Flash flood forecasting: What are the limits of predictability?

Abstract

Flash floods may occur suddenly and be accompanied by other hazards such as landslides, mud flows, damage to infrastructure and even death. In the UK such events are comparatively rare occurring on average only once or twice per year. Warning systems must depend upon the accurate real-time provision of rainfall information, high-resolution numerical weather forecasts and the operation of hydrological model systems in addition to forecast delivery procedures not discussed in this paper. In this paper we review how flash floods are forecast considering the limitations and uncertainty involved in both the meteorological and hydrological aspects of forecasting systems. Data assimilation and the use of ensembles are both key elements across disciplines. Assessing the susceptibility of river catchments to extreme flooding is considered, and statistical methods of estimating the likelihood of extreme rainfall and floods within a changing climate are examined. Ways of constraining flash flood forecasts are noted as one way to improve forecast performance in the future.

NORA–Nowcasting of Orographic Rainfall by means of Analogues

“…In the smallest steep Alpine catchments, heavy orographic precipitation combined with rapid runoff can lead to flash floods, which are characterized by rapid occurrence resulting in a very limited opportunity for warnings to be prepared and issued (Collier, 2007). Catchments of up to 400 km 2 that are capable of generating flood hydrographs with times to peak of up to 6 h are regarded as potential flash-flood catchments (e.g.…”

“…Catchments of up to 400 km 2 that are capable of generating flood hydrographs with times to peak of up to 6 h are regarded as potential flash-flood catchments (e.g. Georgakakos, 1986;Collier, 2007). For example, in the Verzasca Valley, a small and steep catchment (186 km 2 ) located in the southern Alps (Figure 1), the time between a precipitation impulse and the main runoff response at the outlet is of the order of 1-2 h .…”

Saving Lives: Timely Flash Flood Warnings in the UK

“…Sene [2] provides an overview of the methods used to warn people of serious flooding. The requirements for warnings include rainfall and radar data [3], soil moisture deficit data [4] [5], and for some probabilistic forecast methods [6] [7] [8] suggest that improvements in data collection, assimilation and modelling are needed, as are the human responses to flood warnings. Real time flood warning systems are not yet the norm in the UK.…”

A space-time geostatistical approach for ensemble rainfall nowcasting

“…An example of short-term prediction ensemble method is the STEPS (Short Term Prediction Systems) approach ( [7]; [8]). Based on a combination of extrapolation, perturbation and cascading of nowcasting weather predictions, it allows the forecast of an event with a lead time of up to 6 hours ahead.…”