Status:Ongoing analysis Material from: Linus, David L., Ziga


1. Impact

From (


According to Slovenia’s environmental agency ARSO, northern and central parts of the country recorded more than 200 mm of rain within 12 hours from 03 to 04 August 2023. This is more than these areas would normally see during the whole of August, ARSO said.

The rain caused multiple rivers to break their banks, including the Savinja River in Celje where around 4,000 people evacuated their homes.


2. Description of the event

The northern parts of Ljubljana (the airport?) received 201 mm from 3 Aug 06UTC to 4 Aug 06UTC, and almost all fell between 18UTC and 06UTC. One can note that the central city only got 23 mm. More than 100mm was also measured for all stations in northern Slovenia.

The plots below show analyses of MSLP and 6 hour rainfall from 1 August 00UTC to 6 August 00UTC, every 12th hour.

The plots below show analyses of z500 and T850 from 1 August to 6 August 00UTC, every 24 hour.

The tip of the through initiated cyclogenesis in the Gulf of Genoa lead to strong southwesterly advection from the Mediterranean to the Eastern Alps.

Most of the precipitation occurred in the night from Aug 3 to Aug 4. The type of precipitation, which lead to the flooding was prefrontal convective. 

The southwesterly flow encountered the orography in SW Slovenia, underwent forced orographic ascent to the level of free convection, at which the air parcel became unstable.

3. Predictability


3.1 Data assimilation


3.2 HRES

The plots below show 24-hour precipitation (3 August 06UTC - 4 August 06UTC) in observations (first plot) concatenated short forecasts (second plot) and o-suite HRES forecasts with different lead times. There was some hits of extreme precipitation (>100 mm) in the western part of Slovenia but did not extend to the east, as in observations.

3.3 ENS

The plots below shows EFI and SOT for 3-day precipitation (3 August 00UTC - 6 August 00UTC)  from different initial dates. The reason to use 3-day EFI is that the event spent over 3 and 4 August.

The plots below shows EFI for 1-day integrated water vapour (IVT) 3 August  from different initial dates. A very strong water vapour flux was present in the region and was well predicted in the medium-range.

The plot below shows the forecast evolution plot for 14-hour precipitation (3 August 06UTC - 4 August 06UTC) for the station locations (see list below) inside the box (46.0N-46.5N, 13.7E-14.7E) outlined in the HRES plots above. Mean of observations - green hourglass, concatenated 6-hour forecasts - green dot, HRES –red,  ENS blue box-and-whisker, Model climate – cyan box-and-whisker. Ensemble mean as black diamonds. Triangle marks the maximum in the model climate based on 1600 forecasts.

The day was predicted to be wetter than normal in the medium-range, with members exceeding the model climate maximum in most runs. However, also in the shortest forecast the ensemble median was about the 99th percentile of the climate distribution. The absolute value of the forecast (median of ~30 mm) severely underestimated the mean of the observations (125 mm).












3.4 Monthly forecasts

3.5 Comparison with other centres

3.6 EFAS

The plots below show the river discharge for river Lava north of Ljubljana (point Medvode).While already the forecast from 1 August had a hint of a peak in the flow on 4-5 August, the flow was clearly underestimated until the event was observed after 4 August.

4. Experience from general performance/other cases

5. Good and bad aspects of the forecasts for the event

  • Good medium-range prediction of a wet day in Slovenia, probably due to strong water vapour flow and the large-scale forcing.
  • Also the shortest forecast severely underestimated the magnitude and extent of the precipitation. Probably due to the nature of the triggering of the convection(?)

6. Additional material

Link to a report from the National Met service: