The Bureau of Meteorology issued frost warnings for South Australia and Victoria on Tuesday, with temperatures down to minus one degree forecast for Wednesday morning. The South Australian warning covers the Mid North, Riverland, Murraylands and Upper South East forecast districts. The Victorian warning covers the Mallee.
The South Australian bulletin, product IDS21007, went out at 11.16am CST. The Victorian one, IDV23000, followed at 11.46am EST. Both carry the same forecast minimum, and both state that no further warnings will be issued for this event.
The reading at the station is not the reading on the ground. "Temperatures at ground level can be many degrees lower than those recorded by standard weather stations, especially in calm conditions with clear skies," the South Australian warning said.
In New South Wales the Bureau issued a sheep graziers warning for the Snowy Mountains forecast district at 11.03am EST. Cold temperatures, showers and westerly winds are expected through Wednesday, with a risk of losses of lambs and sheep left exposed to them. That bulletin was due to be updated by 5pm EST on Tuesday.
It is the third round of frost warnings across the southeast in less than a week. On 30 July the Bureau forecast severe frost to minus four degrees in parts of Victoria's North East and flagged the risk of damage to crops. Warnings ran again over the first weekend of August.
The conditions that produce these mornings are the ones named in the warning text. Clear skies, light winds and dry air let the ground shed heat overnight without a cloud layer to hold it. A high pressure system parked over the southeast after a cold front delivers all three at once, which is why frost warnings tend to arrive in clusters rather than singly.
The alps have already had the front. The Bureau cancelled severe weather warnings for the Snowy Mountains and for East Gippsland and West and South Gippsland on Monday morning, after gusts to 120km/h at Thredbo Top Station before dawn.
Marine wind warnings were current on Tuesday for Western Australia, Queensland, New South Wales and Victoria, with seven ocean wind warnings active for Metarea 10. The Northern Territory and Tasmania had no warnings.
Both frost warnings lapse with Wednesday morning and will not be reissued. The graziers warning is the one that rolls, with the next bulletin due Tuesday evening and the conditions it describes running through Wednesday.
