Between Friday, 13 and Monday, 16 September 2024, a low-pressure system named Boris brought record-breaking rainfall to central Europe, leading to severe flooding in parts of Austria, Czechia, Slovakia, Poland, Romania and Hungary. The flooding caused widespread damage and disruption with costs estimated in the billions of euros. Sadly, the loss of 27 lives was reported and many people will have been left with mental health issues due to the trauma they experienced.

At ECMWF we issue forecasts four times daily to our Member and Co-operating States and other users, including specialized products which provide guidance on hazardous weather. We also produce forecasts for the European Flood Awareness System (EFAS) under our role as the hydrological computation center for the EU Copernicus Emergency Management Service (CEMS).

Based on these and other sources of information, national meteorological and hydrological centers and other civil authorities within Member and Co-operating States issue weather, flood and other emergency warnings at the national and regional level.

About Storm Boris

In the days before Storm Boris, unusually cold air was driven south across the UK. During 11 and 12 September, this cold air continued on across France into the western Mediterranean to meet warm, moist air to the east, creating a strong temperature contrast across central Europe (Figure 1a).

A low-pressure system formed along the interface and was named Boris by the Italian Meteorological Service (under the EUMETNET storm naming activity) when it first formed over northern Italy on Wednesday, 11 September. It moved erratically east and then north, and a cut-off low developed (Figure 1b). High-pressure ridges built to the northeast and the northwest (Figure 1b), which helped to anchor the system and its very active rainfall zone for several days over central Europe (Figure 1c).

While this synoptic pattern is rare, it is not unprecedented, and it has historically been associated with flooding in central Europe. Similar conditions in July 1997 and August 2002 were linked to devastating flooding.

Exceptional rainfall from storm Boris

The largest three-day rainfall totals (from 13 September 00 UTC to 16 September 00 UTC) occurred over eastern Austria and along the border between Czechia and Poland (Figure 2, top row). A three-day total of 442 mm in the mountains in northern Czechia was the highest value found in observed station data available at the time of writing.

There were also seven fatalities due to short duration extreme rainfall, and associated flash floods, in a topographically complex region of eastern Romania. Floods there were separate from the main rainfall region well to the west, but still attributable to Boris, at an early stage in its lifecycle.

Heavy rainfall and flooding later affected the Emilia-Romagna region of northern Italy, but that was a somewhat separate event, and in this article we focus on the central European rainfall.

Five-day rainfall totals in Austria for the event, compared with previous record rainfall, show that in some places the rainfall associated with storm Boris was almost twice the previous five-day record.

The September 2024 Climate Bulletin from the Copernicus Climate Change Service (implemented by ECMWF on behalf of the EU) provides further information on the precipitation extremes during Storm Boris. The study shows that more than three months' worth of rainfall fell during the event (12 to 16 September) in eastern Austria and on the border between Czechia and Poland.

Providing weather forecasts for our Member and Co-operating States

As early as a week before the event, there were indications from ECMWF's Integrated Forecasting System (IFS) of unusually high rainfall over the 13 to 16 September period, and this signal became generally stronger and more extreme over subsequent days.

This is exemplified by the forecast from Monday, 9 September 00 UTC, which had values of the Extreme Forecast Index (EFI) over 0.8, suggesting the likelihood of unusual rainfall. The EFI provides an indication of the risk of extreme/severe conditions by comparing the IFS ensemble forecast (ENS) with the model climate.

Looking at a forecast for Vienna issued on 13 September (Figure 4), we see daily precipitation totals for three days in a row (Friday 13, Saturday 14 and Sunday 15 September) which are beyond the depicted model climate range (the thin green line on the total precipitation graph shows the 99th percentile of the model climate based on about 2000 model runs representing the previous 20 years). The anomalously cold conditions can also be seen.

In addition to the ECMWF operational IFS, Figure 2 shows examples of forecasts produced for the EU's Destination Earth (DestinE) initiative (4.4 km resolution), the ECMWF Artificial Intelligence Forecasting System (AIFS) (28 km resolution) and the AIFS ensemble (AIFS-ENS) (111 km resolution), here represented by the first perturbed member. The magnitude of the extreme rainfall in the worst-affected regions was best captured in the DestinE and operational ECMWF forecasts. The AIFS forecast captured the large-scale structure of the extreme but underestimated the amplitude, partly due to the lower resolution. As the AIFS ensemble is running with much lower resolution currently, the magnitude was missed but the general pattern was captured.

Of course, the process of evaluation will continue, for Storm Boris and for all the forecasts that make up ECMWF's operational forecasting system, for which collaboration and feedback from our Member and Co-operating States and other users is invaluable.

Flood forecasts through the European Flood Awareness System (EFAS)

EFAS was first developed following the major floods that hit parts of Europe in 2002. Managed by the European Commission's Joint Research Center, EFAS is now part of the EU Copernicus Emergency Management Service (CEMS), and its main aim is to support preparatory measures before major flood events strike, particularly in large transnational river basins, as was the case for storm Boris.

As the computational center for CEMS-Flood, ECMWF's role in EFAS is to generate flood forecasts for Europe. They are produced using an open-source hydrological model, LISFLOOD, with an ensemble of meteorological forecasts (from ECMWF, DWD and the COSMO Limited-Area Ensemble Prediction System consortium) and meteorological and hydrological observations. Forecasts are updated twice per day up to 10 days ahead, with a time step of sixhours and a resolution of approximately 1.4 km.

The first very strong signal of a severe flood associated with storm Boris was suggested by EFAS from the 9 September 12 UTC run, with large parts of the Oder River basin (in western Poland and its border with Czechia and Germany) suggesting river flow exceeding at least a 20-year return period. The flood signal extended to the upper Danube (in Austria) with the 10 September 12 UTC forecast run.

EFAS forecasts from 12 September 00 UTC (Figure 5) show a large region of central Europe with flood peaks potentially exceeding a 20-year return period.

The hydrographs (Figure 6) show the risk of exceptional flood levels—with some ensemble members well beyond the maximum level within the model climatology (shown by the dashed line in Figure 6).

To put these flood levels into context, we can compare the maximum 6-hourly flow during the 12 to 21 September period with the long-term average annual maximum flood (Figure 7 left). In eastern Austria (along tributaries of the Danube) and along the river Oder (on the border between Poland and Czechia and Germany), we see more than 5 times the average maximum flow (for information, Figure 5 shows the location of the Oder and Danube).

Comparing that with the European floods in August 2002 (Figure 7 right) shows clearly the much greater spatial extent of extreme flood levels during storm Boris. Storm Boris produced flood peaks beyond twice the average annual maximum along 8,500 km of rivers—over double the extent of the 2002 floods.

Medium-range prediction for advance warning of extreme events

Clearly, the rainfall and flooding associated with Boris were exceptional, and ECMWF and EFAS forecasts provided an effective early signal of such.

Advance warning of hazardous weather is key to helping countries prepare and minimize impacts, and this is becoming ever more important as climate change increases the frequency and severity of such events.

ECMWF has a key goal of pushing the boundaries of our prediction systems to provide effective forecasts of high-impact weather out to two-weeks or more.