Rain and thunderstorms are spreading across southern and central Australia today as the first big system of spring pushes east. Spring is a season of volatile weather for sub-tropical parts of Australia, as this is when cold air from the south starts to get pushed out by warmer air taking over from the north. This temperature contrast drives up instability in the atmosphere, which can lead to outbreaks of severe thunderstorms and flooding rain. A pool of cold air in the upper atmosphere is pushing north over South Australia today, where it is clashing into a river of warm tropical air flowing south along the jet stream. As expected, rain and storms are developing where these two air masses meet, some which could become severe. Before arriving in South Australia, the system caused a wet and stormy end to winter in Western Australia. Esperance recorded more than 20mm of rain during the 24 hours to 9am today, the heaviest in about three months. Elsewhere in the state, Hopetoun and Warburton both had their heaviest rain in 2 months from the system. Rain and storms are now occurring over northern and western parts of South Australia and the southern half of the Northern Territory today. Severe weather warnings and a flood watch have been issued in these areas, with flooding and damaging winds possible in the coming hours. You can find the latest details on these warnings here: http://www.weatherzone.com.au/warnings.jsp Queensland, New South Wales, the ACT and Victoria are next in line. Cloud and rain is already developing today and will intensify tomorrow as a low pressure system deepens and moves east. Catchments in western and central-southern Queensland and the inland river valleys in New South Wales are under flood watch, with widespread falls of 20-50mm possibly causing flooding from Friday onward. Rain will start contracting towards the east coast and ranges on Saturday as the system reaches the Tasman Sea, before clearing most areas later in the day.