The Bureau of Meteorology had two frost warnings current across southern Australia at 12.40pm on Thursday, one for the Mid North in South Australia and one for the Mallee in Victoria. Earlier in the week the same warning type covered five districts.

A sheep graziers warning was also current for the Snowy Mountains forecast district in New South Wales, where cold temperatures and westerly winds carry a risk of losses among lambs and recently shorn sheep.

The Bureau cancelled its road weather alert for Canberra on Thursday morning. The city's forecast has areas of frost and patchy fog early, with winds turning northwesterly at 15 to 25 km/h through the middle of the day before easing in the evening.

Road weather alerts cover fog, ice or heavy rain expected to affect driving conditions, and are withdrawn once the Bureau judges the hazard has passed. Canberra's went during the morning while the frost and fog in the same forecast were still there, which is the usual sequence rather than a contradiction.

The synoptic pattern behind the run of cold mornings has changed shape rather than broken down. A low pressure system and trough that brought showers and cold air have moved off to the east, and a ridge of high pressure is building across eastern and central Australia behind them. That ridge is what produces frost. Clear skies and light winds let the ground radiate heat away overnight with no cloud layer to hold it in.

The effect now reaches further north than it has this week. The Bureau's national forecast has a frosty start extending up into southern Queensland, even as the number of districts under a formal warning in the south has fallen.

Frost warnings are issued against forecast minimum temperatures at standard screen height. Readings at ground level run colder, and the Bureau's frost bulletins routinely note that the difference can be several degrees on calm, clear nights. A district that drops out of the warning list has not necessarily stopped getting frost.

Western Australia is next in the sequence. Showers were across the south of the state on Thursday and moving toward the coast, ahead of a cold front and trough approaching the southwest. Marine wind warnings were current for Western Australian, Victorian and Tasmanian waters, with three ocean wind warnings active for Metarea 10.

The Northern Territory and Queensland had no warnings current at the time of the Bureau's summary.

Frost warnings are single event bulletins and lapse with the morning they cover. The graziers warning for the Snowy Mountains is the one that rolls forward, and the front approaching Western Australia is the system to watch into the weekend.