Hotspot Watch 28 March 2024

Source: NIWA – National Institute of Water and Atmospheric Research

A weekly update describing soil moisture patterns across the country to show where dry to extremely dry conditions are occurring or imminent. Regions experiencing significant soil moisture deficits are deemed “hotspots”. Persistent hotspot regions have the potential to develop into drought.

Recent rainfall and current soil moisture conditions:

North Island:

• Rainfall totals of 20-40 mm were generally observed in the western and central North Island in the past week.
• However, much of Northland, Auckland, the east coast, and lower North Island only received meagre rainfall totals of 5 mm or less.
• Overall the soil moisture situation across the North Island did not change significantly in the past week, although additional soil moisture decreases were observed in the eastern and lower North Island.
• The driest soils across the North Island, when compared to normal for this time of the year, are found in eastern Northland and the northern Coromandel Peninsula, while the wettest soils for this time of the year are found in western and southern Waikato.
• Hotspots are currently located across most of Northland, northern Auckland, northern Coromandel Peninsula, parts of coastal Gisborne, southern Hawke’s Bay to Wairarapa, and coastal Manawatū-Whanganui to Kāpiti Coast.
• As of 26 March, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in Northland, northern Auckland, northern Coromandel Peninsula, southern Hawke’s Bay, southern Manawatū-Whanganui and Wellington. Very dry to extremely dry conditions are located in much of Northland and Wellington. In addition, parts of eastern Northland are on the precipice of meteorological drought.

South Island:

• Much of the West Coast and Fiordland received 50-100 mm in the past week, with isolated amounts above 125 mm.
• Rainfall in the upper West Coast, interior Canterbury, and Southland was as high as 25 mm.
• However, a majority of the upper South Island and coastal Canterbury only saw meagre totals of 5 mm or less.
• This resulted in small soil moisture decreases in the upper South Island along with small to moderate increases along the West Coast. Elsewhere, little change was observed.
• The driest soils across the South Island, when compared to normal for this time of the year, are found in northern Tasman, while the wettest soils for this time of the year are found in western Southland.  
• Hotspots are currently located across northern and eastern Tasman, Nelson, parts of Marlborough, northern and central Canterbury, and small parts of eastern Otago, although the remainder of Canterbury and eastern Otago are near hotspot status.
• As of 26 March, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in eastern Tasman, Nelson, Marlborough, several parts of Canterbury, and northern Otago. Very dry to extremely dry conditions are located in eastern Tasman, Nelson, western Marlborough, parts of northern and far southern Canterbury, and northern Otago.

Pictured above: Soil Moisture Anomaly Maps, relative to this time of year. The maps show soil moisture anomalies over the past two weeks.

As of 26 March, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in Northland, northern Auckland, northern Coromandel Peninsula, southern Hawke’s Bay, southern Manawatū-Whanganui, Wellington, eastern Tasman, Nelson, Marlborough, several parts of Canterbury, and northern Otago. Very dry to extremely dry conditions are located in much of Northland, Wellington, eastern Tasman, Nelson, western Marlborough, parts of northern and far southern Canterbury, and northern Otago. In addition, parts of eastern Northland are on the precipice of meteorological drought. Please note: some hotspots in the text above may not correspond with the NZDI map. This difference exists because the NZDI uses additional dryness indices, including one which integrates the rainfall deficit over the past 60 days. Changes are therefore slower to appear in the NZDI compared to soil moisture anomaly maps that are instantaneously updated.

The week ahead:

North Island:

• Scattered showers will affect most of the island on Friday (29 March), with the heaviest showers along the east coast.
• The east coast will continue to see rain on Saturday (30 March) while other regions will be mostly dry.
• From Sunday, generally dry weather is expected through Wednesday (3 April).
• Late next week, an arriving front could bring light to moderate rain.
• Weekly rainfall totals are likely to be meagre once again, especially in the upper North Island where amounts are likely to remain below 10 mm. Along the east coast and lower North Island, weekly amounts are likely to be 20 mm or less.
• Due to the expected rainfall in the next week, additional soil moisture decreases are likely in the upper North Island, with generally little change expected elsewhere.
• Current hotspots in the upper North Island will have the best chance to strengthen in the next week, while those located elsewhere may not change significantly.

South Island:

• Other than isolated showers on Friday (29 March), high pressure will bring generally dry weather through Tuesday (2 April).
• A front could bring at least moderate rain to the West Coast during the middle of next week. 
• Weekly rainfall totals of 40-60 mm will occur across much of the West Coast, with 15-30 mm possible in the lower South Island.
• However, meagre rainfall amounts of 10 mm or less are possible in the upper and eastern South Island.
• Due to the expected rainfall in the next week, additional soil moisture decreases will be likely in the upper and eastern South Island, while other regions may see little change.
• The current hotspots in the South Island will likely strengthen and expand in the next week, with new hotspots potentially forming in southern Canterbury and Otago.  

Long-term outlook (through late April):

• The drier (25^th percentile) and middle (50^th percentile) rainfall scenarios both show drier or much drier than normal conditions across the upper North Island and much of the South Island, leading to higher confidence for that outcome.
• In the middle (50^th percentile) scenario and wetter (75^th percentile) scenario, above normal rainfall is signalled in the eastern North Island.
• Very dry soil conditions will affect the upper and eastern South Island in all three rainfall scenarios, with meteorological drought signalled in the upper South Island in the drier scenario.

Pictured above: 35-day forecast rainfall anomaly scenarios (Top), and 35-day forecast dryness and drought scenarios (Bottom). These maps are updated daily at https://niwa.co.nz/climate/seasonal-climate-outlook 

Background:

Hotspot Watch: a weekly advisory service for New Zealand media. It provides soil moisture and precipitation measurements around the country to help assess whether extremely dry conditions are imminent. 

Soil moisture deficit:  the amount of water needed to bring the soil moisture content back to field capacity, which is the maximum amount of water the soil can hold.

Soil moisture anomaly:  the difference between the historical normal soil moisture deficit (or surplus) for a given time of year and actual soil moisture deficits.

Definitions: “Extremely” and “severely” dry soils are based on a combination of the current soil moisture status and the difference from normal soil moisture (see soil moisture maps at https://www.niwa.co.nz/climate/nz-drought-monitor/droughtindicatormaps)

Hotspot: A hotspot is declared if soils are “severely drier than normal” which occurs when Soil Moisture Deficit (SMD) is less than -110 mm AND the Soil Moisture Anomaly is less than -20 mm.

MIL OSI