropics or near the eastern Australian coast. Perth residents usually welcome rain from thunderstorms during the dry season, particularly if it brings some respite from the heat.

Although the natural environment of the coastal plain is not particularly prone to flash flooding, heavy summer rains have a major impact on a population unaccustomed to rain during the warm season. Sufficiently heavy rain that may cause flash flooding is rare and usually requires a moisture source from tropical origins or an unusual combination of features such as a slow moving low-level convergence zone and deep upper-level trough. On 8 February 1992 such a combination led to sustained heavy rainfall in the Perth region and flooding, particularly near the Darling Ranges. Perth registered its highest daily fall of 121 mm while Medina recorded 230 mm (see Heavy Rainfall and Flooding).

Hail

Large hailstones, in excess of 2 cm diameter are infrequent and are usually associated with supercell type storms. Although hail size reports are often quite subjective there have been confirmed reports of hail greater than 3 cm in diameter in Perth. The largest known hail report was 5 cm, just north of Perth at Woodridge on 27 March 1995.

Lightning

A significant consequence of summer thunderstorms is lightning. In Australia, lightning accounts for five to ten deaths and well over 100 injuries annually. In Perth, lightning caused deaths are infrequent, with only one over the last decade. A roof tiler who narrowly escaped death after being hit by lightning in Perth on March 1995 described his ordeal to The West Australian (28 March, 1995):

"I heard a crack, saw a big blue flash, then time just stood still", he said from his bed at Sir Charles Gairdner Hospital. "I was still on my feet but the top half of my body just folded over" ... He told doctors he felt a sharp pain in his neck and ankle - the suspected entry and exit points of the lightning.

Significant damage to electrical and communications equipment and appliances is common. Following lightning strikes in January 1999, for example, 900 fault calls were received by Telstra, amounting to almost $1 million in costs (The West Australian, 25 January, 1999). A frequent problem is lightning-ignited bushfires, particularly if little rain occurs. On 22-23 December 2002, lightning ignited dozens of fires in the metropolitan area creating havoc for fire agencies. Figure 5 illustrates the spectacular show that can be created by lightning. Lightning illuminated the 'shelf' cloud of the severe thunderstorm that caused damage in the northern suburbs.

Warm Season Storms

The thunderstorm threat

Rainfall

Hail

Lightning