Pictured: 112 year-old Moreton Bay Fig (Ficus macrophylla) uprooted at Harold Boas Gardens West Perth. Image: J.Boudville.ON TUESDAY, the WA Bureau of Meteorology (BOM) recorded the lowest pressure in Perth for November since the Mt Lawley observing site was relocated to in 1993.
Adding to this, a severe weather warning was issued for the South West of the State yesterday, including a warning for higher than normal tides and damaging winds up to 100km per hour.
WA BOM expert Neil Bennet says the conditions are certainly “surprising” and warnings are rarely issues for these types of events this time of the year.
“It’s not unusual to see cold fronts coming up during the summer months, but they’re usually very weak, nothing of this magnitude,” Mr Bennet says.
“Usually around about this period the weather pattern has shifted so the semi-permanent ridge of high pressure that circles the globe, sitting around about 30 degrees south, has started to move a little further to the south.
“Typically by February it’s fairly anchored south of the continent, so the movement of that sub-tropical ridge restricts the intrusion of cold air from the Antarctic regions and the cold fronts that are pushing up from the westerly band that runs south of that sub-tropical ridge.
“These ‘travelling lows’ are there all the time, but during the winter months, the ridge itself pushes further to the north and will typically lie somewhere around Geraldton and Carnarvon, and that allows the western air flow to become established.
“Now this pattern [described above] is obviously a seasonal event and the averages have showed that it happens frequently and quite consistently, but on a daily or weekly basis you’re going to get those perturbations within that flow.
“What we saw this week was quite a cold, sharp movement northwards of the cold air, and the displacement of the ridge much further north for a short period of time.
“And a deep low pressure system formed, not too far off the west coast, which had a cold front associated with that low pressure, and that’s why it was so windy.
“So essentially, we got a winter pattern occurring towards the end of spring.”
Mr Bennet says while this kind of weather is highly unusual it cannot be attributed to climate change, but rather can be considered to be a one-off “highly unusual event”.
He also says it should not continue into next week.
“We’re already starting to move back into a more typical summer pattern, and by next week the forecast is shifting back to the 30 degree range.
He also says that the State can expect a typical cyclone season this period.
