Temperatures are soaring in parts of southern Australia today, with many places in SA and Victoria experiencing their hottest day in 10-11 months. So, when will it cool down?

Today's heat is being carried south from the nation's interior by a stream of northwesterly winds, which have developed ahead of an approaching low pressure trough.

With temperature's soaring 10-15 degrees above average ahead of the trough, many people will be eagerly awaiting the arrival of the cooler air in its wake.

The mercury had already pushed over 30 degrees in Adelaide and Melbourne by 9am on Friday and is forecast to reach up to 38 degrees in both cities later in the day.

For Adelaide, this heat will reign until cooler southwesterly winds arrive in the late afternoon, dropping the mercury progressively into the evening. The city is forecast to dip to a low of 18 degrees overnight and a more manageable top of 26 degrees on Saturday.

Melbourne will stay hot into the evening and is likely to still be above 30 degrees around dinner time. A weak change will bring some relief the city and surrounds during the night, although more substantial relief from the heat won't arrive until a stronger change turns up on Saturday, most likely during the late morning or afternoon. Melbourne is forecast to reach 38 degrees today, 27 degrees on Saturday and just 22 degrees on Sunday thanks to this two-phase cool change.

Today's heat and northwesterly winds are also elevating fire danger ratings, with total fire bans in place for a number of districts in both states. Visit these websites for the latest information on bushfires:

Victoria - www.cfa.vic.gov.au

South Australia - www.cfs.sa.gov.au