It's time to locate that umbrella again.
A big low pressure system forming off the NSW coast looks set to bring the wettest weather in months to coastal regions around Sydney.
For the past month, Sydneysiders have enjoyed the longest autumn dry spell in more than 150 years of records, with just 0.6 millimetres of rain landing at Observatory Hill since April 21.
Tuesday will offer one last day of warm, sunny weather, with a maximum of 23 degrees expected, before showers and then rain develop from Wednesday until Saturday.
“Between the Illawarra and the Hunter there will be isolated falls of above 200 millimetres,” Rob Sharpe, a meteorologist with Weatherzone, said. “The heaviest falls are going to be right along the coastal fringe.”
Inland regions, such as the Blue Mountains, will receive 50 millimetres over the next few days. Falls further west will be less - at 15-30 millimetres - but still very welcome after months of well-below-average rainfall had stoked farmers' fears of drought setting in.
"This will be the rain that some people have been waiting for," Brett Dutschke, senior meteorologist at Weatherzone, said.
This week's rain may also signal an end to the dry spell over much of southern Australia. The Bureau of Meteorology's latest rainfall outlook, due out Wednesday, will report that climate models show conditions are becoming more conducive for rainfall.
Of particular note for climate watchers is the so-called Indian Ocean Dipole. The dipole, which measures sea-surface temperature anomalies between the waters off the coasts of Africa and Western Australia, has its greatest influence on south-east Australia over winter and spring, and the signal is encouraging for those hoping for more rain.
“All models are pointing to conditions close to, or negative IOD, which is favourable for rainfall across southern, especially south-eastern Australia,” Andrew Watkins, manager of climate prediction services at the Bureau of Meteorology, said.
Atmospheric moisture is likely to produce cloud over the Timor Sea, which then streams over the continent. If the cloud “interacts with a cold front, you can get some good rainfall out of that”, he said.
The rain, should it come, will reward farmers who have taken the punt in recent weeks to “dry sow” their crops – planting the seeds in dry soils on anticipation of wintry rain to make up for the dismally dry autumn for many rural areas.
Dr Dutschke cautions, though, against hopes rising too high. While some cold fronts will bring decent rains, others may prove less intense than average.
"It'll be a bit of a balance," Dr Dutschke said. "You may get normal moisture levels but weaker-than-normal frontal systems."
"Overall, a lot of the rain is likely to fall in fewer days than we'd see in a normal season," he said. "It might look good in the rain gauge but if it runs off to the low points of the land (without soaking in), it's effectively less rain."
This week’s rainfall outlook update will also see the debut of the weather bureau’s new climate model.
The Predictive Ocean Atmosphere Model for Australia, or POAMA, will shift from predictions primarily based on historical records to a model which captures the dynamics of current climate conditions.
“We’ve kind of rung all the blood out of those stones,” said Dr Watkins, referring to the historical data.
The new model will also use 1981-2010 averages for comparisons in the forecasts, rather than the 1950-1999 period used previously, to reflect how Australia’s climate has warmed over recent decades.
“In other words, when we say it is above or below average, it’s more aligned to what people have experienced over the last generation, or so,” Dr Watkins said. “People do feel their climate is a bit different to mid-last century.”
An example of the shift is that Sydney’s start to 2013 has been 1.4 degrees warmer than normal over the long term but only 0.3 degrees warmer than the average since 2000, Acacia Pepler, a bureau meteorologist, said.
The city’s average maximum of 25.6 degrees between January 1 and May 15, while among the hottest on record, lagged 2004, 2006 and 2010 to this point in the year, she said.
Even if Sydney’s weather falls short of the extremes of some previous years, though, the consistently warm conditions have made their mark in the record books.
Each day of May has been above the average maximum for the month of 19.4 degrees, and the city clocked up 26 consecutive days of 20-plus degree days up until last Thursday, a record for this late in the year.
Weatherzone is owned by Fairfax Media, publisher of this website.