Forecasting of Extreme Stationary Convection - 18/9/2007 Zelezniki Flash Flood

Forecasting of extreme convection in complex terrain is still a challenge for forecasters and numerical models and the predictability in some region could be low. Even with 1 km resolution some steep and narrow valleys are not well represented. Even if model precipitation rates are well forecast, the precipitation maxima could be shifted for several tens of km. Sometimes the catchment response to precipitation is fast and flash flood occurs. Therefore inaccurate forecast of extreme precipitation rates in time and space and eventual flash floods could cause damage and even deaths.

On 18/9/2007 6 UTC the prefrontal south-western moist winds caused queasy stationary convection over the north-western parts of Slovenia which lasted over 12 hours. The maximum measured 24 h accumulation was 303 mm at Vogel Mountain, while the most affected area was along Sora River upstream the town of Zelezniki, where precipitation rates reached up to 70 mm/h and 100 mm in 2 h. Because of the wet surface in the Sora River catchment, it flooded in just 30 minutes and river swept away 70 cars and buses and caused 3 deaths.

The operational LAM model ALADIN at Environmental Agency of Republic Slovenia with 9 km resolution underestimated the precipitation with rates 50 mm/6 h in the region. Also the operational radar underestimated the rainfall accumulation for 50 %. The locations of current automatic weather stations were not well positioned to represent the actual amount of precipitation, also the time frequency of data was too low. Therefore it was not easy to determine the full extent of ongoing event in scope of the operational forecast shift.

In this article the predictability of this extreme event in scope of current forecast operational system is presented. Some parameters which could indicate the event in medium range as ECMWF ensemble forecast are discussed, also all available precipitation information from various LAM and LAM ensembles are compared. To diagnose the potential instability of the atmosphere Udine radiosounding is studied.

To improve the forecasting of such events, the high density network of automatic reporting stations, especially upstream the river, with 10 minutes updating is planned. Also a new radar will be installed. This data will be used in the INCA analysis and nowcasting system, an example will be shown in the paper.