Issued on 3 August 2012

Record-dry July for SW Western Australia

A dry July for the western half of Australia exacerbated rainfall deficiencies in Western Australia. In contrast, above-average rainfall in western New South Wales relieved short-term rainfall deficiencies in that region.

July rainfall was below average for most of Tasmania, South Australia, central Australia, and Western Australia; elsewhere July rainfall was average to above average, with some pockets of highest on record, in northeastern Australia (see July rainfall deciles). For the state of Western Australia it was the third-driest July since comparable records commenced in 1900, and for the southwest of Western Australia (southwest of a line from Jurien Bay to Bremer Bay) July was the driest on record. The southwest region received only 36 mm for the month (against a long-term average of 114 mm) — the previous lowest total July rainfall for this region was 47 mm in 1918, while the third-lowest was 48 mm in 2005. Perth Metro received a record-low 34.6 mm of rain during July (the previous lowest was 61.5 mm in 1876, the year Perth Metro records commenced).

4-month rainfall deficiencies

Severe 4-month (April to July 2012) deficiencies now cover much of the southwest and interior of Western Australia. The area of southwest Western Australia experiencing long-term rainfall deficiencies over periods of two years and longer also increased during July. The southwest region had its driest year on record in 2010, and the two-year period from 2010 to 2011 was the lowest on record in some parts of southwest Western Australia. Short-term (3 month) rainfall deficiencies have become more extensive in northern South Australia and central Australia, however July is part of the dry season in this part of the country.

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Definitions

Lowest on record - lowest in the historical analysis, which runs from 1900.
Severe deficiency - rainfalls in the lowest 5% of historical totals.
Serious deficiency - rainfalls in the lowest 10% of historical totals, but not in the lowest 5%.

Very much below average - rainfalls in the lowest 10% of historical totals.
Below average - rainfalls in the lowest 30% of historical totals, but not in the lowest 10%.
Average - rainfalls in the middle 40% of historical totals.
Above average - rainfalls in the highest 30% of historical totals, but not in the highest 10%.
Very much above average - rainfalls in the highest 10% of historical totals.

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