September 2020 climate report

Summary

Spring has kicked off with a very dry September 2020 for Northland, most of the region only receiving 39% of normal rain. Although winter provided more rain than 2019, the rainfall totals did drop off towards the end of winter in the south of the region. The dry September has pushed Northland's rivers down, now flowing about 40% lower than this time last year, which is a concern as 2019 was one of the driest years on record.

Comparing the recovery of the 2020 drought to the 2010 drought, both events had similar rainfall over winter. In 2010 a dry spring struck Northland, similar to this year. Both droughts had La Niña events over the following summer. In 2010 after the dry spring, the rains begun in December and continued through January. With a La Niña setup, the high pressure systems will be sitting further south than last year, which is good as it allows the low pressure systems to drag warm moist air down from the warmer seas up by the equator. Last year the high was locked right on top of Northland and bouncing off rain that tracked our way.

The key points are:

• Good winter recharge, better than 2019 but a dry finish in the south and a dry start to spring.
• Aquifers are close to average, except:
  • Mangawhai & Ruawai & Russell & Taipa.
• River flows are well below normal across the region.
• Soil temperatures are 2-3 degrees above normal, soil moisture deficits are higher and earlier than normal.
• The 12x month rainfall deficits compared to 2019 show the region is looking:
  • better along the east coast but dry from Bream Bay south
  • worse from Kaitaia south along the west coast.
• Rainfall along the western catchments is currently 20-30% below normal.

Outlook

• La Niña conditions are likely to continue through the remainder of spring.
• The Far North is typically a bit wetter than normal during a La Niña spring and summer.
• Past La Niña events have featured a gradual trend toward wetter conditions in northern New Zealand during late spring and early summer, with an increased chance for a moisture-rich tropical connection to develop.
• October – December 2020 air pressure is forecast to be higher than normal to the southeast and lower than normal to the north of New Zealand. This is expected to be associated with developing La Niña-like north-easterly air flow anomalies, although a westerly flow anomaly, which may be strong at times, is favoured to continue for much of October.
• NIWA continues in “La Niña Alert”, but based on expected trends in the climate system and the likelihood for New Zealand’s weather patterns to become more aligned with La Niña during October, it is likely that NIWA will confirm La Niña conditions during the next month. According to international guidance, the probability for oceanic La Niña conditions is 77%
• NIWA are predicting :

The full probability breakdown

• Temperatures are very likely to be above average (70% chance)
• Rainfall totals are most likely to be near normal (45% chance)
• Soil moisture levels and river flows are likely to be below normal (45% chance).

Rainfall

• September 2020 was a dry month, with most of Northland receiving less than 60mm of rain.
• Purerua Peninsula in the northern Bay of Islands recorded the least amount of rain with only 11.6mm, just 9% of the long-term median for the area.
• Waimamaku at Weka Weka Road recorded 139.5mm and Tutāmoe 107mm of rainfall, the only two sites that exceeded 100mm.
• The regional average of median rainfall was 39%. Poutō Point rarely gets the highest rainfall but in September 2020 it had the highest percentage of the median at 76%.

September 2020 rainfall maps

One year rainfall deficit percentage values

SPI Index map for 12 months to the end of September 2020

River flow

• River flows are very low, the map below show most of Northland rivers are running at 20-60% of the normal September flows.
• If no significant rain falls over spring, with above normal temperatures predicted the rivers will likely reach drought flows in January & February 2020, when typically drought flows are not observed until March.

Flow Map September 2020

Groundwater

Northland aquifer status