One of the messiest weather patterns so far this year is now tracking around New Zealand and it means a changeable forecast – for some, a highly repetitive one. Messier doesn’t mean stormier – but it does mean we’re not in a settled, calm period.
Over winter we had a season of “big” weather systems from at least two large low-pressure systems that engulfed the entire nation for several days at a time, to the most powerful high-pressure zone ever recorded in New Zealand. Big weather systems can take weeks to move through.
Fast-forward to September and while this new season looks to also be a season of “big”, this month in particular also looks to be the month of “changeable”. That’s because weather systems (anticyclones and depressions) are moving through much faster now – classic spring!
There are two things standing out about our weather pattern this month: One, just how very powerful the storms over the Southern Ocean are this year. In recent weeks most storms have had air pressure between 920 and 950hPa, which does happen at this time of year but it’s the frequency and large number of storms reaching this depth that is standing out to WeatherWatch this year. And two, the warmer-than-average airflows in the mix.
The warmer-than-average weather won’t be consistent – the volatile weather south of NZ and the generally faster-moving, more chaotic pattern we have means NZ will still get regular cold injections – but because things are moving fast from west to east, these cold air injections may not linger long, and may struggle to go north – limiting northern frosts.
Storms south of NZ will send several cold fronts into NZ so the lower South Island in particular may feel winter this year is taking longer to ease, while those in northern NZ may be talking more about how fast the grass is now growing as an early spring starts to establish itself.
High pressure may cross NZ giving us a pause from this all – and a return to cold nights with a heightened frost risk. At the time of writing this, high pressure was forecast to cross NZ on Monday September 3 – but modelling was conflicted about if it would linger very long or directly affect both main islands. Outside of that, many highs may skirt to our north for a time, allowing for more cold fronts to come in from the west and south.
Due to the dominating westerly flow, rainfall over the next 15 days looks to be mostly on the West Coast with over 300mm likely around mid to south Westland, and around 100mm for parts of the western North Island, like Taranaki and Waitomo. Gisborne and Northern Hawke’s Bay may have the lowest totals.
Upcoming Highlights:
• Monday/Tuesday kicks off colder with a southwesterly flow across NZ
• Wednesday/Thursday has milder west to northwest winds returning
• Another cold front moves up NZ on Friday/Saturday
• Most wet weather in the west to southwest of both main islands
In Focus Podcast: Full Show | 13 September
