Autumn climate report 2018

24 Jul 2018, 12:14 PM

Current situation

The marine heatwave over the Tasman Sea and La Nina were still lingering on through early autumn with NE winds and humid conditions leading to a warm March 2018. Then in April 2018 a switch in high-pressure patterns lead to some cold fronts from the SW. Through April and May 2018 there were two rain event in northland totalling over 100mm for the day.

A wet day is defined as rain over 1mm, during the autumn period most rain gauges had 25-40 wet days. The wettest place over autumn in Northland was the Waimakuku Valley. The NRC rain gauge recording 52x wet days and 715mm of rainfall. The Waimakuku Valley is encircled by the Parataiko range to the south and shadowed by Mt Te Raupua (781m) to the north, northlands highest peak, this area is real rain magnet.

Autumn air temperatures continued to run above average, NIWA have indicated this is the fifth warmest start to any year on record, as highlighted in the graph below.

Maps displaying 2018 Monthly Temperature Anomalies.

The warm temperatures placed January to June 2018 as the fifth warmest start to any year on record, sitting behind 1998, 1937, 1999, and 2016, based on NIWA's Seven Station Series that began in 1909.


Soil temperatures were above average for March and April for most of the NIWA climate stations in Northland.

Outlook

Seasonal climate outlook July - September 2018

The international climate models indicate neutral conditions through most of winter with a possible shift to El Nino in September 2018. For Northland, winter temperatures, flows and rainfall are forecast to be close to average, follow the link below for the full NIWA breakdown:

Go to the Seasonal climate outlook July - September 2018 on NIWA's website

Rainfall

For Autumn 2018, rainfall totals in Northland were close to average for most of the region. Breaking the rainfall totals down into months:

  • March 2018 was wet for all of Northland
  • April 2018 was below average from the Bay of Islands north along the east coast including the Far North
  • May 2018 was below average for the southern half of Northland

On 28 April 2018, a thunder system delivered 141mm of rain to the Ngunguru area, 78.5mm falling in three hours, causing a sharp rise in the Ngunguru River. Fortunately the flood peak arrived on a low tide, keeping the Ngunguru road open.

Median Rainfall Map – Autumn 2017                Median Rainfall Map – Autumn 2018

Median Rainfall Map – Autumn 2017.  Median Rainfall Map – Autumn 2018.

Monthly maps

Median Rainfall Map - March, April, and May 2018

Median Rainfall Map - March, April, and May 2018.

Flows

Northland monthly flow maps for Autumn 2018

River flows map March 2018.  River flows map April 2018.

Groundwater

The nine automatic groundwater monitoring stations across Northland all show water levels close to average for the autumn period.

Groundwater levels

Groundwater systems Status for April 2018 Status for May 2018 Status for June 2018
Aupouri Average Average Average
Taipa Average Average Average
Russell Average Average Average
Kaikohe Average Average Average
Whangārei Average Average Average
Mangawhai Average Average Average
Ruawai Average Average Average

Poroti groundwater system (water level)

The Poriti system shown in the graph below is above the average water level expected for May 2018.

It's interesting to note the average water level for the month of June 2018 is the second highest since records began in 1980, following on from the hottest summer on record.

The highest water level for the month of June was recorded in 1998, this was after a significant El Nino event which saw the warmest temperatures ever recorded from January to June.

If the winter rains deliver the expected totals, the aquifer system will be in good shape for the 18/19 summer. The graph below shows water level at Poriti Springs for the previous three years, the red line indicates the normal water level for May, the red dots show the actual levels for 01 May each year.

Graph displaying groundwater levels at Poroti August 2015-July 2018.

Soil moisture deficits

All NIWA stations are recording soil moisture deficts close to zero, which means the soils are at capacity for holding water.