Abstract:
The development of the Severe Weather Impact Forecasting System (SWIFS) for flash flood
hazards in South Africa is described in this thesis. Impact forecasting addresses the need to
move from forecasting weather conditions to forecasting the consequential impact of these
conditions on people and their livelihoods. SWIFS aims to guide disaster managers to take early
action to minimise the adverse effects of flash floods focussing on hotspots where the largest
impact is expected. The first component of SWIFS produced an 18-hour probabilistic outlook of
potential occurrence of flash floods. This required the development of an ensemble forecast
system of rainfall for small river basins (the forecasting model component), based on the
rainfall forecast of a deterministic numerical weather prediction model, to provide an 18-hour
lead-time, taking into account forecast uncertainty. The second component of SWIFS covered
the event specific societal and structural impacts of these potential flash floods, based on the
interaction of the potential occurrence of flash floods with the generalised vulnerability to flash
floods of the affected region (the impact model component). The impact model required an
investigation into the concepts of regional vulnerability to flash floods, and the development of
relevant descriptive and mathematical definitions in the context of impact forecasting. The
definition developed in the study links impact forecasting to the likelihood and magnitude of
adverse impacts to communities under threat, based on their vulnerability and due to an
imminent severe weather hazard. Case studies provided evidence that the concept of SWIFS
can produce useful information to disaster managers to identify areas most likely to be
adversely affected in advance of a hazardous event and to decide on appropriate distribution of
their resources between the various hotspots where the largest impacts would be. SWIFS
contributes to the current international research on short-term impact forecasting by focussing
on forecasting the impacts of flash floods in a developing country with its limited spatial
vulnerability information. It provides user-oriented information in support of disaster manager
decision-making through additional lead-time of the potential of flash floods, and the likely
impact of the flooding. The study provides a firm basis for future enhancement of SWIFS to
other severe weather hazards in South Africa.
