Introduction

In May 2003, parts of the Hunter and Sydney metropolitan coast received well above average rainfall. Monthly totals in excess of 300mm were received in some coastal suburbs of Sydney, the Illawarra coast and just north of Newcastle. Several stations recorded their wettest May on record, including 545 mm at Cronulla, which was the highest fall in NSW for the month.

However, little of this rain extended west of the Great Dividing Range, where falls for the month were in the lowest 10% of historical readings, or the lowest on record, particularly in the eastern half of the plains, and on the western slopes. Parts of the Northwest Slopes near Narrabri, Inverell and Walgett recorded almost no rain at all.

The rains came after a protracted dry spell lasting many months, following a substantial collapse of the moderate El Nino event that had prevailed for the previous 14 months or so.

The first 10 days of the month over southeastern Australia had been dominated by high pressure moving slowly east-southeast from the Great Australian Bight towards southern NSW before finally weakening and contracting northeast. This latter movement took place with the occurrence of a change in the upper atmospheric configuration over South Australia, which allowed the start development of the systems that were to bring the rainfall. A slow moving upper trough, which had developed west of Perth early in the month, moved east and amplified in the Bight on the 11^th, with a surface trough forming to its east on the South Australian/NSW border. In succeeding days the upper system continued to move east northeast slowly while high pressure reformed south of Adelaide, moving towards the southern Tasman Sea.

This set the stage for a very moist infeed of easterly winds into the surface and upper systems. It would then be only a matter of time before the usual east coast low would develop and this happened on the morning of the 16^th, just south of the Queensland border. The system moved slowly south, developing a secondary centre west of Lord Howe Island, before being absorbed by a deep low that had formed in the Indian Ocean in a tropical dip. The collapse of the eastern system was a classical example of the breakdown of these systems and brought a brief burst of gale force westerlies across the southeast of NSW and Victoria with some light snowfalls at higher levels.

Click here to see the surface and upper air charts.

The slow progress of systems caused widespread rain for nearly a whole week with totals exceeding 300 mm in places. Audley, in southern coastal Sydney, received 365 mm, including 108 mm in 3 hours on the 13^th from a thunderstorm. At Forster on the mid north coast, there were 183 mm in 24 hours on the 15^th, the highest for May since the commencement of records in 1896. Nearby, Taree recorded 43 mm in an hour on the same day. Jervis Bay in the Illawarra recorded 131 mm that day, its second highest for May.

As already mentioned, the rain generally failed to make any impact west of the Divide, but as the upper trough moved over Wilcannia (Upper Western division) it delivered 46 mm from a thunderstorm, the wettest May day there since 1981.

The following stations received their highest May rainfall on record:

One feature I noticed over 2 days of this event was on the Sydney radar. For several hours, heavy rain areas developed continually across the northern suburbs (Turramurra and Frenchs Forest for example) and moved directly south across inner suburbs all the way to the Illawarra. With echoes showing yellow and patchy green it is small wonder impressive totals built up in a short space of time and places like Turramurra and Frenchs Forest received over 100 mm in only 6 hours or so.

From a statistical point of view the monthly totals probably are pretty ordinary events that would recur every 10-20 years or so. But the fact that a significant proportion of it fell in short bursts of only several hours, I would suggest ramps up the probability of such an event by a factor of 10 or more.

Of the Sydney stations for which I keep ongoing records, Sydney, Seven Hills and Turramurra each respectively had their 8^th, 5^th and 3^rd wettest May on record. These totals respectively fall within the top 6, 10 and 5 percent of readings for each station for May, with records going back 145, 54 and 70 years respectively.

Following a long period in which El Nino dominated the weather over Australia, and the values of the Southern Oscillation Index having come back from moderately to strongly negative values through most of 2002, such a rain event would not be all that unusual in terms of its synoptic situation. However the intensity of the rainfall over short periods of time was something less out of the ordinary, but, in historical terms, set no new precedent; many such events have occurred before and this was, in simple terms, just another correction. This is no surprise given Australia’s special quality of being a land of droughts and flooding rains, and often at the same time as each other!