October 2020 climate report

Summary

• For most of the region, October 2020 drier than a normal when compared to a typical month of October.
  
  
• This is the second dry month in row, and we are seeing the impacts of this, with river flows and groundwater levels low for this time of year.
  
  
• A long term (12-month period) rainfall deficit of -20% remains in Kaitaia and on the west coast (Opononi, Dargaville and Ruawai). These areas have been receiving less rain from a predominantly north-easterly airflow. On a positive note, these deficits are less extreme than those recorded on the east coast this time last year.
  
  
• NIWA have confirmed that La Nina conditions have begun, and this will be the main driver of weather patterns through Spring and Summer. This would typically indicate a wet summer for Northland, but the prediction is for near normal rainfall over the next quarter. NIWA note that this is not likely to be a typical La Nina, and the confidence in the long-range forecast is low. The summer months are “feast or famine” by nature, with Northland relying heavily on storms from the north drawing tropical moisture onto Northland. The frequency of these storms is usually higher in La Nina conditions, but there is also a high degree of chance involved. One or two big storms can be the only difference between a wet or dry summer.
  
  
• There is a high likelihood of hotter than normal conditions over the next three months. This means there will be elevated water loss to evapotranspiration less water staying in soils and reaching rivers and aquifers. We need “above normal” rainfall for “normal” river flows and groundwater levels to occur.
  
  
• Soil moisture is low for this time of year and similar to last year, largely due to the lack of rainfall through September and October and warmer than normal temperatures. Low soil moisture is a good indicator that without significant rain rivers could drop rapidly leading into summer as, they did around this time last year.
  
  
• The general feel is that there is a good chance of Northland receiving the rainfall it needs to get through summer without a repeat of last summer’s drought conditions, but we need to be mindful of the points above and that the risk of dry conditions still exists.

Outlook

September - November:

The long-range forecast for November 2020 to January 2021 is for near normal rainfall for Northland, but it should be noted that there is high uncertainty in the forecast. La Nia conditions have now set in, which would usually lead to more North-Easterly winds and wet conditions for Northland, but forecast atmospheric conditions suggest this may not be a typical La Nina, and it is difficult to forecast the next three months with any certainty.

Temperatures are forecast to be higher than normal over the next three months, so although rainfall may be normal, the water lost to evapotranspiration may be higher than normal leading to low soil moisture and low river flows.

NIWA Temperature, Rainfall, Soil Moisture and River Flows probabilities for November to January 2020

Rainfall

October 2020 was the second dry month in a row following a very dry September. Most of the region received little rainfall through October, with the exception of some storm rain in the Waitangi and Hatea catchments on the 13th. Northland region received on average 67mm of rain in October compared to the 94mm we would usually expect. The Waitangi at Wiroa Road gauge had the most rain with 125mm for October and Waimamaku at Weka Weka Road was just behind with 124mm. Whakapara at Puhipuhi and Waima at Tutamoe were the only other gauges topping 100mm for the month. On the low end was Whangārei Harbour at Marsden Point with 29mm and up at Cape Reinga with 31.8mm. The heavier falls were mostly around the higher country while the low rainfall areas were in the south, the far north and the eastern coastal fringe

The NRC SPI drought index indicates that over both the previous 12 months and previous 6 months, the west coast and Kaitaia has been a lot drier than the east, with “Moderately Dry” to “Severely Dry” conditions over the last 12 months as a whole. 12-month rain deficits in Kaitaia, Opononi, Dargaville and Ruawai are between -20% to -26%, which is significant, but not severe as this time last year when the east coast had 12 month deficits of up to -43%.

Rainfall % median maps for August 2020 to October 2020

SPI Index map for 6 and 12 months to the end of October 2020

One year and six-month rainfall deficit percentage values

River flow

For October 2020, river flows were below normal in most areas, with most falling into the category “Low” (10th to 25th percentile, so 10-25% of the historical October flows were as low as they were in October 2020 at these stations). Some Stations fell into the categories “Very Low” and “Extremely Low” (0-10th percentiles, so only 0-10% of the historical October flows were as low as they were in October 2020).

Flow Map October 2020

Groundwater

Groundwater levels are below normal in all aquifers except the Aupouri system. Most Notably, Ruawai is extremely low for the month of October. This reflects the minimal rainfall in this area over the last three months.

Northland aquifer status

Groundwater percentiles map for groundwater levels recorded during October 2020

Soil moisture deficits

NIWA water balance modelling indicates soil moisture deficit is below average at all NIWA climate stations except Whangārei. This is influenced by low rainfall over September and October, as well as warmer than normal temperatures. “SMD is calculated based on incoming daily rainfall (mm), outgoing daily potential evapotranspiration (PET, mm), and a fixed available water capacity (the amount of water in the soil 'reservoir' that plants can use) of 150 mm” (NIWA).

Soil moisture deficit at NIWA climate stations

NIWA Soil Moisture Deficit Plots