(and it has been like that for 178 years!)
There were more blackouts in South Australia a couple of days ago following a wild storm. In a report in the Adelaide Advertiser, SA Power Networks spokesperson Paul Roberts is quoted:
“This is just another example of the unprecedented weather in the last six months,” Mr Roberts said, referring to bouts of wild weather that have hit power supplies hard this summer and the preceding spring.
21mm of rain was measured at the Kent Town gauge.
Just how “unprecedented” is Adelaide’s weather over the past few months? I couldn’t find any records for the number of severe storms, so for a proxy I have made do with rainfall data from West Terrace and Kent Town in Adelaide. The overlap period has very similar rainfall recordings so I joined the two series to give a record starting on 1 January 1839. That’s 178 years of data.
When thinking about “unprecedented”, we need to check amount, intensity, and frequency.
Firstly, a few plots to give some context. How unprecedented was Thursday’s storm?
Fig. 1: Rainfall for the first 21 days of January compared with Days 1 – 21 of every year
Note Thursday’s rainfall had less rain than four previous occasions on this day alone, and 20 or so in previous Januarys.
Fig. 2: Rainfall for each day of 2016 compared with each day of every year:
Note the December storm had extreme rain (for Adelaide) but not a record.
Amount and intensity has been higher in many previous years. 141.5mm was recorded on 7 February 1925.
Fig. 3: 7 day average rainfall over the years:
The topmost dot shows the maximum 7 day average for each year. 2016 got to 13.4mm on 4 October- multiply by 7 to get the weekly total rain. Note there were many wet and dry periods all through the record.
21mm of rain fell in a severe storm on Thursday, so I arbitrarily chose 20mm as my criterion for heavy rainfall in one day as a probable indicator of stormy weather. I am the first to admit that 20mm might fall steadily all day and not be at all associated with wild winds, and wild winds can occur without any rain, but bear with me.
Fig. 4: Rain over 20mm throughout the year:
There seems to be no increase in amount or intensity of rain at any time of the year.
Fig. 5: Frequency:
Note 2016 had 7 days with above 20mm in 24 hours. That’s the most since… 2000, when there were 8 days- and many previous years had 7 or 8 days, and 1889 had 9. So no increase in frequency.
However, Mr Roberts was referring to the last six months, spring and summer. So let’s look at rain events over 20mm from July to December, firstly amounts recorded:
Fig. 6: July to December Rain over 20mm:
Nothing unusual about 2016.
Fig. 7: Frequency of heavy rain July – December:
1973, 1978, and 1992 had the same or more days with over 20mm.
I now restrict the count to spring and summer only:
Fig. 8: Spring and Summer frequency:
Not unprecedented: 1992 had one more. Add in last Thursday’s event to make them equal.
Adelaide has a long climate record, showing daily rainfall has varied greatly over the years. There is no recent increase in amount, intensity, or frequency for the whole year, or for the last six months or four months. Spring and summer rainfall in 2016 was not unprecedented, and to the extent that spring and summer falls over 20mm are a proxy for storms, there is no evidence for an increase in wild weather. This is normal. Get used to it, Mr Roberts, and make sure the electricity network can cope.