A high-pressure system is trapping moisture over the state of New South Wales and causing thunderstorms and humidity, but it should not be a cause for concern, the Bureau of Meteorology says.
Sydney had been battered by storms the past week, with Ku-ring-gai, Hornsby, Warringah, Camden and Hawkesbury among the worst hit areas.
Meterologist Mohammed Nabi, of the Bureau of Meteorology, told SBS the wild weather was caused by moisture trapped over the state.
"What we’ve had over the last is few days is a lot of moisture sitting over the entire state and it hasn’t had a chance to move through to the east, which it normally does, thanks to a large high that is straddling the Tasman Sea," Mr Nabi said.
"That high-pressure system is blocking the moisture and holding it over much of New South Wales."
He said such moisture was usually cleared by a front travelling over the state but that hadn't happened this time as fast as expected.
December is typically the stormiest month of the year, but in the past two years New South Wales has had fewer thunderstorms than normal, making the recent weather seem even more unusual.
Listen to the full interview with the Bureau of Meteorology's Mohammed Nabi:
Mr Nabi said that while days of consecutive storms were rare, they were not unheard of.
"Going back to 2007 there was a run of five days of consecutive thunderstorms from the 3rd to the 7th," Mr Nabi said. "In 1992, the same happened between the 21st and the 25th and in 1955 we had another run of five days of stormy weather from the 24th to the 28th of December.
"It’s not completely out of the blue, it’s not completely uncommon but it doesn't happen all the time. And then if we get a couple of years when we don’t get this happening it changes our perception of the whole thing and we think 'December shouldn’t be like this'."
He said the strong humidity felt in parts of the state was caused by "slow moving" air dragged down from the tropics.
"The only way to clean that up is if a big front comes up," he said.
"And for that front to be able to move anywhere, we don't want a high-pressure system sitting over the Tasman Sea."
But the Bureau could not predict when such respite would come.
"If we get a couple of years when we don't get this happening it changes our perception of the whole thing and we think 'December shouldn’t be like this'."
"What we'll probably get overnight tonight is a southerly push," Mr Nabi said.
"What we really want to happen is for that high to erode away over the Tasman Sea and move further out to east of New Zealand somewhere."
He said the summer season was normal so far.
"To my mind it's a warmish summer; a typical summer," he said.
"What's throwing us all off, as far as our perceptions go, is that in the last couple of years we haven't had this in December. Compound that with the fact that it's been a slightly longer run of the same weather every day."
He said the rest of the country was experiencing average conditions at the moment, and it was only NSW experiencing unusual weather patterns.
The Bureau of Meteorology said there was a 70 per cent chance of showers and a thunderstorm on Monday afternoon, with light winds becoming south to south easterly 15 to 20km/h late in the day.
Wild weather is forecast for most of the week, however the BOM said it was not expected to be as "intense" as it has been in recent days.
-With AAP
