Hotspot Watch 19 November 2020

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

Meagre rainfall amounts of 10 mm or less were observed across most of the upper North Island during the past week, and typically less than 25 mm in the lower half of the island. However, pockets of 30-50 mm occurred in Taranaki and South Wairarapa. This rainfall distribution resulted in small to moderate soil moisture decreases across nearly all of the North Island, although a moderate increase was observed in South Wairarapa. The driest soils across the North Island, when compared to normal for this time of the year, are found in northern Waikato and western Northland. Meanwhile, the wettest soils for this time of the year are located in Hawke’s Bay.
No hotspots are currently found in the North Island. In the South Island, rainfall amounts of 30-70 mm were generally found along the West Coast, with pockets up to 100 mm. However, east of the Southern Alps, weekly rainfall amounts were generally less than 25 mm, with some locations receiving less than 10 mm. Small to moderate soil moisture decreases were observed nearly everywhere in the South Island, with the exception of Fiordland, where minor increases were observed. The driest soils in the South Island compared to normal for this time of the year are now located in coastal Waitaki District, while the wettest soils for this time of the year are found in Nelson and interior Tasman and Marlborough.
A hotspot is currently found in Waimate and coastal Waitaki District.

Outlook and Soil Moisture

High pressure is expected to bring dry weather to the North Island on Friday and Saturday, before a weak front delivers light rainfall amounts to most locations on Sunday (22 November). Scattered afternoon showers and isolated thunderstorms will be possible on Monday, before dry weather returns on Tuesday. However, indications are that a significant low may arrive in the middle of next week, producing widespread moderate to heavy rainfall amounts across most of the North Island. Weekly rainfall totals may reach 25-50 mm across most of the North Island, with larger amounts in isolated areas.

Due to the expected rainfall over the next week, much of the North Island will likely see at least small soil moisture increases, with some locations possibly seeing moderate increases. No new hotspots are expected to form in the next week.

Showers will affect Southland on Friday before a more potent low brings rain to the West Coast and lower South Island on Saturday. After a mostly dry Sunday, afternoon showers and isolated thunderstorms may bring localised moderate rainfall on Monday (23 November). During midweek, low pressure may deliver moderate rainfall to at least the upper half of the South Island. Weekly rainfall totals may reach 25-40 mm across much of the upper South Island, but 25 mm or less is likely from southern Canterbury to Southland.
Significant soil moisture changes are not expected during the next week, although small increases may occur in the upper South Island, with at least minor decreases expected in the lower South Island. This may result in a strengthening of the current hotspot located in Waimate and Waitaki districts.

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.

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

New Zealand Drought Index (NZDI)

As of 16 November, the New Zealand Drought Index (NZDI) map below shows that dry or very dry soils are located in several primarily coastal areas, but meteorological drought is not currently found in New Zealand. 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.