Catastrophic fire danger is likely in parts of South Australia today with hot dry and gusty northerly winds. Fire danger is likely to reach Catastrophic levels today in the Mt Lofty Ranges and Lower South East, with Extreme levels likely across much of SA today and Victoria tomorrow. The reason for the elevated fire danger is an approaching low pressure trough. This trough is drawing very hot northerly winds across the south of the nation, with wind gusts a risk to exceed 90km/h in Victoria tomorrow. The air is also very dry, with humidity dropping below 10% at times across most of each state. Any fires that spark up in these two states today or tomorrow are likely to be fast moving and uncontrollable. Relief from the heat will slowly arrive on Saturday, with a southwesterly change reaching Adelaide mid morning and then Melbourne in the late afternoon or early evening and later further inland. Thunderstorms are likely to develop in SA and VIC tomorrow near the cool change, with lightning a risk to start new fires. On Sunday wind speeds will be lower, temperatures will be cooler and humidity will increase markedly, making fires easier to contain. This will give firefighters a chance to get on top of any out of control fires before a slightly weaker spell of heat moves into both states on Tuesday and Wednesday next week.