Australia Weather News

Australia is collectively sweating through an unusually muggy start to summer; a weather pattern being partially driven by the warmest waters on the planet off our tropical coastline.

Sea surface temperatures surrounding Australia were the highest on record in spring, and are currently simmering up to 32 degrees Celsius off the Top End and Kimberley coasts.

This abnormal offshore warmth is supplying the atmosphere with abundant moisture, creating an environment primed to fuel another widespread outbreak of thunderstorms across south-east states this weekend.

The humidity is not only a key ingredient for thunderstorms, but is also behind Sydney's mysterious sea fog on Friday which cloaked the city's coast.

Flash flood risk from Melbourne to central NSW

The hot, humid conditions prevailing across much of Australia are the perfect set-up for thunderstorms, and development is likely today in a broad band from the north, through the interior to Melbourne and Sydney.

Storms should be most intense over south-east states where humid northerly winds will clash with an approaching cold front and severe thunderstorms are likely from Victoria, through New South Wales to far south-west Queensland.

A combination of very high instability and an overlying jet stream could even result in long lasting supercell storms over the eastern Murray Basin — the most dangerous type of thunderstorm which are capable of producing destructive winds and giant hail.

Supercells are possible in the areas around Albury, Hay, Griffith, Wagga Wagga and Wangaratta, before the greatest storm threat shifts to central NSW later this afternoon.

The system is also producing heavy rain, and overnight totals included 49 millimetres in Canberra, our capital's heaviest rain in four years.

While storms are possible in Melbourne, the city lies just outside the zone which could see severe weather due to a stabilising south-west change arriving around lunchtime, however Sydney will remain hot and humid all day, reaching the mid-30s across most suburbs, and could see brief damaging wind gusts and heavy rain.

Cooler and drier air behind the front will flush out the heat from Victoria and southern NSW by Sunday, however storms will still develop on Sunday from about Newcastle to Brisbane, along with much of Australia's northern tropics, interior and Western Australia.

Record warm seas fuelling wet December

Just a week into the month and already this December is wetter than average over parts of central Australia and the Murray-Darling Basin, including 106mm at Nyngan — more than double the town's average.

On Friday storms dumped 47mm in just one hour on Nundle, 34mm on Hume Dam and generated a wind gust of 93 kilometres per hour at Bourke.

And there's more rain on the way next week. 

The storms over the interior will shift gradually east through Monday and Tuesday, reaching the northern NSW and southern Queensland on Wednesday.

In total over the next five days almost the entire country will see rain, and in a repeat of the past few weeks, areas of above 50mm are likely in multiple states along with the Northern Territory.

While bursts of lightning and thunder are common in November and December, the current frequency and extent of storms is more reminiscent of January when heat and humidity typically peak — our national average rain is 51mm in December but rises sharply to 80mm in January.

So why have mid-summer conditions arrived early? 

It's a direct result of northerly winds from the tropics, however the humidity of the southward blowing air is being enhanced by above average sea surface temperatures.

The vast majority of Australian waters are currently warmer than normal with the greatest deviations up to 3C off the Pilbara coast, and since warmer waters increase evaporation and convection, additional moisture ends up in the atmosphere.

Long-range modelling shows simmering waters around Australia will continue through January and February, one of the key influences behind the forecast of a hot and wet summer.

Pre-Christmas cyclone possible

The combination of very warm seas and a pulse of tropical convection (called an MJO) has lifted the threat of cyclones this December.

The lower limit for cyclone formation is a water temperature of 26.5C, and already our tropical waters are well above the threshold this wet season, including broad swathes of ocean above 30C from the Gulf of Carpentaria to the Kimberley.

A cyclone already formed near the Cocos Islands in late November and further cyclones may develop from tropical lows during the coming days — one west of Christmas Island and another off Western Australia's north coast.

The Bureau Of Meteorology's cyclone forecast currently rates the WA system at a 25 per cent chance of becoming a cyclone by Monday night, increasing to a 40 per cent by Thursday, and while it's too early for an accurate forecast of its path, the most likely scenario is a track well offshore.