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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.
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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
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