Hotspot Watch 15 December 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:

• Meagre rainfall totals of less than 10 mm were observed in much of the North Island in the past week.
• Pockets of rainfall totals of 10-20 mm were observed in the eastern and lower North Island.   
• This resulted in moderate soil moisture decreases in a majority of the North Island, although some western regions observed large soil moisture decreases.
• The driest soils across the North Island, when compared to normal for this time of the year, are found in southern Waikato and western Wellington, while the wettest soils for this time of the year are found in Cape Reinga and parts of Gisborne and northern Hawke’s Bay.
• A hotspot is now in place in coastal Manawatū-Whanganui and the Kapiti Coast.
• As of 12 December, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Wellington.

South Island:

• Rainfall amounts of 30-70 mm were generally observed in the West Coast in the past week, with 100 mm or more in Fiordland.
• Much of northern Canterbury and Southland observed amounts of 15-30 mm, but meagre rainfall was observed in the upper South Island along with interior and South Canterbury.
• This resulted in small soil moisture decreases in much of the South Island in the past week, although small increases were observed in northern Canterbury and Banks Peninsula.
• The driest soils across the South Island, when compared to normal for this time of the year, are found in parts of Marlborough Sounds and western Southland, while the wettest soils for this time of the year are found in the upper West Coast and northern Canterbury.
• In the past week, the previous hotspot located in Banks Peninsula dissipated, the hotspots in Marlborough Sounds and Nelson remained in place, while conditions remained near hotspot criteria in South Canterbury, parts of Otago, and Southland.
• As of 12 December, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Wellington, Tasman, Nelson, Marlborough, North Canterbury, Banks Peninsula, South Canterbury, Otago, coastal Southland, and Stewart Island. Very dry conditions are now located in southeast 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 12 December, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in parts of Wellington, Tasman, Nelson, Marlborough, North Canterbury, Banks Peninsula, South Canterbury, Otago, coastal Southland, and Stewart Island. Very dry conditions are now located in southeast Otago. 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:

• This weekend (16-17 December) will be mostly dry, although some showers will affect Wellington and the western North Island.
• Monday and Tuesday will have a better chance for showers and isolated thunderstorms, particularly in the central North Island.
• High pressure will then bring generally dry weather late next week. 
• Weekly rainfall totals will again be meagre in many regions, especially in the upper North Island and east coast where amounts may be less than 10 mm. Pockets of 10-20 mm are possible from Taranaki to Wellington.
• Due to the expected rainfall in the next week, moderate soil moisture decreases are again likely in the upper and eastern North Island, with minor decreases possible in western regions.
• The current hotspot in coastal Manawatū-Whanganui and the Kapiti Coast may not change substantially in the next week, while a new hotspot may form in parts of Auckland.

South Island:

• The upper South Island will see some rain on Saturday, with additional heavy rain for much of the West Coast on Sunday (17 December).
• High pressure will then provide generally dry weather from Monday to Wednesday (18-20 December).
• Another front may bring moderate rain to the western and lower South Island late next week.
• Weekly rainfall totals of 70-100 mm are possible in the West Coast, but much lighter amounts of 20 mm or less are expected east of the Alps.
• Due to the expected rainfall in the next week, soil moisture decreases will be likely east of the Alps, with smaller changes likely in the upper South Island.
• The current hotspots in Marlborough Sounds and Nelson may strengthen slightly in the next week, while new hotspots may form in parts of the lower South Island.  

Long-term outlook (through mid-January):

• The drier (25^th percentile) and middle (50^th percentile) rainfall scenarios both favour widespread drier or much drier than normal conditions across both islands through mid-January.
• Even in the wetter (75^th percentile) scenario, below normal rainfall is still favoured in the upper and eastern North Island, although western New Zealand shows a chance for above normal rainfall in the wetter scenario.
• Very dry soil conditions could affect large swaths of both islands in the drier rainfall 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