Wednesday, December 6, 2023

Record heat a possibility this summer

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It takes a very particular set-up to break 40degC in New Zealand, says Phil Duncan.
Departure from Normal Rainfall for Spring and Summer shows spring starting to dry out, but summer looking even drier for the north. However, it also shows rain for the West Coast and Southland. Credit: 
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By Philip Duncan, WeatherWatch

Hot air and a lot of bluster – no, it’s not the upcoming election, it’s the outlook for late spring and summer for many (but not all) parts of New Zealand. 

This week is a classic mid-September week. As we approach the spring equinox this coming Saturday (at precisely 6:49pm) it’s normal to be getting windier westerlies, but the growing El Niño component may add a bit more oomph to that westerly flow. 

This airflow means Canterbury, Marlborough, Nelson, Wellington, Wairarapa, Hawke’s Bay, Gisborne, Bay of Plenty, Waikato, Auckland and Northland are all at risk of a drier-than-usual spring and summer.

But NZ may end up being part of two extremes this summer – one a drier and hotter airflow out of Australia, the other a cooler and wetter airflow out of the Southern Ocean. 

Southland’s forecast this week is very different to that for most of NZ. It’s colder, bleaker and wetter.  There may be another 40mm this week for Southland, with daytime temperatures struggling to climb much into the double digits. 

Contrast that with Hastings in Hawke’s Bay which has highs in the mid-20s this week and overnight lows in double digits. That is a remarkable difference between the two islands and highlights the fact that half of our weather comes from the Southern Ocean area.

So Southland and the West Coast look to lean wetter and cooler this spring, with Otago the buffer region between wet in the west and super dry in the east. The western North Island may also be cooler at times this spring with cloudier weather off the Tasman and the odd showery day – although rainfall totals are expected to continue to be below normal in much of the North Island even if you do have regular showers.

This summer is when El Niño will be peaking, and that – along with the peak heat of summer –  could mean NZ has a record that may be broken: Our hottest ever day. 

Rangiora in Canterbury holds the record maximum of 42.4degC recorded in 1973, with Christchurch recording 41.6degC in that same year. More recently, Timaru reached 41.3degC on Waitangi Day in 2011. 

It takes a very particular set-up to break 40degC in New Zealand: we need high pressure but we also need a nor’west airflow out of central Australia; hot air that may start off in the mid to late 40s or even 50degC. A nor’wester over the Tasman Sea loses about 10degC due to the cooler waters then, as that nor’wester blows over mountains and ranges, it heats up even more in the east (friction makes heat – the same way you get carpet burn if you jump onto the floor too fast on your hands and knees!). 

So eastern areas do have some chances this summer of record hot days. But remember, the Southern Ocean weather pattern is like the cold tap to El Niño’s tap of hot air.

We just need Mother Nature to be the hand in the bath to mix those airflows up and make them milder.

Highlights this week

• More wintry weather in Southland, single-digit highs possible

• Wet on the West Coast

• Eastern New Zealand (both islands) into the late teens to mid-20s

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