East Coast Low 6 - 7 September 2006
A significant September Weather event
Siva Sivakumar Auburn Council's storm water Drainage Engineer recorded 110 mm of rain at his place in Toongabbie from 9.30 pm till 6.30 am and calculated this out:-
Storm period from 9.30 pm to 6.30 am.
Duration 9 hours.
Rainfall 110 mm.
Rainfall intensity 12 mm / hour. This is closer to the 1 in 5 year rain event.
If the rainfall occurred within a 6 hour duration period, the effect would have been closer to 1 in 20. Hence the same amount falling in 6 hours would produce this effect.
I have since been advised that in Auburn, due to the lower amount of rainfall, the effect was a standard 1 in 1 year event.
When the rainfall figures are analysed from AWS (Automatic weather stations) the effect is:-
Liverpool 65 mm
Berowra 45 mm
Prospect Dam 69 mm
Richmond 89 mm
Cronulla
76 mm
Kings Langley 86 mm
Blacktown 100 mm
Parramatta 107 mm
Toongabbie 111 mm (Siva's recording is in line with this).
Merrylands 107 mm
No other area in western Sydney received this single 100 mm fall and it was limited to two local government areas being Blacktown and Parramatta.
It caused flash flooding and significant overland flows. It commenced with a thunderstorm with small hail (pea sized hail for a few minutes at least where I was at Quakers Hill). The rain was then sustained over 9 hours with isolated rumbles of thunder and sheet lightning throughout the night until at least 4 am Thursday morning where I live.
This is a significant weather event in which it is likely to be the wettest single September day from a single rain event in 124 years since September 1883 (For the month of September that is).
The city also received a fall of 100 mm. Most other falls in the metropolitan area varied widely between 45 mm and 80 mm.
This single rain event (In which a fall of 100 mm occurring in September) may even be the first such event to occur in this part of Sydney (Western Sydney) since European settlement but this needs to be quantified.
Further:
The 5 day day rain event from 6 September to 11 September produced the final tally (Samples only from AWS):-
Marsfield 212 mm
Merrylands 184 mm
Parramatta 191 mm
Blacktown 160.6 mm
Glenmore Park (Penrith) - 78 mm
Richmond 141 mm
Liverpool 105 mm
Sydney 176 mm
This is a sample of AWS. It shows the significance of this event. History buffs have found evidence that this 5 day weather event is the wettest such event Sydney has experienced during the month of September since 1859 (147 YEARS AGO). This indicates the rarity of such a rain event.
It is also interesting that the heavier falls were away from the coastal suburbs breaking the normal trend in which the coast receives most rain.
Warragamba Dam wall received 51 mm indicating that the falls tapered off towards the inland.
Harley Pearman
