Hotspot Watch 8 December 2022

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.

Facts: Soil Moisture

In the North Island, most areas received minimal rainfall of less than 10 mm in the past week. However, much of Northland, northern Auckland, the Coromandel Peninsula, Gisborne, and northern Hawke’s Bay saw amounts of 20 mm or more, with pockets of 30-50 mm observed in these regions. This resulted in small to moderate soil moisture decreases, with the largest decreases observed in central and southern parts of the island. The driest soils across the North Island, when compared to normal for this time of the year, are found in coastal Wairarapa, while the wettest soils for this time of the year are found near Cape Reinga and parts of Auckland.

No hotspots are currently located in the North Island.

In the South Island, rainfall amounts of 30-70 mm were observed in Fiordland, with 15-30 mm observed in much of Canterbury, coastal Otago, parts of Southland, and the lower West Coast. However, minimal rainfall of 10 mm or less occurred in the top of the South Island, most of the West Coast, and interior Otago and Southland. This generally resulted in moderate soil moisture decreases across most of the South Island, although some increases were observed in southern Canterbury and southern Fiordland. The driest soils in the South Island, when compared to normal for this time of the year, are located in Marlborough Sounds, Nelson, the lower West Coast, and near Dunedin, while the wettest soils for this time of the year are found near Kaikōura.

No hotspots are currently located in the South Island. As of 6 December, the New Zealand Drought Index (NZDI) map below shows that dry conditions are located in a very small portion of north-central Canterbury.

Outlook and Soil Moisture

In the North Island, an area of rain and isolated thunderstorms will move through on Friday (9 December). During the weekend, a slow-moving front will bring waves of rain primarily to the western North Island, and this rain may be moderate to heavy at times. High pressure will bring mostly dry weather on Monday (12 December), with more scattered showers and isolated thunderstorms during the middle of next week. Weekly rainfall totals of 50-80 mm could be widespread in the central and western North Island, with the chance for isolated amounts of 100 mm in western regions. Meanwhile, amounts of 30-50 mm will be possible in the upper North Island, with lesser amounts of 15-30 mm along the east coast.

Due to the expected rainfall in the next week, many locations are likely to see small to moderate soil moisture increases, especially in western regions. However, small decreases may occur along the east coast. There is a small chance that a new hotspot could form in coastal Wairarapa during the next week.

In the South Island, moderate to heavy rain will impact the West Coast on Friday and Saturday (9-10 December), with some unsettled weather also impacting the top of the island. Meanwhile, Saturday and Sunday afternoons could also feature showers and thunderstorms in Otago and Southland. During early-to-mid next week, high pressure will result in mostly dry weather for the South Island. Weekly rainfall totals may reach 100-150 mm in parts of the West Coast, with 30-50 mm in Tasman and perhaps parts of Otago and Southland where the strongest showers and thunderstorms occur. Meanwhile, the east coast is likely to see minimal rainfall amounts of 15 mm or less.

Due to the expected rainfall in the next week, soil moisture levels will likely increase in much of the West Coast, with localised increases also possible in the lower South Island. However, moderate decreases may occur along the east coast. There is a small chance that a new hotspot could form in eastern Otago during the next week.

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