September 2017 - Climate report

9 Oct 2017, 3:16 PM

Current situation

September was a month of extremes for parts of the country. The Tasman sea has been active with persistent frontal systems tracking in from the west.

Additional to rain, the Foehn north westerlies brought warm air across from the Tasman, which caused higher temperatures along some parts of the east cost of New Zealand.

For Northland the extremes were:

  • Kaikohe: recorded 89 kph, the 4th highest maximum wind gust since 1986
  • Whangārei: recorded 22.9°on 25 September 2017, which is the 4th highest maximum September temperature since 1967
  • Dargaville: the average low temperature at night for September 2017 was 10.5°, which is the 3rd warmest recorded since 1943

Most Northlanders feel like they had a very wet winter, which is supported in the graph below. Comparing the number of wet days each month (orange) to the long-term average (blue), August and September 2017 show over double the expected rain days for Whangārei. Over summer the number of wet days may continue to run above the expected average with the arrival of the La Nina system. A wet day is defined as a day with more than 5mm of rain.

Rain days at the Whangārei MetService Station

Graph displaying rain days at Whangarei MetService Station.

Rainfall

The Northland Regional Council monitoring stations recorded rainfall totals above average for the winter period as expected, but the Bay of Islands and Whakapapa catchments were actually below the expected rain totals for the period over July, August and September. Like the previous winters of 2015 & 2016 no significant storms struck the Northland region.

Winter rainfall maps

Median Rainfall Map – Winter 2016 and 2017

Winter 2016 percentage of median rainfall map.  Winter 2017 percentage of median rainfall map.

Monthly rainfall maps

Median Rainfall Map – July, August and September 2017

Median rainfall maps - July August September 2017.

Compared to the previous two winters the west coast, especially the south Hokianga which consistently had more rain each month. The gauge in the Hokianga harbour at Opononi recorded 35% more rain than expected over the last 12 months, the Ruawai gauge also recorded well above the expected rainfall, shown below in the rainfall deficit table below.

Rainfall deficits

October 2016 - September 2017 rainfall deficits (mm)
Rainfall
stations
Expected annual
rainfall
Actual rainfall for
September 2016-2017
Deficit
(mm)
Deficit
(%)
Kaitaia 1367 1389 22 2
Kerikeri 1056 1705 1598 -107 -6
Kaikohe 1569 1817 248 16
Whangārei 1523 1389 -134 -9
Opononi 1196 1619 423 35
Dargaville 1211 1298 87 7
Ruawai 992 1242 250 25
Tara 1697 1617 -80 -5

The Waimamuku Valley potentially has one of the highest annual rainfall totals in Northland. We now have an automatic rain gauge operating in the Valley, displayed on the NRC rivers & rainfall website:
www.nrc.govt.nz/riversrainfalldata

River flows

The winter flow maps highlights the higher than normal flows across the western region painting a similar picture to the rainfall maps. The Kaihu and Punakitere catchments had mean monthly flows that were 134% and 128% of normal.  At the other end of the scale the Whakapara, Ngunguru  and Waiotu catchments had mean monthly flows down at 60 to 65% of normal flow, these catchments will be monitored through spring and summer.

Northland monthly flow maps for Winter 2017

River flows July-September 2017.

Drilling down into the western catchments, the Kaihu river from January through to September 2017 returned 118153 mega litres to the ocean compared to the expected amount of 99416 mega litres, which is a lot considering there has been no significant floods in the Kaihu.

  • Mega litres are a term or unit used to describe large volumes of water, 1x mega litre = 1x million litres.

In plain English the Kaihu River has discharged 118 billion milk bottles of water this year.

The graph below shows a distinct absence of floods in the Kaihu River over the last three-year period. The annual flood for the Kaihu Gorge river station is 150m³/s, which has not been exceeded since 2014.

Graph displaying annual maximum flow at Kaihu Gorge.

Groundwater

The consistent rain this year has topped up the groundwater systems across the region, with water levels average to above average for September, which is ideal leading into summer. The table below shows the status of each groundwater system.

Groundwater levels

Groundwater Systems

Status for September 2017

Aupouri

Average

Taipa

Average

Russell

Average

Kaikohe

Average

Whangārei

Average

Mangawhai

Average

Ruawai

Average

The Poroti groundwater system has lifted to normal  levels observed before the summer period. The graph below highlights the impact the dry winters had on the system over the 2015 and 2016 period. The red line indicate the average water level expected in September.

Poroti groundwater system (water level)

Poroti groundwater system (water level) graph.

Soil moisture deficits

Soil moisture deficits graph - Dargaville.

All NIWA stations are recording soil moisture deficit at average levels, the graph above shows the soil moisture deficit at the NIWA climate station in Dargaville.

The Northland Regional Council is currently looking to develop a soil moisture network aross Northland to help the public manage through drought events and collect data for climate change analysis. Any additional information related to soil moisture in Northland would be appreciated, for further information please contact the Water Resources/ Hydrology Manager at the Northland Regional Council.

Outlook

The international climate models have shifted in the last month. They are now indicating a 50% chance of a La Nina event persisting through to December 2017. In the coming months from October to December 2017 we may see low pressure systems to the northwest of New Zealand and highs sitting south and east of the country, this sets up for northeast airflow carrying moisture, increasing the chances of heavy rainfall tracking onto the east coast.

What La Nina means for New Zealand:

  • Slightly warmer sea temperatures
  • Cloudier weather, especially coastal areas
  • Higher risk for afternoon showers, especially inland
  • Wetter than average for many North Island and some western and northern South Island areas.
  • An uptick in easterlies
  • A slight increase in an ex-cyclone passing by New Zealand or directly impacting New Zealand
  • Generally warm & possibly more humid

From October-December 2017, NIWA have predicted temperature, rainfall, soil moisture and river flows to be most likely average to above average across Northland. It's interesting to note in the summary table below, that the chance of below average temperature is down at 10%.

  Temperature Rainfall Soil moisture River flows
Above average  60  40  40  40
Near average  30  35  35  35
Below average  10  25  25  25