December 2024 climate report

Key take away from this report

The region is moderately dry, with more than 10 catchments recording below their mean annual low flow, and is expected to continue to worsen.

Summary

• Northland recorded on average 50% of the expected rainfall for December, with more than half of sites monitored receiving less than 100% of their expected December total.
• Majority of river flows range from “low” – “extremely low” mainly in the central and southern areas of Northland.
• Groundwater levels are varied but are generally all “below normal”.
• “Above normal” or “near normal” temperatures and rainfall are expected between January and March, with “near normal” or “below normal" flows and soil moisture levels.
• However, January is likely to be dry, with similar patterns to December.
• There are currently low levels of hydrological drought conditions in Northland as of 10 January 2025.

Rainfall

• During December there were no significant rainfall events across the region; however, some small areas did record isolated storms in the second week of the month and around Christmas. The region was drier than expected, recording 55 mm on average, which is 50% of the average December rainfall (Figure 1). The Rainfall we did receive was mainly in the far north, and northeast coast, and in the high ranges of Waimamaku. The minimum rainfalls recorded were 23.5 mm in Dargaville, 24 mm at Paparoa and 25.5 mm at Kai Iwi Lakes. On the other extreme the maximum rainfalls recorded were 130 mm on the Purerua Peninsula, 125 mm at Waimamaku and 110.5 mm at Waihopo. Compared to the site average for the month of December, Paparoa held the minimum with 25% of expected rainfall, followed by 29% in Dargaville and 32% at Kai Iwi lakes. The maximum percentages recorded were Purerua Peninsula with 153% and Waihopo with 138% of their December average.
• The standard Precipitation Index places the sites rainfall in relation to their individual average and indexes between -3 (extremely dry) and 3 (extremely wet), this allows all sites in the region to be compared to each other without bias. On a 3-month scale (October – December) the region ranges between “normal” to “moderately Dry” (Figure 2), however on a 6-month and 12-month scale the region is “moderately dry” to “severely dry” other than the Poutō Peninsula which is “normal”.

Figure 1: December rainfall distribution for Northland in mm, and in percentage of median.

Figure 2: Standardised rainfall for Northland on a 3-month scale (October – December) and a 6-month scale (July – December).

River Flows

River flows in most of Northland’s primary monitored catchments ranged from “below normal” to “Extremely low” for December (Figure 3).

The west coast which had the lowest rainfall amounts are the lowest, with less than 5% of their expected monthly flows. The areas around Kāeo are currently in “normal flows” ranging between 40-60% of their December average, which is congruent with the rainfall for the month mainly being focused in this area.

Many of the rivers in the central and southern areas of Northland are at or below their Mean Annual Low Flow (MALF), and are continuing to steadily drop, throughout January this pattern is expected to continue as no significant rainfall is currently predicted.

Figures 3 and 4.

Groundwater

Groundwater levels in Northlands primary monitored aquifers ranged from normal to extremely low for November. This variation is due to rainfall distribution through the region, and aquifer characteristics. The areas of most concern are around the Whangārei district.

Figure 5: Groundwater levels in Northland’s primary monitored aquifers for December

Climate Outlook for January – March 2025

Summary of climate forecasts from NIWA and MetService

Throughout January Northland can expect similar conditions to December, with near average temperatures, and south-eastly winds. However, for Northland due to a ridge of pressure sitting on the region we are likely to remain drier, with the weather systems blocked to the south of the country.

Between January to March temperatures are likely to be “near average” or “above average”, while rainfall, soil moisture and river flows are likely to be “below average” or “near average” (Table 1). There is only a 40% chance of a La Nina event being declared, however with higher than average sea surface temperatures and other climate drivers, heavy rain is likely to occur if a low pressure/ ex-tropical cyclone develops.

The forecast for the next ten days in Northland is for mainly fine weather, with some small, isolated showers in places, winds will be mainly southerlies (Figure 6).

For more information visit the following links:

• Seasonal climate outlook January-March 2025 (NIWA)
• Get weather emails from MetService

Table 1: Forecasted likelihood of above, near or below average climate conditions for Northland from January – March 2025

(Expressed as % likelihood).

Figure 6: MetService Northland Regional Forecast, issued 9 January 2025

Current Drought / Dry Conditions

On the 5 January 2025, the NIWA Drought Index (NZDI), categorised the majority of Northland as “Very Dry”, and north of Kaitaia as “extremely dry” (Figure 7).

The index is based on the Standardised Precipitation Index, the Soil Moisture Deficit, the Soil Moisture Deficit Anomaly, and the Potential Evapotranspiration Deficit, with a value less than 0.75 considered normal – wet.

Figure 7: NIWA New Zealand Drought Index produced January 2025

Compared to previous summers, Northland is currently matching the conditions recorded in January of 2020 and 2021, in all three districts of the region (Figure 8).

Figure 8: NIWA drought index tracking for the Far North, Whangārei and Kaipara districts from 2018 - 2024

Predicted conditions for January

NIWA’s drought prediction tool combines rainfall, soil moisture, and evapotranspiration to estimate the risk of meteorological drought, over the next 35 days.

The forecast from 5/1/2025 to 8/2/2025 predicts a moderate likelihood of “very dry” conditions in the west in its median scenario (50th percentile), and a minor risk of the whole region being “very dry” (90th percentile) (Figure 9).

The rainfall prediction for the region for the same period is that Northland will be dry – very dry, receiving 50 – 80% of the expected rainfall for January, with some areas receiving less than 50% (Figure 10).

The NIWA35 forecasting tool can be rerun every week, and is most reliable for the two weeks after the forecast: NIWA35 forecasting tool at shiny.niwa.co.nz

Figure 9: Drought risk assessment for Northland from NIWA35 from January 5th. There is a moderate risk of dry conditions in the west (50th percentile), and minor risk of very dry conditions (90th percentile only).

Figure 10: Rainfall prediction for Northland from NIWA35 from January 5th. There is a moderate risk of dry conditions – very dry conditions (50th percentile).

Northland Drought Risk Assessment

The drought risk assessment (Table 3) provides a visual summary of the current conditions of Northlands rainfall, river flows, and groundwater levels and the current state of water resources. It also incorporates NIWA’s Climate Outlook and Drought Index, and MetService Forecasts.

In summary Northland region has experienced lower than normal rainfall for the 2024 calendar year. Consequently, most river flows and groundwater levels through the region are currently lower than normal for this time of year.

NIWA’s climate outlook for summer predicts rainfall to be normal to above normal with moderate confidence between January - March, river flows to be about normal to below normal, and soil moisture below normal.

There is likely to be no significant rainfall events in January.

Northland is currently in a moderate level of meteorological drought (low rainfall) and a low level of hydrological drought (low river flows, groundwater levels, and reservoir levels etc.).

Compared to early December, the dry conditions are worsening with 17 rivers having fallen below MALF.

There is a risk Northland could experience an increasing level of hydrological drought (lower river flows, groundwater levels and water reservoirs etc.) over the next three months.

Table 3: Drought Risk assessment matrix

Tropical Cyclone Outlook

Summary of NIWA and MetService Outlooks

On average at least one ex-tropical cyclone (TC) passes within 550km of New Zealand (NZ) each year. This season the risk is considered normal-elevated.

If an ex-TC tracks close to NZ, there is a near equal probability of it racking to the east or west of the North Island. An ex-TC entering the NZ region could produce significant rainfall, severe winds, hazardous marine conditions and coastal damage.

Figure 11: Number of predicted named tropical cyclones interacting with an island group for the 2024-25 season