Hello
Is there a SW to NE Storm path through Western Sydney?
Reading through the forum, I have noted that this topic has not been covered too well. Since moving to Sydney in 1997, I noted on two occasions being 3/11/2000 and 16/2/2002 two particular thunderstorms that moved up from the SW causing significant property damage across western Sydney. They were both severe thunderstorms or supercells. They moved in a NE direction taking roughly the same path.
Recently, I have come across a saying being 'The Oaks to Dee Why Supercell or storm Path". I have found evidence and material on it and I would like to open up a discussion thread on this to find out what the views are to see if this is true.
Researching this topic, I have found a summary of these storms going back to 1947 (As best as possible) but concentrating on the most damaging of all storms.
1) 1 January 1947 - A storm moves in a NE direction from SW Sydney. Hail was around 8 cm and damage was 3 million pounds (Equated to $47 million in today's currency). 100 were injured. There was property damage in the CBD.
2) 9 November 1976. A thunderstorm causes $131 million damage. 10 people are injured. Hail the size of golf balls and winds of 96 km/ h hits large areas of western Sydney moving in a NE direction.
3) 10 February 1978. A thunderstorm moves in a NE direction leaving 8 people injured. Winds reach 141 km / h. A tornado is reported in the Drummoyne and Hunters Hill area. Damage bill tops $44 million.
4) 3 October 1986. A thunderstorm drops 6 cm hail over suburbs of western Sydney and 10 people are injured. The damage bill tops $161 million.
5) 18 March 1990. A thunderstorm moves from a SW direction to NE dropping 8 cm hail. Liverpool, Bass Hill, Auburn are hardest hit. The hail swathe stretches from Camden to Narrabean. Severe damage to 14,000 homes, 9,000 vehicles and winds of 109 km / h over Bankstown. Damage is around $384 million.
6) 21 January 1991. Another thunderstorm moves from a SW direction to NE leaving 100 injured and a fatality. Hail 7 cm. Power loss to 164,000 homes and winds anywhere from 118 km / h to 230 km / h. Damage estimates range from $226 million but I have seen reports suggesting it was up to $680 million.
7) 12 February 1992. A thunderstorm drops 7.5 cm hail over western Sydney from Toongabbie to Girraween which indicates a NE direction. Some 3,000 homes and 7,000 vehicles damaged. Damage was $118 million.
8) 28 October 1995. A thunderstorm moves from SW to NE dropping hail at Cobbitty, 6.3 cm at Theresa Park and 4.5 CM at Parramatta. A study later undertaken by Sandra and Russell shows that the damage ellipse was 10 x 25 kilometres through Western Sydney and the preferred damage path was to the left side of the storm path (Left moving away from someone but to the right if moving towards someone).
9) 3 November 2000. A tornadic thunderstorm tracks from SW to NE bringing 7 cm hail. I have seen damage loses in the order of $250 million.
10) 16 February 2002. A thunderstorm tracks from SW to NE (Same path) bringing hail up to 6 cm. There were some 8,000 requests for help. I have seen damage bills around $280 million.
This history goes back as far as there are records that I can possibly find. If there are more, then I cannot find them despite extensive research. There are 10 known events since 1947 that I can find and 9 such events since 1976.
Given that until 2008 (32 years) from 1976, it does suggest an average return frequency of around 3.55 to 3.6 years specific for these big damaging storms events.
Looking at available radar images provided by the Bureau of Meteorology and historical analysis, they all seem to start in SW Sydney, track through a certain path through western Sydney and then track NE. That places certain suburbs in the firing line such as Liverpool, Fairfield and Parramatta.
In a study by Sandra S Schuster (2005) she has found evidence that from 1791 to 2003 there had been 1,570 storms in Sydney that has produced hail and the average hailstone is around 3.8 to 4 cm (Rather large) but most occur between 3 pm and 7 pm.
When looking at these thunderstorms heading from SW to NE, many seem to produce large hail around 6 to 8 cm in size at somepoint during their lifetime. The damage path always seem to be over the urban areas. Thus what is going on and why?
A stormy Sydney Forecast - October 2005:
Computer modelling by the Macquarie University physical geographers suggests that the removal of the natural vegetation from Sydney's SW over the past 200 years may have affected the weather patterns creating more dangerous thunderstorms (Specific to severe thunderstorms and supercells).
Professor Andy Pittman and Anna Gero have suggested that the basin could experience more of the extreme thunderstorms in coming years because of changes in land use patterns in the landscape in SW and western Sydney.
It is found that the clearing of land for agriculture and then housing has already had an impact on weather patterns. There are fewer trees to impede the storms progress in areas that have been cleared. Storms which brew over the SW do move NE across the city (Across an urban corridor). They travel towards the city significantly faster than storms originating in other areas.
Unstable conditions and convection are created as sea breezes moving across a landscape smoothed by clearing were able to accelerate.
With a majority of storms, there is no impact however, storms moving from the SW to NE across the city are accelerated (Professor Pittman 2005). Urban surfaces are capable of altering natural weather systems through the urban heat island effect (Parramatta City Centre also has a role in this).
Researchers created a virtual city in the computers (modelling) to represent Sydney's CBD. An intense convective storm occurred that did not occur under natural land cover conditions (vegetated areas).
Storms triggered by cold fronts did not respond to changes in land surfaces. Local convective storms such as those that occur in Sydney (Summer afternoons and evenings) have been found to be highly sensitive to the triggering mechanism associated with land surfaces.
The evidence is emerging to suggest that the urban heat island effect has a role to play as well. Reading a paper from Sciencedirect "The impact of land cover change on Storms in the Sydney Basin" 30 June 2006, it is found that:-
a) Storms travelling over the smoother agricultural land in SW Sydney experience an increase in velocity and in a special case, the dense urban surfaces of Sydney's city core appears to trigger an intense convective storm.
b) Parramatta City Centre has also been found to aid in the development of a more intense convective storm through the heat island effect.
More and more evidence is emerging that urban areas alter climate and can help to intensify a thunderstorm through the urban heat island effect. Studies by Balling and Burian (1987), Jauregui and Romales (1996) and others have shown that urban areas can influence storm initiation, intensity and motion.
Under certain conditions, a sea breeze will dictate the location of a storm cell and initiation and its travel path.
In Sydney, the basin is relatively flat, bounded to the north, west and south (To some extent the east) by hills which creates enough relief for a basin. Sydney is found to be frequented by storms during spring and summer (usually early afternoon to early evening).
Computer models have been used for SW triggered storms, frontal storms and isolated convective storms.
Incredibly from the numerous computer models, it is found that a storm initiating in SW Sydney travel over agricultural regions in their youth but as it does so, horizontal velocity is increased due to the presence of smoother surfaces. These storms do in fact travel NE. Strong winds, downdrafts and very heavy rain occurs but the storm moves at a rapid pace (Nothing to slow them). The cloud cell accelerates and there is less surface drag being exerted on the flow allowing winds to pass unimpeded. This type of storm is likely to cause the big damage claims (As highlighted early in this post).
Isolated convective storms:
These storms are relatively common in Sydney. These are isolated convective cells that are sensitive to inland propagation of the sea breeze. Instability is usually greater over the inland area than coastal areas.
The triggering mechanism for this storm is related to the dynamics forced by the sea breeze front. These are a typical layer convergence line where the convergence of advancing low level moisture advected inland by the sea breeze provides uplift and additional low level moisture capable of transporting warm humid air high into the atmosphere leading to a thunderstorm. Wind shear, buoyancy, moisture depth and strength of the convergence also contributes.
Storms in the isolated convective storm category are highly sensitive to the triggering mechanism which is associated with an advancing sea breeze and the convergence of moisture occurring at its leading edge.
The models in the study show the sea breeze late in the day across much of Sydney's eastern areas but the western areas remain unaffected by the sea breeze.
It seems that the two go hand in hand being the Heat island effect and the position of the sea breeze. In addition, it is also found that land surfaces in western Sydney is 0.6 C warmer than natural surfaces averaged for pre storm simulations.
It appears that exceptional conditions (With the above two in mind) are required for this thunderstorm (Being a severe thunderstorm or supercell for western Sydney). Historically, only 1 such storm per season is documented in any given season.
When does it occur:
Looking at records (I have gone back as far as possible to 1947 and can only find 10 such damaging storms on western Sydney) emanating from the SW portion of Sydney from the Oaks region or nearby areas. (This discusion should exclude the 1999 event, 2003 event and 2007 event as the storms were on separate pathways). I cannot find any record between 1947 and 1970 or earlier records. There are records from 1970 onwards with the first documented one in 1976. If this was used as a base date then since 1976, there have been 9 such storms giving an average return frequency of 3.55 years.
However, interestingly, looking at when they occur, I bought a book called Extreme Weather by H Michael Mogil 2007 which shows all the La Nina and El Nino years from 1950. When plotted against this, it does suggest that such a thunderstorm in Sydney (Being a damaging severe storm or supercell from the SW to NE) strikes towards the end of a La Nina event or as the Southern Oscillation is tending towards a neutral pattern.
We are now moving into a post La Nina phase of the ENSO cycle which provides some food for thought. Could this storm path fire a damaging supercell this upcoming spring or summer??
Any thoughts and discussion on this topic.
Sources
Bureau of Meteorology (Archives and records 1970 - 2008).
Extreme Weather H Michael Mogil, 2007.
The Impact of Land Cover Change on Storms in the Sydney Basin (Macquarie University 2006).
Macquarie University (2005).
A Stormy Sydney Forecast (Lyn Danninger 2005).
Sandra Schuster (2005).
Spillane and Dixon (1969).
Harley Pearman
