Hotspot Watch 15 March 2024

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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.

Recent rainfall and current soil moisture conditions:

North Island:

  • Meagre rainfall was widespread across the entire North Island in the past week, with most locations receiving 5 mm or less. Isolated pockets up to 15 mm were observed in the Coromandel Peninsula and parts of the Central Plateau.
  • This resulted in moderate soil moisture decreases across the entire North Island.   
  • The driest soils across the North Island, when compared to normal for this time of the year, are found in eastern Northland, while the wettest soils for this time of the year are found in western and southern Waikato.
  • Hotspots are currently located across the Far North and eastern Northland, far northern Auckland, coastal Gisborne, southern Hawke’s Bay to Wairarapa, and parts of western Wellington.
  • As of 13 March, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in Northland, Auckland, part of the Coromandel Peninsula, eastern Bay of Plenty, and much of the lower North Island. Very dry to extremely dry conditions are located in much of Northland and Wellington.

South Island:

  • Much like the North Island, a majority of the South Island saw only meagre rainfall in the past week, with 5 mm or less observed in most of the upper and eastern South Island.
  • Pockets of the upper West Coast, central Canterbury, and Southland received up to 20 mm.
  • Weekly rainfall totals in the lower West Coast and Fiordland were much higher, however, with generally 50-100 mm observed.  
  • This resulted in small to moderate soil moisture decreases across most of the upper and eastern South Island, with little change observed elsewhere.    
  • The driest soils across the South Island, when compared to normal for this time of the year, are found across parts of Tasman, Nelson, Marlborough Sounds, and Canterbury, while the wettest soils for this time of the year are found in western Southland.  
  • Hotspots are currently located across much of the upper and eastern South Island, including eastern Tasman, Nelson, parts of Marlborough, nearly all of Canterbury, and eastern Otago.
  • As of 13 March, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in eastern Tasman, Nelson, Marlborough, northern and southern Canterbury, and eastern Otago. Very dry to extremely dry conditions are located in Nelson, Marlborough, parts of northern and far southern Canterbury, and northern Otago.

Soil moisture anomaly map (mm) at 9am on 7 March 2024. [NIWA]

Soil moisture anomaly map (mm) at 9am on 14 March 2024. [NIWA]

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 13 March, the New Zealand Drought Index (NZDI) map below shows that abnormally dry conditions are currently found in Northland, Auckland, part of the Coromandel Peninsula, eastern Bay of Plenty, much of the lower North Island, eastern Tasman, Nelson, Marlborough, northern and southern Canterbury, and eastern Otago. Very dry to extremely dry conditions are located in much of Northland, Wellington, Nelson, Marlborough, parts of northern and far southern Canterbury, and northern 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.

New Zealand Drought Index (NZDI) - 13 March 2024 [NIWA]

The week ahead:

North Island:

  • The bulk of the week’s rainfall will likely occur today and tonight (15 March) as a front moves across the North Island.
  • From Saturday (16 March), high pressure moving into the region will result in mostly dry weather this weekend and through much of next week.
  • However, the eastern North Island may see scattered showers on Monday and Tuesday (18-19 March).
  • Weekly rainfall totals of 15 mm or less again look to be widespread across the North Island in the next week, with the potential that parts of the upper North Island and much of the lower North Island will see less than 10 mm.
  • However, parts of the central North Island could receive larger totals of 20-30 mm.
  • Due to the expected rainfall in the next week, additional soil moisture decreases are likely in the upper and lower North Island, while central areas may see little change overall.
  • Current hotspots across the upper and lower North Island will have a good chance to strengthen and expand in the next week, although those in Gisborne may not change significantly.

South Island:

  • Areas of light to moderate rain are moving north across the island today (15 March), but high pressure will begin moving in on Saturday (16 March).
  • A few showers will affect the eastern and lower South Island on-and-off from Saturday to Tuesday (16-19 March), but other regions will remain mostly dry.  
  • A better chance for some rain may reach the lower West Coast by late next week.
  • Weekly rainfall totals of 25-50 mm will be possible along the West Coast, with pockets of 15-25 mm in the eastern and lower South Island.
  • However, the upper South Island again looks to see meagre rainfall totals that will likely be less than 10 mm.   
  • Due to the expected rainfall in the next week, moderate soil moisture decreases are likely in the upper South Island, but rest of the island may only see small decreases.
  • The current hotspots in the upper South Island will likely strengthen and expand in the next week, with those in Canterbury and Otago likely seeing little change overall.

Key Messages for the Top of the South Island:

  • From October 2023 through April 2024, Blenheim recorded just 88 mm of rain – the lowest rainfall over that period since at least 1941.
  • Many parts of the region experienced six or seven consecutive months with below normal rainfall, depending on the location.
  • This severe rainfall deficit culminated in a precipitation drought for parts of Marlborough, Nelson, and Tasman.
  • Read the government’s declaration of a medium-scale adverse event here.

Long-term outlook (through mid-April):

  • The drier (25th percentile) and middle (50th percentile) rainfall scenarios both show drier or much drier than normal conditions across much of the country, leading to higher confidence for that outcome.
  • Even in the wetter (75th percentile) scenario, below normal rainfall is still forecast in the western North Island, but near normal rainfall is most favoured across the country in that scenario.
  • Very dry soil conditions could affect the lower North Island and eastern South Island in all three rainfall scenarios, along with parts of the eastern North Island and more of the upper South Island in the drier scenario.

Rainfall anomaly next 35 days from 13 March 2024 [NIWA]

Risk of areas experiencing dryness or drought within 35 days from 13 March 2024 [NIWA]

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.

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