Winter climate report 2018

29 Oct 2018

Current situation

The wet end to summer and slightly above average rainfall over autumn led to wet ground conditions and combined with the drizzly days, gave the impression of a wet winter but the rainfall numbers tell a very different story.

The Far and Mid-North experienced below average rainfall for winter overall; Kaitāia has a 226mm rainfall deficit for the previous six months. Rainfall deficits are calculated by comparing the actual rainfall totals to the expected amount, the difference is either a deficit or surplus. A major rainfall event during spring is required to make the shortfall up before summer arrives. Some of the Far North and Bay of islands rivers are running below normal flows and soil moisture deficits are well below the average. These catchments will need to be monitored closely if the dry weather continues to persist through spring.

Winter rainfall map 2018:

Map displaying rainfall for Winter 2018.

Outlook

NIWA is predicting a shift toward ‘weak’ El Niño conditions leading into summer. The NIWA climate models are leaning towards average to below average conditions leading into summer. NIWA has projected 75% chance of average to below average rainfall with an 80% chance of average to below average river flows and soil moisture levels until December 2018. See the table below for more detail.

NIWA Regional predictions for the October – December 2018 season table:

  Temperature Rainfall Soil moisture River flows
Above average 30 25 20 20
Near average 45 40 40 40
Below average 25 35 40 40

Tropical cyclone outlook

NIWA, MetService, Australian Bureau of Meteorology, MeteoFrance and the Pacific Island National Meteorological Services prepare the Tropical cyclone outlook. They are predicting near-normal tropical cyclone activity overall during November-April for New Zealand, but an elevated risk in the Cook Island and Austral Island regions, see below for more details.

Tropical Cyclone risk map:

Map displaying areas at risk of tropical cyclones during November 2018 - April 2019.

NRC Standardised Precipitation Index (SPI) maps

In 2016 NIWA completed a report for NRC describing the impact of climate change on Northland which can be found on the NRC website: www.nrc.govt.nz/climatechangeprojections 

In the report, NIWA indicated over the coming years Northland will see an increase in droughts hitting the region. In response NRC developed a warning system to assist catchments, agriculture, industry and water users at a localised scale, helping with early detection of dry weather and showing the severity. Droughts can come on quite quickly, with one month of extremely low rainfall, but often with the severe droughts, it is gradual slow process, which can be hard to detect until it has the region in its grips.

When a catchment is experiencing dry conditions, the yellow and orange colours appear on the SPI map, the circles show the severity of the event. It should be noted that the Ministry for Primary Industry is the authority responsible for the declaration of a drought. The maps are an information tool to assist the communities of Northland. The maps will be run every month. Below are maps from two recent droughts in Northland.

Drought 2010:                                                                   Drought 2014:

Map showing severity of Northland's drought 2009-2010. Map showing severity of Northland's drought 2013-2014.

SPI Map definition: The SPI calculation for winter (JJAS) was based on the long-term precipitation record at 25 sites across the region. Monthly precipitation aggregates at one and four months were used to compute monthly and seasonal SPI, respectively. The value of SPI is indicative of dryness or wetness: positive SPI values indicate observed precipitation is above median (wet condition), whereas negative SPI values indicate precipitation is below median (dry condition).

Rainfall

June 2018:
Rainfall totals throughout Northland in the first month of winter were higher than long term medians in most areas (around 120% of medians). These totals were influenced by two weather events, the first near the beginning of the month and the second on 20-21 June. The second event caused some flooding over SH1 at Whakapara.

July 2018:
July rainfall totals were unusually low throughout the region, at around 54% percent of expected rainfall based on long term medians, particularly dry in the north of the region, with NRC northern gauges recording only around 29% of expected rainfall for the month.  The northern half of the region had a Standardised Precipitation Index (SPI) classification of severely dry through his month.

August 2018:
August experienced drizzly days, but no significant weather events. Rainfall throughout the region was around 65% of typical August rainfall with higher rainfall to the northwest and lower rainfall totals to the south and east during this month. The southern and eastern areas of the region had an SPI classification of moderately dry conditions through this month.

The dry conditions through July, August and September are evident in the table below showing most of the region in rainfall deficit.

Six month rainfall deficits table:

Rainfall stations Expected annual rainfall Actual rainfall for April - September 2018 Deficit (mm) Deficit (%)
Kaitāia 820 594 -226 -23
Kerikeri 1056 997 875 -122.4 -12
Kaikohe 921 886 -34.6 -4
NRC Water Street 816 839 23 3
Tara 932 813 -119 -13
Opononi 748 696 -52 -7
Dargaville 725 634 -90.6 -12
Ruawai 656 633 -23 -4

Median Rainfall Map July - September 2018:

Median Rainfall Maps July - September 2018.

NRC Monthly SPI Maps:

Monthly SPI Maps - July-Sept 2018.

River flow

The format of the river flow map has changed to align with the rainfall and SPI maps, historically if a river was running low, the flow maps showed the entire catchment as low, which is correct with a catchment of similar geology, but Northland catchments are a mixture of geologies. Now the dots represent key river station and their flow status.

As mentioned above, the September flows reflect the below average rainfall over winter in the Mid and Far North.

In the Far north, rivers running at 40-60 percent of normal flow include:

  • Victoria
  • Takahue
  • Te Puhi

In the Bay of Islands region, rivers running at 40-60 percent of normal flow include:

  • Waitangi
  • Maungaparerua
  • Waiaruhe

Further southwest, rivers running 60-80 percent of normal flow include:

  • Mangakahia,
  • Opouteke
  • Kaihu

River flows map - September 2018.

Groundwater

Northland's aquifers are close average in water level reading, except for the systems in the Mid-North and Whangārei region, see the table below for more details:

Groundwater Systems Status for February 2018
Aupouri OK
Taipa OK
Russell Below average
Kaikohe Below average
Whangārei Below average
Mangawhai OK
Ruawai OK

Soil moisture deficits

NIWA Climate Station Soil moisture deficit (mm) as of 24/10/2018
Kaitāia -75
Kaikohe -15
Kerikeri -40
Whangārei -10
Dargaville -70
Warkworth -30

Northland NIWA Climate Stations soil moisture deficits:

Soil moisture deficit graph for Kaitaia.

Soil moisture deficit graph for Kerikeri.

Soil moisture deficit graph for Kaikohe.

Soil moisture deficit graph for Whangarei.

Soil moisture deficit graph for Dargaville.

Soil moisture deficit graph for Warkworth.