Take note of seasonal weather forecasts

by Henk Cerfonteyn

Weather forecasts, especially for short-term periods, have certainly become part of our daily routine, be it for work, recreation, or just general information. Seasonal forecasts on the other hand are seemingly not that familiar, probably due to the fact that some uncertainty exists on the accuracy and therefore the relevancy of these types of forecasts. While seasonal forecasts take on a relatively longer time span, normally 90 days, these forecasts can actually be applied when planning for longer periods. While the benefits of these forecasts are likely to be underestimated, this article approaches the matter by presenting some useful, however, elementary comments on the application of seasonal forecasts.

Apart from issuing weekly forecasts, the South African Weather Service (SAWS) releases monthly updates on seasonal climatic conditions, under the caption Seasonal Climate Watch (SCW) (www.weathersa.co.za).

The content of seasonal forecasts will most probably vary between organisations, though certain issues remain reasonably similar. Seasonal forecasts tend to refer to the El Niño–Southern Oscillation (ENSO), which evidently plays a large role in global forecasts. The current SCW warns that the emerging El Niño conditions during spring could result in below normal rains for the summer rainfall areas of the country.

Seasonal forecasts normally take on the form of maps depicting forecasts for the different periods in terms of rainfall and temperatures. The SCW refers to three periods (September-October-November, October-

November-December, and NovemberDecember-January), whereby each of these periods is expressed in terms of rain, maximum and minimum temperatures.

It stands to reason that the closest period to the release date will be the most accurate of the three periods. Since seasonal forecasts are released monthly, the subsequent periods move to earlier positions as the months progress, which makes it possible to compare to what extent a particular forecast (period) has changed from a previous one.

Seasonal forecasts can guide farmers in terms of emerging climatic outcomes. While short-term forecasts show what the week has in store in terms of specific measurements (e.g. rain in mm), seasonal forecasts present a longer view of climatic conditions expressed as above or below normal, (i.e. compared to long-term averages). Short-term forecasts can change, therefore to stay updated it is necessary to review short-term forecasts on a routinely, preferably daily, basis. Similarly, seasonal forecasts, which normally take a 90-day period into account, should at the least be reviewed monthly.

The Department of Agriculture, Forestry and Fisheries (DAFF) issues the National Agro-meteorological Committee (NAC) Advisory (Agri-Outlook at www.elsenburg. com) monthly. These advisories contain a brief version of the current SAWS SCW forecast with strategies to mitigate possible agricultural risks. Seasonal forecasting has certainly become part and parcel of agriculture; farmers are therefore encouraged to adopt these forecasts on a more routinely basis.

AP

April-May-June (AMJ) 2019 seasonal precipitation prediction without skill taken into account (left), as well as skill masked out (middle). Also included is the climatological average for AMJ (right, in mm) calculated over the period 1979-2009.

Source: http://www.weathersa.co.za/ media/data/longrange/gfcsa/scw.pdf