Monday 15 February 2021 is the release of a new version of EFAS (version number 4.1). It changes the visualisation of the ERIC reporting points layer on the EFAS website, and the criteria for issuing flash flood notifications. These changes lead to an improvement by reducing the overall number of flash flood notifications that are issued, whilst minimising the impact on the forecast skill.
ERIC Flash Flood reporting points are displayed on the website according to the following criteria:
- Size of the catchment is <= 2000 km2
- Surface runoff index exceeds the mean annual maximum surface runoff index in at least 4 ensemble members
Prior to the release of EFAS 4.1, the following additional criteria were applied to decide at which of these ERIC Flash Flood reporting points a flash flood notification should be issued:
- Catchment is part of the Conditions of Access (CoA)
- Exceedance probability of the surface runoff index value associated with the 5 year return period is >= 10%
- Lead time to start of the event <= 48 hours
If more than one ERIC Flash Flood reporting point satisfies the above additional criteria in a given administration region, then only the point with the greatest exceedance probability of the 5 year return period is selected. It is at this selected reporting point that a flash flood notification is issued. In addition, the selected point is displayed on the EFAS website as an enlarged inverted triangle, whilst the other ERIC Flash Flood reporting points are displayed in a smaller size.
However, these criteria have led to a large number of flash flood notifications being generated. The sheer number of notifications suggested a possible increase in false alarms. Therefore, the ERIC flash flood notification criteria were re-evaluated against flash flood observations.
Re-Evaluation of Flash Flood Notification Criteria
The re-evaluation of the ERIC flash flood forecasts was performed against observations for the period 1st October 2020 until 31st January 2021. Flash flood observations were obtained from FloodList.com, European Severe Weather Database (ESWD), the National Meteorological Service of Slovenia (ARSO) and the EFAS hydrological database.
A range of different criteria for issuing flash flood notifications were tested. For example, different exceedance probability thresholds of the 5 year return period were tested. Additional criteria were also tested using upstream area thresholds of 1000 km2 and 2000 km2.
The results showed that increasing the 5 year return period exceedance probability threshold to 30% greatly reduced the number of notifications whilst only having a slight reduction in the forecast skill (Figure 2).
More details about the re-evaluation methodology can be found in the EFAS version 4.1 wiki page.
The findings from the re-evaluation mean that for EFAS version 4.1 the criteria for plotting the ERIC Flash Flood reporting points are unchanged. However, the criteria for selecting at which ERIC Flash Flood reporting points flash flood notifications should be issued, have been changed to the following:
- Catchment part of Conditions of Access (ex-MoU)
- Probability of exceeding the 5 year return period ERIC threshold is >= 30%
- 30% probability exceedance of the 5 year return period ERIC threshold occurs within a leadtime <= 48 hours from the forecast time
- Actual lead time to the earliest predicted peak is > 0 hours (actual lead time = time difference between the current time when the forecaster analyzes the forecast (CET or CEST) and the predicted peak of the event (UTC))
The following changes have also been made to the ERIC reporting points layer in the EFAS version 4.1 release:
- Enlarged ERIC reporting point triangles on the EFAS website will only be shown where the flash flood notification criteria are matched exactly. Previously, enlarged triangles were also shown for lead times up to 60 hours, this was to highlight events which may have started at a lead time of 48 hours.
- Bugfix to the method used to calculate the lead time of the ERIC reporting points.
- The generation of the ERIC reporting points includes a spatial index file which aims to improve the speed with which the layer is shown on the EFAS website.
More information about the EFAS version 4.1 release is available here.