Hotspot Watch 26 March 2021

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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, high pressure resulted in minimal rainfall nearly everywhere during the past week. Most locations received less than 5 mm, with many receiving no rainfall at all. Only coastal Hawke’s Bay and Gisborne received larger amounts (generally up to 15 mm), due to scattered showers. This resulted in moderate soil moisture decreases nearly everywhere in the North Island. The driest soils across the North Island, when compared to normal for this time of the year, are found in western Northland, western Waikato, East Cape, and Tararua District, while the wettest soils for this time of the year for the North Island are found near Cape Reinga.  

Hotspot conditions are currently found in southern Northland, most of Auckland, northern Waikato, East Cape, Napier south to Wairarapa, interior Manawatū-Whanganui, and the area around Wellington City. The New Zealand Drought Index (NZDI) map below shows that widespread dry to very dry conditions are in place across most of the central and eastern North Island. A small area of extremely dry conditions is also located in coastal Gisborne.

In the South Island, high pressure resulted in minimal rainfall during the past week. Nearly all locations received between 0-5 mm. This resulted in small to moderate soil moisture decreases across all of the South Island. The driest soils in the South Island compared to normal for this time of year are located in coastal Hurunui District and southeastern Southland, while the wettest soils for this time of the year for the South Island are found in lower Fiordland.

Hotspots are currently in place near Nelson, across much of eastern Marlborough, northern Canterbury, Banks Peninsula, central Canterbury, and much of Clutha District. The New Zealand Drought Index (NZDI) map below shows that widespread dry to very dry conditions are in place across the northeastern South Island as well as parts of Otago and Southland. Extremely dry conditions are also found in coastal northern Canterbury.

Outlook and soil moisture

High pressure will continue to control the North Island’s weather through the upcoming weekend. However, a front will arrive on Monday (29 March), bringing showers and areas of steady rain to many locations. Locally heavy rain will be possible from Tuesday through Thursday along with scattered thunderstorms. A few more showers may occur on Friday (2 April). Weekly rainfall totals could be quite variable depending on where showers and thunderstorms occur, but amounts of 40 mm or more will generally be possible in the upper and central North Island, with 25 mm possible in southern areas. The east coast may miss out on the heaviest rainfall, with amounts there possibly less than 20 mm.

The more substantial rainfall expected in the next week should result in at least small soil moisture increases for large swaths of the North Island, with some locations perhaps even seeing moderate increases. However, little improvement may be observed along the east coast. The current hotspots in the upper and western North Island are likely to weaken during the next week, while those in the east may not change significantly.

In the South Island, areas of heavy rain will affect the West Coast this weekend (27-28 March), with lighter showers reaching the east coast. After a few showers on Monday, low pressure may deliver more substantial rain on Tuesday and Wednesday (30-31 March). An approaching front may bring additional showers on Friday. Weekly rainfall totals will be significant along the West Coast, where amounts may exceed 100 mm in many areas. Amounts in the 30-50 mm range may also be observed in Nelson, Tasman, and the lower South Island. Lighter amounts of 25 mm or less may occur in eastern Marlborough and Canterbury.

Due to the expected weekly rainfall, many locations around the South Island will likely see soil moisture increases during the next week. However, increases may only be small along the east coast. Current hotspots in the lower South Island may weaken during the next week, while those in eastern Canterbury and Marlborough may not change substantially.

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.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, high pressure resulted in minimal rainfall nearly everywhere during the past week. Most locations received less than 5 mm, with many receiving no rainfall at all. Only coastal Hawke’s Bay and Gisborne received larger amounts (generally up to 15 mm), due to scattered showers. This resulted in moderate soil moisture decreases nearly everywhere in the North Island. The driest soils across the North Island, when compared to normal for this time of the year, are found in western Northland, western Waikato, East Cape, and Tararua District, while the wettest soils for this time of the year for the North Island are found near Cape Reinga.  

Hotspot conditions are currently found in southern Northland, most of Auckland, northern Waikato, East Cape, Napier south to Wairarapa, interior Manawatū-Whanganui, and the area around Wellington City. The New Zealand Drought Index (NZDI) map below shows that widespread dry to very dry conditions are in place across most of the central and eastern North Island. A small area of extremely dry conditions is also located in coastal Gisborne.

In the South Island, high pressure resulted in minimal rainfall during the past week. Nearly all locations received between 0-5 mm. This resulted in small to moderate soil moisture decreases across all of the South Island. The driest soils in the South Island compared to normal for this time of year are located in coastal Hurunui District and southeastern Southland, while the wettest soils for this time of the year for the South Island are found in lower Fiordland.

Hotspots are currently in place near Nelson, across much of eastern Marlborough, northern Canterbury, Banks Peninsula, central Canterbury, and much of Clutha District. The New Zealand Drought Index (NZDI) map below shows that widespread dry to very dry conditions are in place across the northeastern South Island as well as parts of Otago and Southland. Extremely dry conditions are also found in coastal northern Canterbury.

Outlook and soil moisture

High pressure will continue to control the North Island’s weather through the upcoming weekend. However, a front will arrive on Monday (29 March), bringing showers and areas of steady rain to many locations. Locally heavy rain will be possible from Tuesday through Thursday along with scattered thunderstorms. A few more showers may occur on Friday (2 April). Weekly rainfall totals could be quite variable depending on where showers and thunderstorms occur, but amounts of 40 mm or more will generally be possible in the upper and central North Island, with 25 mm possible in southern areas. The east coast may miss out on the heaviest rainfall, with amounts there possibly less than 20 mm.

The more substantial rainfall expected in the next week should result in at least small soil moisture increases for large swaths of the North Island, with some locations perhaps even seeing moderate increases. However, little improvement may be observed along the east coast. The current hotspots in the upper and western North Island are likely to weaken during the next week, while those in the east may not change significantly.

In the South Island, areas of heavy rain will affect the West Coast this weekend (27-28 March), with lighter showers reaching the east coast. After a few showers on Monday, low pressure may deliver more substantial rain on Tuesday and Wednesday (30-31 March). An approaching front may bring additional showers on Friday. Weekly rainfall totals will be significant along the West Coast, where amounts may exceed 100 mm in many areas. Amounts in the 30-50 mm range may also be observed in Nelson, Tasman, and the lower South Island. Lighter amounts of 25 mm or less may occur in eastern Marlborough and Canterbury.

Due to the expected weekly rainfall, many locations around the South Island will likely see soil moisture increases during the next week. However, increases may only be small along the east coast. Current hotspots in the lower South Island may weaken during the next week, while those in eastern Canterbury and Marlborough may not change substantially.

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)

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.  

Soil moisture anomaly map 9am 25 March 2021 [NIWA]

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

As of 24 March, the New Zealand Drought Index (NZDI) map below shows that widespread dry to very dry conditions are in place across most of the central and eastern North Island, northeastern South Island, as well as parts of Otago and Southland. Small areas of extremely dry conditions are also located in coastal Gisborne and northern Canterbury. However, 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.

Drought Index map 26 March 2021 [NIWA]

 

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