The ups and downs of autumn

By Daniel Corbett, Metservice weather ambassador

The summer certainly had its ups and downs and the autumn has had its more settled moments too. The La Nina that influenced the summer weather patterns has faded away but hints of it still remain in the atmosphere. This and other factors have played a part in autumn's mix of weather.

March arrived with a bang, literally, it was in the form of a weather bomb on March 2. The explosive cyclogenesis of the low-pressure system allowed the pressure to drop dramatically, which saw it classified as a weather bomb. It was good upper level dynamics in the westerlies that helped bring about the rapid cyclogenesis.

After a settled couple of weeks the weather pattern shifted to allow warm moist air to feed in from the tropics across New Zealand. A low developed in this flow and it slowly moved down to New Zealand.

Parts of Northland received 200-300 millimetres of rain from this low. Weta, in Northland, picked up 345 millimetres during the heavy rain event which saw rain rates up to about 30 millimetres per hour at times.

The upper-level winds in the subtropical jet also buckled, developing into an upper level low to the west of Northland. This clockwise flow of air continued to feed in tropical moisture as well as preventing the initial low and rain band from clearing away to the east.

It lingered across parts of Northland for over 24 hours.

Rainfall was also extensive elsewhere. Parts of Gisborne-Hawke's Bay picked up 100-200 millimetres of rain and there was widespread flooding with Kaoe in Northland being cut off for a time.

A large high dominated the weather across New Zealand for several days at the end of March and then another surge from the tropics began to spread south. This final surge was far more devastating for Fiji.

The continuous rain for days on the monsoonal trough brought over 500 millimetres of rain to parts of Fiji. A tropical low sat close to Fiji for a couple of days and then Cyclone Daphne developed to the southwest of the island.

Daphne passed south of Fiji where it, along with the first tropical low, began to move south out of the tropics at the beginning of April. The first tropical low moved down to New Zealand at the start of April, but Daphne was hot on its trail. An upperlevel low to the northwest of the North Island allowed the tropical low and its moisture to slowly spread in across the North Island. Daphne was moving at a good clip and kept trucking southeast away from New Zealand. It did help elongate and thin the band of moisture from the first tropical low.

In turn, the rain across the North Island was patchier, but, in a sustained southeast flow, the rain accumulated across the Gisborne and Hawke's Bay region. By the morning of 4 April 179 millimetres had fallen in the Gisborne ranges in Tepuia. There were some places that received over 300 millimetres of rain during the period. The rain shadow effect brought some fine days during that period from Waikato up to Auckland.

The unsettled spell was then replaced with a building anticyclone just in time for Easter. Through much of the rest of April anticyclone was the word as they came in numbers and were large when they arrived. They brought some fine dry

days especially the very large 1039 high during the third week of April. A blocked upper flow allowed it to linger for well over a week. There were many fine and warm days and some were even cloudless in places.

A series of active fronts then brought spells of unsettled weather with wet and windy conditions and even some snow to lower levels across the far south of the South Island at the end of the month. The active front that ushered in the much colder air and snow down South brought over 100 millimetres of rain to the western upslopes of the South Island and lower North Island.

There were strong to severe gales associated with the tight pressure gradient from the active weather system. A wind  gust of 153 kilometres (73 knots) was recorded near Wellington just before the front pushed through.

From La Nina to neutral territory

Conditions in the central Pacific are now in neutral territory, in-between a La Nina and an El Nino, which usually  indicates more variability in our weather. It also means our weather pattern may change every few weeks from one scenario to another.

In the shorter term, with remnants of La Nina still present, the anticyclones will continue to track across central and

northern parts of New Zealand over the next few weeks. Their appearances over New Zealand will bring the accompanying scenario of settled weather with light winds and frosty mornings as these anticyclones depart to the east of New Zealand.

When this happens northern areas become vulnerable to lows that may form near Norfolk or Lord Howe Island, bringing north-easterly winds to New Zealand along with a few periods of wind and rain.

Our general winter scenario involves cold fronts and associated troughs of low pressure moving on to New Zealand  from the Tasman Sea, bringing occasional rain and episodes of north-westerly, then south-westerly wind.

So enjoy the fine weather with the anticyclones, but keep an eye out for active troughs from the Tasman Sea.

- Hamilton News

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