As the longest (and, possibly, coldest) night of the year approaches, Tuesday evening’s flip from icy evening to mild morning was the perfect illustration of how Canberra’s winter nights can drastically vary from the forecast.
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Bureau of Meteorology forecaster Sean Carson said temperature predictions for Canberra’s days were generally quite accurate, but winter nights in the capital presented some unique challenges.
Early on Tuesday evening, Canberra’s north was hit with a short, sharp thunderstorm that dumped significant amounts of hail over parts of the capital.
“It doesn’t surprise me in hindsight. It wasn’t really forecast, showers were forecast. The difference between a shower and the cloud growing a bit taller and turning into a thunderstorm, it’s not a huge real difference in it,” Mr Carson said of the hail.
But rather than the cold, minus 3 night predicted, the temperature stayed well above zero, hitting a relatively balmy low of 2.9 degrees, which Mr Carson said was the result of a steady breeze that was originally forecast to ease.
“We’ve been under a southerly breeze now since last Friday, with a low out on the Tasman Sea. We thought it would’ve moved a little bit further out into the Tasman Sea last night and the winds would have dropped off, and we’d get those cold temperatures,” he said.
“It did drop out at Goulburn, it got down to minus 3 there this morning. But the 20km per hour wind basically just kept the atmosphere well mixed [in Canberra].”
Mr Carson said wind was one of the most significant factors in Canberra’s night time weather conditions, as a breeze could help mix the atmosphere and keep temperatures higher in certain areas.
“The night time temperatures can be hugely variable. The difference between Tuggeranong and the Canberra Airport, which are our two main gauges, can vary by as much as 10 degrees in a single night,” he said.
“Any given night, with a cold night, just a little bit of wind in places, calm in other areas, temperatures can be hugely variable right across all of Canberra … But when it’s completely still everywhere, completely cloud free, then you can get pretty much uniform cold everywhere.”
Southerly breezes tended keep Tuggeranong a little warmer and the airport and Canberra’s north colder overnight, while northwesterly winds had the opposite effect. Winds become more prevalent from late July to mid-October, but Mr Carson said the capital was still susceptible to freezing nights during that time if the conditions were right.
Canberra’s coldest night on record was minus 10 degrees, recorded in July in 1971. For it to get that cold, the air needs to be very still and dry – conditions that often follow a cold front.
Thus far, the 2013 winter has been relatively mild. There have been nine nights below freezing (on track for an average 13 June days below zero), but the average minimum temperature is sitting at about 1.7 degrees, compared to the long-term average of 1 degree.
But Friday morning looks set for the coldest temperature of the year so far, with a forecast minimum of minus 5 degrees – just in time for the winter solstice, and coinciding with the Vinnies CEO Sleepout.
So long as the wind drops, Canberra will get some practice on Thursday morning, with minus 3 forecast, while on the other side Saturday looks to get down to minus 4 degrees, before a string of below-freezing nights next week.
