Tuesday 1 May 2007 - Monthly Climate Summary for Sydney - Product code IDCKGC15L0

Sydney has cool, wet April

Main points:

  • Cool days, especially in the west
  • Cool to mild nights
  • Above average rainfall in the east, below average in the west
  • Two severe thunderstorms over eastern suburbs caused flash flooding
  • Below average sunshine

Temperature

Although much of NSW recorded above average temperatures during April, the Sydney area experienced cool conditions with below average temperatures.  This was due to a combination of above average rainfall and persistent, cool S-SE winds.  The average maximum (daytime) temperature at Observatory Hill was 23.0°C, or 0.2°C below the historic average1of  23.2°C.  Slightly cooler days were experienced in the western suburbs where average maximum temperatures were about 1°C lower than coastal suburbs. The average maximum temperature at Richmond of 22.5°C was the 7th lowest for April since records commenced in 1929.

Nights were also cool to mild during April.  The average minimum temperature at Sydney (Observatory Hill) of 15.6°C was 0.4°C above the historic April average1.  However, average minimum temperatures in far western suburbs were below the historic average and about 4°C cooler than coastal suburbs.  At Camden the average minimum of 11.2°C was the lowest in the Sydney area.

The highest temperature at Sydney (Observatory Hill) during April was 27.1°C on the 11th while the lowest temperature was 12.6°C on the 26th.   The lowest maximum temperature was 17.3°C on the 25th, which is the lowest for April in 8 years and the coldest Anzac Day since 1948.  Corresponding extremes in the Sydney area ranged from 30.4°C at Sydney Olympic Park on the 15th to 6.3°C at Camden Airport on the 11th.

Rainfall

The total April rainfall of 179.6 mm at Sydney Observatory Hill was well above the historic April average of 125.7 mm.  There were 12 rain days which is average for April.   Rainfall totals varied greatly over the Metropolitan area during the month with heavy rain falling along the coast while western suburbs recorded only light falls. Monthly totals were highest in the eastern suburbs with Rose Bay recording a total of 386.8 mm, its wettest April since 1988.  In the west the lowest April total was only 32 mm at St Clair.  The two major rain events during April (4th to 8th and 23rd to 25th) were caused by moist on-shore S-SE air streams and cold air in the upper atmosphere.  Typically with S-SE air flows, the associated rainfall does not penetrate very far inland and this was the case in this instance.

Unfortunately the rain also failed to reach the Warragamba catchment with monthly totals over much of the catchment around 20 to 40 mm.   However, the smaller Woronora catchment to the south of Sydney and closer to the coast recorded quite heavy rain during April with a total of 195 mm.  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

Other phenomena

Three thunderstorms were recorded over Sydney metropolitan area during April with 2 storms at the official site at Sydney Observatory Hill (historic average 1storm). Slow moving thunderstorms with heavy rain affected the eastern suburbs on the 4th and 22nd causing local flash flooding, especially in the Double Bay area where shops and houses were damaged by floodwaters.  At Rose Bay, 98 mm was recorded on the 22nd, which included 42 mm in 30 minutes. This intensity has a recurrence interval of around 10 years.  A similar slow moving thunderstorm at Rose Bay on the 4th produced 96 mm in less than 3 hours.  A combination of plentiful moisture from S-SE winds and an upper low pressure trough contributed toward the severe thunderstorms.  Sunshine averaged 6.6 hours per day which is below the April average of 6.9 hours per day and a marked contrast with April last year which was the sunniest on record (averaging 9.3 hours per day).  A dense fog in the early hours of the 20th reduced visibility in the CBD area to 150 metres for several hours and only 100 metres at Sydney Airport and at Richmond.

Further information

Mike De Salis
NSW Climate Services Centre
Bureau of Meteorology
Phone (02) 9296 1542
Email reqnsw@bom.gov.au

Extremes for the Sydney area

Hottest day
Highest temperature
 30.4°C at Sydney Olympic Park on the 15th
Warmest days on average
Highest mean daily maximum temperature
 24.4°C at Penrith
Coolest days on average
Lowest mean daily maximum temperature
 21.8°C at Terrey Hills
Coldest day
Lowest daily maximum temperature
 16.1°C at Springwood on the 25th
Coldest night
Lowest temperature
 6.3°C at Camden Airport on the 11th
Coolest nights on average
Lowest mean daily minimum temperature
 11.2°C at Camden Airport
Warmest nights on average
Highest mean daily minimum temperature
 15.6°C at Sydney (Observatory Hill) and Sydney Airport
Warmest night
Highest daily minimum temperature
 18.6°C at Sydney Airport on the 12th
Wettest overall
Highest total rainfall
 386.8 mm at Rose Bay (wettest April since 1988)
Driest overall
Lowest total rainfall
 32.0 mm at St Clair
Wettest day
Highest daily rainfall
 98.0 mm at Rose Bay for 24 hours to 9am on 23rd
Highest wind gust
 67 km/h at Sydney Airport on the 6th

Summaries

Summary statistics for the Sydney area for April 2007
Maximum temperatures
April 2007
(°C)
Minimum temperatures
April 2007
(°C)
Rainfall
April 2007
(millimetres)
Mean for
April
Diff
from
normal
Highest for
April
Mean for
April
Diff
from
normal
Lowest for
April
Total
for
April
Normal
for
April
Rank of
April
Fraction of
April
normal
Sydney (Observatory Hill) 23.0 −0.2 27.1 11th 15.6 +0.4 12.6 26th 179.6 125.7 high 143%
Parramatta 23.4 −0.5 29.1 15th 13.2 +0.4 10.5 13th 115.9 85.1 high 136%
Richmond 22.5 −1.6 27.4 15th 11.3 −0.8 8.0 11th 56.6 70.3 normal 81%
Camden 23.5 −0.7 28.2 15th 11.2 −0.2 6.3 11th 36.8 54.1 normal 69%
Terrey Hills* 21.8 26.8 15th 13.9 10.8 26th 137.0
Springwood* 21.9 26.2 5th 11.9 9.0 11th 44.2 73.8 normal 59%

* new stations, no historic average temperatures available and no historic average rainfall available for Terrey Hills.

Summary phenomena for Sydney (Observatory Hill) for April
 

April 2007

Normal for April

Number of rain days

 12

 12

Average sunshine hours per day*

 6.6

 6.9

Average humidity at 3pm (%)

63

59

Number of thunderstorms

 2

 1

Number of windy days*

 2

 3

* recorded at Sydney Airport


Notes

This statement has been prepared based on information available at noon on Tuesday 1 May 2007.  The data has been quality controlled but it is possible that results will change as new information becomes available.

1Averages:  historic average temperatures for Observatory Hill and Richmond are based on the period 1961 to 1990 which is a convention of the World Meteorological Organisation.  Average temperatures for Parramatta are based on all available data, 1968 - 2006 and Camden 1972 - 2006.  Rainfall averages and extremes for Observatory Hill are based on all years of record 1859 - 2006.

Normals are long-term means based on observations from all available years of record, which vary widely from site to site. They are not shown for sites with less than 10 years of record, as they cannot then be calculated reliably.   The median is sometimes more representative than the mean of "normal" rain.

The Rank indicates how rainfall this time compares with the climate record for the site, based on the decile ranking (very low rainfall is in decile 1, low in decile 2 or 3, normal in decile 4 to 7, high in decile 8 or 9 and very high is in decile 10).  The Fraction of normal shows how much rain has fallen this time as a percentage of the long-term mean.