This animation layer shows precipitation accumulations forecasted for the upcoming five days. For the first few hours of the forecast, the timestep is 1 hour, thereafter the timestep is 6 hours.

These precipitation accumulations have been obtained by seamless blending of the:

  • 20-members ensemble of 1-h accumulation generated with the algorithm for probabilistic nowcasting by extrapolation of radar observations SBMcast (Berenguer et al., 2011) applied to the gauge-adjusted OPERA radar composites over Europe (2 km, 15 minutes; Park et al. 2019)
  • 51-members ensemble of 1-h accumulation forecasts obtained by Numerical Weather Prediction (NWP) with the ECMWF Ensemble Prediction System (EPS)

The blending technique (Wong et al. 2009) is applied for the first 6 hours, and applies a phase shift and bias correction to blend radar nowcasts into NWP using a hyperbolic tangent weighting function. For lead times beyond 6 hours, the product relies only on the NWP forecasts. At each timestep this layer shows the 80th percentile of rainfall from the forecast ensemble.

The colorscale of the layer (Fig. 1) relates to the rainfall accumulation within the timestep that is being visualised. For the first 6 hours of the forecast the layer shows the hourly rainfall accumulation, thereafter the accumulation is 6 hourly. The length of the timestep that is currently being visualised is displayed in a box just above the animation slider. Note that the colourscale does not change to accomodate for the change in the forecast timestep, therefore users may notice what looks like an increase in rainfall accumulation when moving from the 1-hour to the 6-hour timestep.

Figure 1. 80th percentile of the hourly blended precipitation ensemble forecast at 15:00 UTC on 2022-09-05

Visualising within the EFAS Webviewer

The layer can be viewed after selecting it from the Flash Flood layers tab within the EFAS webviewer. An animation slider box will appear in the bottom left of the screen and by default the data from the most recent forecast will be loaded. A drop down menu just above the animation slider box can be used to select a different forecast date and time within the past 5 days.

The buttons on the left of the animation slider can be used to step forwards and backwards in time for each timestep of the forecast. The play button will play the animation in a continuous loop. When animating each timestep, it can take a few seconds to load the next timestep, this is shown by a loading icon which appears within the animation slider which will disappear once the next timestep has been loaded. 

References

Berenguer, M., Sempere-Torres, D. and Pegram, G, 2011: SBMcast - An ensemble nowcasting technique to assess the uncertainty in rainfall forecasts by Langrangian extrapolation. Journal of Hydrology, 404(3), 226-240

Park, S., Berenguer, M. and Sempere-Torres, D, 2019: Long-term analysis of gauge-adjusted radar rainfall accumulations at European Scale. Journal of Hydrology, 573, 768-777

Wong, W., Yeung, L., Wang, Y. and Chen, M, 2009: Towards the Blending of NWP with Nowcast - Operation Experience in B08FDP. WMO Symposium on Nowcasting, 30 Aug- 4 Sep, Whistler, Canada.