Forecasted Fire Danger: January 1 2000

National Climate Centre - NIWA

Issued 10 October 1999

Map of likely fire risk for the late December 1999/early January 2000 period based on projected seasonal climate forecasts. This is a scenario of a plausible, but not all possible outcomes, based on seasonal climate trend assumptions. This map will be updated on 10 November 1999.

NIWA October 1999 Outlook

The weight of evidence for October indicates stronger westerlies in the south Tasman Sea, and to the south of New Zealand, extending at times onto the South Island. Anticyclonic anomalies are expected over much of the Tasman Sea, extending across the North Island to the east. The impacts of La Niña are expected to strengthen in October. On average during October in La Niña episodes more lows occurs in the south Tasman Sea with more westerlies and north westerlies over the country.

Although the anticyclonic anomalies of September are likely to continue into October, the latitude of these are expected to shift north with strongest anomalies lying between 35 and 40° S. Guidance suggests stronger westerlies and north westerlies at times over the South Island. This is consistent with recent anticyclone tracks across the north of the country, and more westerlies over the south.

Above average temperatures are expected to continue in all districts, especially in the region from central Marlborough to North Otago. However, nearer to normal temperatures are likely in the west of the South Island.

The anticyclonic westerly pattern is expected to produce above average precipitation in the west of the South Island. Below average precipitation is likely in the Bay of Plenty and east of the North Island, and the east of the South Island including Otago. In other regions of New Zealand guidance is less clear, but near or below average precipitation appears likely.

October Fire Weather Conditions

1. Below average rainfall. Below average rainfall is expected in the east of both islands during October, from Bay of Plenty to Otago. Rainfall is most likely to be below average in Gisborne, Hawke’s Bay, central Marlborough, South Canterbury and eastern areas of Otago as far south as Dunedin. Other areas of lower than normal rainfall include eastern parts of Nelson, and South Otago, and the south west of the North Island.

2. High temperatures. Warm north westerlies are expected at times over the South Island, and could produce high temperatures on several days in the east of the South Island from central Marlborough to North Otago. These warm north westerlies could also produce high temperatures from Gisborne to Wairarapa on some days during the month. Northern districts of the North Island (Northland, Auckland, Waikato and Bay of Plenty) could expect above average temperatures.

3. Low humidity. Foehn westerly and north westerly winds bring with them low humidity. This factor then is expected to be most accentuated when these winds are blowing, especially in the east of the South Island from central Marlborough to central and eastern Otago. Lower humidity could also occur at times in during these conditions in the east of the North Island. Under the more anticyclonic conditions expected over the North Island, inland areas could also experience days with lower humidity later in October.

4. High winds. Higher wind speeds are expected at times, mainly in inland and eastern areas of the South Island, in particular from inland South Canterbury to Southland. There may be stronger winds in other parts of Canterbury, and central Marlborough, but less frequent. Over the North Island the chances of stronger winds are lower, although the odd north westerly event may affect the southern North Island, including Wellington and Wairarapa.

The FWI is expected to increase particularly in inland areas of Otago and parts of South Canterbury to become very high. High fire risk is likely to develop in other areas between central Marlborough and eastern Otago, and spread in the east of the North Island in Gisborne and Hawke’s Bay. In the Bay of Plenty, Wairarapa, drier parts of Wanganui and Manawatu and South Otago, the FWI in the medium risk range could develop. A low FWI is expected in other parts of New Zealand during October.

NIWA October – December 1999 Outlook

The influence of La Niña is expected to strengthen during the remainder of 1999. Positive sea temperature anomalies near New Zealand are likely to persist for much of the period. The persistent anticyclonic circulation anomalies seen at New Zealand longitudes for much of the last year also appear likely to continue through the rest of 1999. As is typical of past La Niña situations, some strengthening of the westerlies is likely in the first half of the period, with a slackening again later. Above-normal temperatures are likely in all districts. Average to above average rainfall is expected over much of the North Island, especially in the north and east. Average to below average rainfall is expected over the South Island. The driest regions are expected to be inland South Canterbury and Otago.

Late December 1999/early January 2000 Fire Weather Conditions

The assessments below are an indicative scenario of the relative intensity of the contributing factors on the basis of the expected climate trends over the October – December 1999 period. Climate prediction is an emerging science and has naturally high uncertainties. These assessments will be revised in early November and early December as the evolution of La Niña with the progression of spring becomes clearer. These projections will be revised in early November.