Monday 2 July 2007 - Monthly Climate Summary for Sydney extremes - Product code IDCKGC1xx0

Sydney has wettest June for 57 years

Summary

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Rainfall

The total June rainfall of 510.6 mm at Sydney Observatory Hill was very much above the historic June average¹ of 128.1 mm and the 2nd wettest June since records commenced in 1859.  The wettest was 642.7 mm in 1950 (the highest for any month in Sydney).  This June was the wettest month in Sydney since February 1990 (when 630.6 mm was recorded).  In the 14 days from 7th to 20th, 466 mm was recorded at Observatory Hill, the wettest fortnight for 17 years and the 10th wettest fortnight in 149 years of record.  There were 16 rain days during the month, well above the historic average of 11 rain days and the highest for June since 1989 (when 17 days). Rainfall totals for June were heaviest in eastern and northern suburbs and lowest in the far south-west. Metropolitan rainfall ranged from 510.6 mm at Sydney Observatory Hill to 209 mm at Camden Valley in the far south-west.

Much of the rain was caused by 5 systems known as "east coast lows" that affected Sydney during June. (Further information about east coast lows can be found at: http://www.bom.gov.au/nsw/sevwx/facts/ecl.shtml ).  The NSW coast would normally expect significant impacts from about 1 or 2 east coast lows per year, and although the number of occurrences is highly variable from year to year, 5 in one month is rare. The high number in June was due to the persistence of conditions that were particularly favourable for their development, including:   persistent strong high pressure systems south of the Bight which directed several bursts of cold air over NSW during the month leading to cooler surface temperatures and instability in the upper atmosphere; and above average sea surface temperatures in the Tasman Sea which provided high levels of moisture and also combined with the cooler land temperatures to create a strongly enhanced temperature gradient along the coast. In addition, east coast lows and the conditions that favour their development are more common during transition periods between the drier El Niņo events and wetter La Niņa periods. Although we have not yet reached the formal threshold for a La Niņa many global climate computer models are forecasting a La Niņa to develop in the next few months.

The Warragamba Dam catchment area received heavy rainfall during June, raising storage levels significantly. Total June rainfalls of 200 - 300 mm were recorded in the lower catchment with up to 393 mm at Katoomba on the catchment divide.  Even the upper catchment recorded monthly totals of nearly 200 mm in the Goulburn area.  Recent rainfall in Sydney's catchments can be found on the Sydney Catchment Authority web site at: http://www.sca.nsw.gov.au/dams/rainfall.html.

Temperatures

The average maximum (daytime) temperature at Observatory Hill was 16.8°C, or 0.9°C below the historic average¹of 17.7°C. This is the coldest average maximum for June since 1989 (when 16.7°C). The western suburbs recorded slightly cooler daytime temperatures. Springwood (elevation 362 metres) recording maximum temperatures more than 2°C lower than on the coast. Both Bankstown and Camden recorded their lowest average maximum temperatures on record for June.

Nights were also cool along the coast with slightly below average minimum temperatures. The average minimum at Sydney (Observatory Hill) of 9.4°C was 0.2°C below the historic June average.¹ However slightly above average minimum temperatures were recorded in most suburbs away from the coast.

The cold conditions during June were in part due to several bursts of cold southerly air over Sydney during the month combined with greater than average cloudiness and rain.

The highest temperature at Sydney (Observatory Hill) during June was 20.7°C on the 2nd while the lowest temperature was 4.9°C on the 23rd.  Corresponding extremes in the Sydney area ranged from 21.0°C at Sydney Olympic Park on the 2nd to −2.0°C at Richmond on the 23rd.

Some sites had their lowest June mean daily maximum temperature on record.

Wind

Associated particularly with the east coast low events, Sydney experienced windy conditions in June. Winds were strongest along the coast, with the prevailing direction between W and S.  There were 3 days of gales (average speed 63 km/hr or more), the highest number in June since 1975.  Normally Sydney would expect 1 gale every 3 years in June. There were also 7 strong wind days (average speed 40−62 km/hr) which is well above the historic average of 4 strong wind days.  The highest wind gust in the Sydney area during June was 106 km/hr at Fort Denison on the 8th.

Other phenomena

Generally cloudy conditions persisted during June with sunshine averaging only 4.7 hours per day, the lowest since 1998. The historic average for June is 6.1 hours per day. There were two thunderstorms during the month (historic average 1storm). There was local flooding in and around Sydney on the 9th and 16th. Strong winds caused considerable damage in coastal suburbs, bringing down trees and power lines and blowing beach sand a hundred metres inland.  Huge seas and a heavy SE swell generated by the east coast lows caused severe erosion along several Sydney beaches with wave heights up to 14 metres recorded off Sydney on 9 June.

Further information

Mike De Salis
NSW Climate Services Centre
Bureau of Meteorology
Phone (02) 9296 1555
Email mikede@bom.gov.au