Climate and weather update

Agriculture

Climate and weather update

Published: 20 October 2016

General

Very little change in terms of climate events and rainfall outlooks for the season took place in the past month.     

La Niña

The current status of sea surface temperatures in the Equatorial Pacific and expected development, still favours neutral to weak La Niña conditions. La Niña is the positive phase of the El Niño Southern Oscillation and is associated with cooler than average sea surface temperatures in the central and eastern tropical Pacific Ocean. There are however some indications of a stronger La Niña development. This scenario is expected to last until the autumn of 2017 with a close to zero probability for redevelopment of El Niño conditions.

Southern Oscillation Index (SOI)

The SOI is the atmospheric component of the Niño -areas. Current SOI values are biased towards more La Niña type of atmospheric conditions with the SOI for the past two months in positive territory (>5). This is very significant for this time of the season to present positive prospects for summer rain. 

Indian Ocean

The Indian Ocean Dipole Index (IOD) is now for more than four consecutive months in a negative phase. This trend seems to ease somewhat but will most probably remains negative to neutral for most of the summer. This is a very positive signal for improved rainfall conditions for Southern Africa.

Expected rainfall conditions for the next months

The combined effect of neutral to weak La Niña conditions and the negative IOD, is a strong signal for favourable rainfall conditions in mid- to late summer.

  • Eastern summer crop production areas (KZN, Gauteng, Mpumalanga and Eastern Free State): In contrast to previous forecasts, very little rainfall was received before 15 October. The current indications indicates that plant rain is expected before 31 October (later than initial indications) that will enable farmers to plant within the optimum planting window of between 1 October and 15 November but there is some concern about timing. Conditions also look favourable for the rest of the season. If 20-50mm does not occur before the end of October, it can upset the planting dates for good yields.
  • Central, northern and western summer crop production areas (Central to western Free State, Limpopo and Northwest Province): Rainfall conditions seem to improve in the second part of October, becoming more favourable in November and December. It is expected that farmers will be able to plant within the acceptable planting window before 15-31 December. Rainfall conditions for the February to April 2017 period also seem favourable. 
  • Western Cape: Some rain is still expected over the Southern Cape in the second part of October that can cause problems with the harvesting process and quality of winter crops.
  • Irrigation and water supply: Conditions still remains critical. Replenishment of surface and subsurface water sources usually do not occur with the first rain event, especially with the current poor status of reservoir levels and drought. Sufficient runoff and rain for improvement of water sources is expected at the earliest only in December.

Risk factors for crop production

  • Wind: It is expected that very windy and stormy weather conditions will prevail in the next months. It may pose a risk during the emerging phase of early plantings.
  • Hail: There is a high correlation between rainfall in the first part of the summer and occurrence of hail. If the expected favourable conditions for rain materialise, it will most probably increase the risk for hail.
  • Heat wave conditions: In contrast to the previous season is it not anticipated that prolonged periods of high temperatures will prevail in the mid- to late summer. Hot and windy conditions may however occur in the period until October/November.

Summary

  • Forecasts for neutral to La Niña conditions still remain valid with borderline La Niña conditions currently present.    
  • The expected rainfall in the first part of October over the eastern summer crop production areas did not occur but conditions are still favourable for rain in the second part of October. There is however now some concern because of time running out but rain in the last part of October will still be in time to enable planting of crops before the middle of November (the last planting date in many areas).
  • Cumulative rainfall forecasts until the end of December 2016 from the accuweather.com website indicate that between 300-400mm of rain is possible over the eastern areas (Mpumalanga, Eastern Free State, and KZN) while between 150mm and 250mm is possible over the central, south eastern and northern parts (Free State, Northwest, Eastern Cape, Limpopo and eastern parts of the Northern Cape). Between 30-50mm is expected for the western parts of the Northern Cape.