EVALUATION OF THE UKMO MODELS RAINFALL FORECASTS FOR OCTOBER 1991 OVER KWAZULU-NATAL, SOUTH-AFRICA

Daily and six-hourly rainfall forecasts provided by the 20-level globa l gridpoint prediction model of the UK Meteorological Office have been evaluated against hourly automatic weather station rainfall data for KwaZulu-Natal. Assessments tt ere made of the results of time-lagged a verage forecasts, which were arithmetical averages of successive numer ical prognoses of weather elements for a specific period of time. The model fitted best in the medium range (3-5 days) and least over one to two days. Monthly rainfall predictions were overestimated by the medi um-range forecasts and underestimated by the one-day forecasts, The po or model forecasts in the short term may be caused by inadequate accou nting for the moisture field during the initialization phase of the mo del. In general there was no significant difference in forecast abilit y between the 00:00 and 12:00 model run. However in diurnal mode very- short-range afternoon and evening forecasts from the 12:00 run were le ss successful than those from the 00:00 run for the same period. Time- lagged average forecasts show potential in forecasting.