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Two layers show the probability of exceeding the 5-year return period in the EFAS forecasts at different lead time ranges:

• 5-year exceedance < 48 h indicates the probability of flooding in the following 48 h, a lead time range for which formal notifications are not issued.
• 5-year exceedance > 48 h indicates the probability of flooding from lead time 2 to 7 days, when formal notifications are issued.

Figure 1. Example of the layer Flood Probability > 48 h on the forecast of March 23 2024 at 00 UTC.

Total probability

The flood probability layers are based on total probability, which means that the hydrological forecasts based on the four different Numerical Weather Prediction (NWP) models (see EFAS Meteorological forecasts) (ECWMF-HRES, ECMWF-ENS, COSMO-LEPS, DWD-ICON) need to be combined. This combination is based on the NWP probabilistic skill measured in terms of the Brier score (BS):

\text{BS} = \frac{1}{T}\sum_{t=1}^{T} \left( P_{obs,t} - P_{pred,t} \right)^2

where, 

T

is the number of time steps, 

P_{obs,t}

is the observed probability of exceedance, and 

P_{pred,t}

 is the predicted probability of exceedance at a specific time step

t

. Brier scores were computed for every NWP and lead time using the historical archive of EFAS v4 forecasts and reanalysis. The resulting Brier scores were converted into weights by inverse distance weighting:

w_{nwp,lt} = \frac{BS_{nwp,lt}^{-p}}{\sum_{i=1}^{4} BS_{i,lt}^{-7}}

where,

w_{nwp,lt}

 is the weight assigned to a specific NWP and lead time, and 

BS_{nwp,lt}

is the Brier score of that NWP at that lead time.

Image Modified

Figure

2.

Distribution

of

weights

over

NWP

models

and

lead

time.

DWD

stands

for

DWD-ICON,

HRES

for

ECMWF-HRES,

COS

for

COSMO-LEPS

and

ENS

for

ECMWF-ENS.

Orange

colours

represent

deterministic

models

and

blue

colours

probabilistic

models.

As shown in the figure above, the assignment of weights gives priority to probabilistic NWPs over deterministic counterparts, particularly on the ECMWF-ENS model, which proved higher skill. The predominance of ECMWF-ENS increases with lead time.