1 September 2006

MONTHLY AND SEASONAL CLIMATE SUMMARY - NSW REGIONAL OFFICE

NSW Winter and August 2006: Warm days, cold nights and dry west of the divide

Winter 2006

Main points:

• Large diurnal temperature range - warmer than average days, cooler than average nights
• 10th warmest winter on record for statewide average maximum temperatures
• 12th coldest winter on record for statewide average minimum temperatures
• Coastal NSW recorded average to above average rainfall
• Central/southern slopes of the Great Divide and southern border areas of the state received below to very much below average rainfall
  

August 2006

Main points:

• Large diurnal temperature range - warmer than average days, cooler than average nights
• 5th warmest August on record for statewide average maximum temperatures
• 12th coldest August on record for statewide average minimum temperatures
• Coastal NSW recorded average to above average rainfall
• Most of NSW west of the divide recorded below to very much below average rainfall
• Parts of western NSW recorded lowest August rainfall totals on record

Winter 2006

Temperatures:
NSW experienced large diurnal temperature ranges during winter with warm days and cool nights. The statewide average maximum temperature was 0.82ºC above the historical average¹ of 16.1ºC , making this the 10th warmest winter on record (statewide averages calculated 1950-2006). Maximum temperatures were above average over much of the east and south of the state with a band of very much above average temperatures stretching along the almost the entire western slopes of the Great Divide. This distribution of above average temperatures is a result of easterly wind anomalies across the state during much of winter. Southern areas on the western slopes of the divide which usually are affected by strong cool westerlies which ascend over the ranges producing cloud and rain instead frequently experienced dry, warm, descending easterly winds. The combination of these warm, dry easterly winds and warm northwesterly winds from the abnormally hot, dry interior resulted in high maximum temperatures on the south west slopes of the Great Divide. Small areas of the southern Riverina and Central West Slopes recorded their highest average winter maximum temperatures on record (See Map 1).

The statewide average minimum was 0.52 ºC below the average of 4.2 ºC, making this the 12th coldest winter on record (1950-2006). Below average minumum temperatures were recorded across most of the south and west of the state. Very much below average winter minimum temperatures were experienced in the Lower Western, Riverina, Sydney Metropolitan, Illawarra and Central Tablelands. The far south coast was the only area of the state to record above average minimum temperatures (See Map 2). The distribution of very much below average temperatures is a result of the dry conditions and clear night skies during winter. The lack of cloud has enabled much stronger radiative cooling overnight than would be normally expected, resulting in much lower than average minimums across large areas of the state.

Rainfall:
Winter rainfall was varied throughout the state in 2006. The central and southern slopes west of the Great Divide and the southern border regions of the state recorded below average winter rainfall. The southern half of the Southwest Slopes received very much below average rainfall. This distribution of below average rainfall is a result of weaker than average westerly wind flow across the state with a below average frequency of rain bearing winter cold fronts. The Upper Western and areas along the entire coast of NSW recorded above average winter rainfall. This pattern is a result of high pressure systems positioned much further south than normal through most of winter. Moist onshore easterly winds on the northern side of the high pressure systems brought with them enhanced coastal winter rainfall (See Map 3). The statewide average rainfall for winter 2006 of 105.6mm was slightly below the average of 115.1mm.

August 2006

Temperatures:
As for the winter season overall, there were strong diurnal temperature ranges across the state in August. Cold nights and very warm days were widespread. August was a very warm month for maximum temperatures across the entire state. Southern coastal districts and the far north east of the state reported above average temperatures whilst the remainder of the state experienced very much above average temperatures during August (See Map 4). The statewide average maximum temperature for the month was 2.03ºC above the average of 17.1ºC, making August 2006 the 5th warmest on record (records from 1950-2006).

The statewide average minimum temperature for August was 0.63ºC below the average of 4.7ºC, making it the 12th coldest on August record for minimum temperatures (1950-2006). Below average temperatures were experienced across most central and southern districts of the state and into the Northwest Slopes and Plains. Very much below average minimum temperatures were experienced across the Central Tablelands, Central West Slopes, Southern Tablelands, Southwest Slopes and the Riverina. An area in the far south of the Riverina and Southwest Slopes recorded the lowest average August minimum temperature on record (See Map 5). The distribution of very much below average temperatures is a result of clear night skies during August in the areas that would on average experience cloudy conditions with rain bearing winter fronts. The lack of cloud has enabled much stronger radiative cooling overnight than would be normally expected, resulting in much lower than average minimums.

Rainfall: (See Map 6)
August was a very dry month across the state to the west of the Great Divide. Vast areas of the south and west of the state experienced very much below average rainfall with small areas in the far west experiencing lowest August totals on record. In contrast all areas to the east of the divide experienced average to above average rainfall for the month. The Northern Rivers recorded very much above average rainfall during August. The contrasting coastal/inland rainfall distribution is a result of weaker than average westerly wind flow across the state with a below average frequency of rain bearing winter cold fronts. High pressure systems were positioned much further south than normal through August. Moist onshore easterly winds on the northern side of the high pressure systems brought with them coastal rainfall. The statewide average rainfall for August was 15.6mm compared to average of 40.6mm, making this the 13th driest August on record.

This statement has been prepared based on information available at 12 PM on 1 September 2006. Although every effort has been made to check them, it is possible that results may change as new information becomes available.

A note about deciles: Deciles are used to give an element (in this case rainfall or temperature) a ranking. Deciles are calculated by arranging the totals in ascending order (from lowest to highest) then splitting them into 10 equal groups (thus the groups are called deciles). The first group would be in decile range one, the second group in decile range two, etc up to the highest annual totals (highest 10 per cent) being in decile range 10.

¹Averages: Averages are based on the period 1961 to 1990 which is a convention of the World Meteorological Organisation.

Further information: Contact Clinton Rakich 9296 1610, email: reqnsw@bom.gov.au

A directory of Bureau services is available on poll fax: 1800 630 100

The latest rainfall and temperature maps are available here
Climate averages for New South Wales localities are available here
Daily Weather Observations for selected New South Wales localities are available here