Hotspot Watch 23 November 2023

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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:

  • Substantial rainfall of 40-80 mm was widespread across the central and eastern North Island in the past week, along with Taranaki. Isolated amounts above 100 mm were also observed in these areas.
  • However, much less rainfall occurred in much of Northland, Auckland, Manawatū-Whanganui, and Wellington, where generally less than 20 mm fell.   
  • This resulted in moderate soil moisture increases across large portions of the North Island.
  • The driest soils across the North Island, when compared to normal for this time of the year, are found in Kapiti Coast, while the wettest soils for this time of the year are found along the east coast and in western Bay of Plenty.
  • No hotspots currently exist in the North Island.
  • As of 20 November, the New Zealand Drought Index (NZDI) map below shows that no unusually dry conditions are currently found across the North Island.

South Island:

  • Rainfall amounts of 75-100 mm were observed in Fiordland in the past week, with 25-40 mm in the rest of the West Coast.
  • Amounts of 15-30 mm occurred from southern Canterbury to Otago, with only 5 mm or less in much of Marlborough, northern and central Canterbury, and lower Southland.
  • This resulted in moderate soil moisture decreases across a majority of the South Island.   
  • The driest soils across the South Island, when compared to normal for this time of the year, are found in much of coastal Southland, while the wettest soils for this time of the year are found in northern Canterbury and Tasman.
  • In the past week a new hotspot formed in parts of coastal Banks Peninsula and Selwyn District.
  • As of 20 November, the New Zealand Drought Index (NZDI) map below shows that no unusually dry conditions are currently found across the South Island.

Soil moisture anomaly (mm) at 9am 22/11/2023 [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 20 November, the New Zealand Drought Index (NZDI) map below shows that no unusually dry conditions are currently found across the country. 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.

NZ drought index 20/11/2023 [NIWA]

The week ahead:

North Island:

  • Rain will move across the North Island on Friday (24 November), with areas of heavy rain continuing in Gisborne and parts of Hawke’s Bay on Saturday and Sunday (25-26 November).
  • Early next week will be mostly dry, but scattered showers and thunderstorms may arrive on Wednesday (29 November).
  • A more significant rain event may impact the North Island late next week.
  • Weekly rainfall totals greater than 100 mm may occur in Gisborne and parts of Hawke’s Bay, with 25-50 mm possible in the rest of the east coast and the Central Plateau.
  • However, much less rainfall is forecast from Northland to northern Waikato, where less than 15 mm is expected.
  • Due to the expected rainfall in the next week, parts of the east coast and central North Island will likely see at least small soil moisture increases, but small decreases are favoured in the upper North Island.
  • No hotspots are expected to form in the North Island in the next week.

South Island:

  • Scattered showers will affect much of the east coast on Friday (24 November), with dry conditions along the West Coast.
  • Dry weather is expected during the weekend, followed by isolated showers on Monday (27 November).
  • Showers or rain will again be possible on Tuesday, followed by another day or two of dry weather.
  • Weekly rainfall totals of 30-70 mm are generally expected in the West Coast, with 20-40 mm possible in northern Canterbury, Otago, and Southland.
  • Elsewhere, less than 20 mm of total rainfall are likely.
  • Due to the expected rainfall in the next week, soil moisture levels may decrease slightly in parts of the upper and eastern South Island.
  • The current hotspot along the central Canterbury coast may strengthen slightly in the next week, while Marlborough Sounds may begin to approach hotspot criteria.

Long-term outlook (through late December):

  • The drier (25th percentile) and middle (50th percentile) rainfall scenarios both favour drier or much drier than normal conditions in the upper North Island and lower South Island.
  • The wetness in the eastern North Island is mostly tied to the expected heavy rainfall in the next few days.
  • Even in the wetter (75th percentile) scenario, below normal rainfall is still favoured in parts of the lower South Island.
  • Unusual dryness and drought are not expected in the next 35 days.  

Rain anomaly next 35 days from 20/11/2023 [NIWA]

Risk of areas experiencing dryness or drought in the next 35 days from 20/11/2023. [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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