Hotspot Watch 30 November 2023

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:

• Substantial rainfall of 60-100 mm was observed across Gisborne and northern Hawke’s Bay in the past week.
• However, much less rainfall occurred elsewhere across the North Island, where totals were generally less than 25 mm.   
• Particularly meagre rainfall was observed in Northland, Auckland, and northern Waikato.
• This resulted in small to moderate soil moisture decreases across a majority of the North Island, although little change was observed in Gisborne and Hawke’s Bay.
• The driest soils across the North Island, when compared to normal for this time of the year, are found in Kapiti Coast, while the wettest soils for this time of the year are found in parts of Gisborne and Hawke’s Bay.
• No hotspots currently exist in the North Island.
• As of 27 November, the New Zealand Drought Index (NZDI) map below shows that no unusually dry conditions are currently found across the North Island.

South Island:

• Rainfall amounts of 30-50 mm were observed in much of the West Coast and Fiordland, along with portions of northern and central Canterbury.
• Elsewhere in the South Island, rainfall amounts of 25 mm or less were generally observed.
• This resulted in at least minor soil moisture decreases across much of the South Island, although minor increases were observed in central Canterbury.    
• The driest soils across the South Island, when compared to normal for this time of the year, are found in Marlborough Sounds and coastal Southland, while the wettest soils for this time of the year are found in western Tasman.
• In the past week, the hotspot located in coastal Banks Peninsula and Selwyn District dissipated, while a new hotspot formed in Marlborough Sounds.

As of 27 November, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Nelson, Marlborough, North Canterbury, and Banks Peninsula.

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 27 November, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Nelson, Marlborough, North Canterbury, and Banks Peninsula. 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 be possible on Friday and Saturday (1-2 December), but many parts of the North Island will remain dry.
• Moderate to heavy rain may overspread much of the island on Sunday and Sunday night.
• There is currently some uncertainty regarding the timing and amount of rainfall expected next week, but periods of at least moderate rain are possible from Tuesday (5 December).
• Weekly rainfall totals of 25-50 mm are forecast across a majority of the North Island, but localised higher amounts may occur.
• Due to the expected rainfall in the next week, a majority of the North Island is likely to see little change to soil moisture levels, or small increases.
• No hotspots are expected to form in the North Island in the next week.

South Island:

• Other than isolated showers, mostly dry weather is expected through Saturday morning (2 December), followed by rain for many regions through Sunday. Heavy rain will affect the West Coast during this time.
• After showers on Monday (4 December), mostly dry weather is expected through Wednesday morning.
• During the middle of next week, another front could bring lighter rain to the West Coast.
• Weekly rainfall totals of 100-200 mm are generally expected in the West Coast, with 30-50 mm possible in interior areas.
• However, lighter amounts of 25 mm or less are forecast along the east coast, Otago, and Southland.
• Due to the expected rainfall in the next week, soil moisture decreases may occur in the lower South Island, with minor increases possible in the West Coast.
• The current hotspot in Marlborough Sounds may not change significantly in the next week, while coastal Southland could approach hotspot criteria.

Long-term outlook (through late December):

• The drier (25^th percentile) and middle (50^th percentile) rainfall scenarios both favour drier or much drier than normal conditions in the upper and eastern North Island and eastern South Island.
• Even in the wetter (75^th percentile) scenario, below normal rainfall is still favoured in the lower South Island.
• Near normal to above normal rainfall is favoured for the North Island in the wetter scenario.

Very dry or meteorological drought conditions are unlikely to develop in the next 35 days, but cannot be ruled out under the driest-case scenario for parts of the South Island.  

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