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Rainfall thwarts hotspots
Wairarapa is in for a cool and cloudy Christmas this year.
The National Institute for Water and Atmospheric Research [Niwa] meteorologist Ben Noll said an onshore wind flow would bring extra clouds and prevent a hot Christmas in the eastern North Island.
“The warmest weather looks to be in the northern and western North Island. The eastern South Island will have low clouds, particularly during the morning, and cooler than average temperatures.”
Noll said warmer-than-normal coastal sea temperatures in New Zealand and the southwest Pacific meant that Niwa forecasters would be keeping a close eye on the tropics right through the silly season.
“While no tropical cyclone development is expected in the near future, that could change early in the New Year.”
MetService said Masterton could expect a few showers to begin Christmas Day before becoming fine.
It said there would also be fresh southerlies dying out later in the day.
The high for the day was forecast at 23 degrees, with a low of 11 degrees.
MetService said there was a 50 per cent chance of one millimetre or more rain on Christmas Day.
Boxing Day was set to be cloudy with northwesterlies, with a high of 26 degrees and a low of 11 degrees.
MetService’s extended forecast showed showers and light winds emerging next Tuesday and Wednesday.
New Year’s Eve was set to have a few showers and light winds, with a high of 23 degrees and a low of 11 degrees.
MetService said there was a 60 per cent probability of one or more millimetres of rain on New Year’s Eve.
What about the summer holidays?
Yesterday, the sun reached its highest position in the sky, marking the Southern Hemisphere’s longest day of the year. The day is known as the summer solstice and means the astronomical summer has begun.
Niwa forecaster Nava Fedaeff said the point at which summer begins depended on who you talked to.
“While astronomical seasons are based on the position of Earth in relation to the sun, meteorological seasons are split into groups of three calendar months, where the start of summer is defined as December 1.”
She said meteorologists had been using the meteorological convention to define seasons since the late 1700s.
“Calendar months align closely with temperature patterns, so this has made it much easier for us to calculate statistics and analyse trends.”
Fedaeff said while both the astronomical and meteorological definitions were valid, Niwa forecasters had also turned to Mother Nature’s weather patterns, looking at the warmest 90-day period of the year to provide another perspective on when summer in New Zealand would begin.
“And it turns out, the date varies considerably.”
She said “summer” had started as early as November 22 in 2017 and as late as January 5 in 1976.
“On average, when summer is defined in this way, it begins on December 14.”
Fedaeff said while summer start dates jumped around when looking at the warmest 90-day period every year, no clear trend was emerging that summer was getting earlier or later.
“The summer period is getting hotter though – an expected result, given our warming climate.”
She said this summer had got off to a very warm start.
“December is currently on track to be in our top 10 warmest on record, but it has also been very wet in places, and some may argue that is not that summer-like at all.”
Hotspots rained away
Late-year rainfall has kept hotspots at bay, but dry and very dry conditions can still be seen in Wairarapa.
In its last hotspot update for the year, Niwa said that a series of rainbands, as well as isolated thunderstorms, led to moderate-to-heavy rain totals observed in most areas.
Very heavy rain was seen in parts of Taranaki, the Tararua Range, the Central Plateau and the western Raukumara and Te Urewera Ranges.
Niwa said the normal west-east divide of rainfall was not as pronounced as last week, with more than 100mm recorded in parts of Wairarapa and 69mm recorded at Napier.
It said this was about 1.5 times the December monthly normal.
While mountain ranges picked up the biggest rainfall totals, Niwa said significant rainfall was also seen in other places like Stratford in Taranaki, which recorded 184mm of rain in the last week.
It said Te Puke recorded 160mm and Turangi recorded 134mm.
Niwa said hotspots in southern Northland, northern Auckland, near Whakatane, and southern Hawke’s Bay had all dissipated.
The New Zealand Drought Index [NZDI] map showed that dry and very dry conditions were located in Wairarapa as of December 14.
Niwa said the rainfall led to widespread soil moisture increases across most of the North Island, with substantial increases about Bay of Plenty, parts of Waikato, Auckland, and Northland.
“The driest soils across the North Island, compared to normal for this time of the year, are found in pockets around the Bay of Plenty, southern Coromandel, and coastal Whangarei.
“The wettest soils for this time of year are found from Wellington to about the Manawatu or Rangitikei region.”
Earlier this month, Niwa said the driest soils across the North Island were found in eastern Wairarapa when compared with normal for the time of year.
Niwa said this week looked to be much drier and settled than last.
It said areas of rain and showers could develop yesterday, but totals were looking light.
“Some eastern areas of the North Island may experience a completely dry week, while areas in the west and those about the ranges may only receive 5mm to 20mm of total rainfall.”
Niwa said because of the expected rainfall this week, soil moisture levels were likely to decrease for most of the North Island, with a re-emergence of small hotspots possible in Northland and around the Coromandel.
