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September 20, 2023,
11:00 UTC

Heavy rain, Flooding
Greece, Libya
04 - 11 September 2023

Satellite imagery of storm Daniel
over the Eastern Mediterranean,
Source: NASA Worldview

A textbook omega weather pattern developed over Europe in the first days of September 2023, with the ridge bringing well-above-average temperatures to Central Europe. The two accompanying cut-off lows brought heavy rains to the Iberian Peninsula and Greece. In Greece, record-breaking precipitation of more than 900 mm/48 h fell.

Flooding Greece - September 04 - September 07, 2023

In the first days of September 2023, the remains of the former Hurricane FRANKLIN neared the Iberian Peninsula, which amplified the ridge residing over Europe. Both up and downstream of the ridge, troughs expanded southwards. On September 03, 2023, the trough above the Iberian Peninsula brought heavy rains to parts of Spain. Within a 24 h-period up to 155 mm of rain fell. The highest precipitation totals fell close to the capital city of Madrid, with flash flooding being observed across the city.

7-day mean 500 hPa geopotential over Europe (left) and 7-day 500 hPa geopotential anomaly over Europe (right), 08.09.2023, source: PSL NOAA

In the following days, the upper-level trough over the Eastern Mediterranean intensified. Being situated southeast of Greece, the upper-level trough induced mid-level north easterlies over the Aegean Sea. After the summer, the heated Aegean Sea provided abundant moisture and therefore latent heat for recurrent thunderstorm formation. In the second half of September 04, 2023, strong thunderstorms started to form over the Aegean Sea. Above the Aegean Sea, the mid-level winds converged, further enhancing the potential for recurrent thunderstorm formation. Due to the mid-level north easterlies, the thunderstorms moved onshore into Greece.

500 hPa Geopotential and sea surface pressure over Europe, 01.09.2023 - 08.09.2023, source: wetter3

As the winds and the convergence were very persistent, banded thunderstorms similar to lake-effect snowfall continued to impact the same region, leading to extreme precipitation totals. In the night from September 04, 2023, to September 05, 2023, the heaviest precipitation was observed, within the night more than 754 mm of rain fell in Zagora. As a reference, Zagora has an average annual precipitation of around 750 mm. In the following hours, the weather station at the town of Zagora stopped transmitting observations, likely as a result of the river flooding induced by the extreme precipitation totals in such a short time frame. Heavy precipitation continued throughout the day of September 05, 2023, and the first half of September 06, 2023. Torrential downpours also affected the port of Volos, leading to widespread flooding in the city. Thunderstorm activity decreased over the Aegean Sea, with the heavy rains ending in the second half of the day of September 07, 2023.

Radar Imagery Greece 04.09.2023 22 UTC - 07.09.2023 15 UTC, source: EMY

From September 04, 2023, to September 07, 2023, the accumulated precipitation totals reached more than 500 mm at many locations across the central part of Greece. At Zagora, the re-reported precipitation data suggests that the storm-related precipitation totals reached 1096 mm. The second-highest precipitation total was observed at the weather station in Portaria recording 885 mm of rain, though due to a suspected power outage, the weather station recorded no data on September 06, 2023.

Heavy rains were also observed in Bulgaria with precipitation totals reaching more than 300 mm on September 05, 2023. On the same day, a severe thunderstorm formed over the Metropolitan area of Istanbul, bringing torrential rains and severe flash flooding to the city. As the cut-off over the Eastern Mediterranean moved westwards towards Sicilia, the thunderstorm activity over the central part of Greece diminished.

The exceptional precipitation totals led to extreme flooding in large parts of Central Greece, Satellite reconnaissance suggests that the flooded area reached more than 70,000 ha, as many rivers reached extreme flooding stages. At least 22 deaths have been reported in Southeast Europe associated with the extreme precipitation of Storm Daniel. Many smaller towns in Greece are still cut off from the outside world. Especially, in the region around Volos where the highest precipitation totals were observed, extensive damage to the infrastructure and buildings must be expected. Further, the flooding will impact the agricultural production of Greece as the central part of Greece is fertile land.

Visible Satellite Imagery of the central part of Greece (left), 08.09.2023, and satellite-based reconaissance of flooded areas in the central part of Greece (right), 09.09.2023, source: NASA Worldview

Ultimately, the precipitation totals by Storm Daniel are the worst in the recorded history of Greece and correspond to more than the average annual precipitation in this region. The special synoptic pattern over the Eastern Mediterranean and the well-above-average ocean temperatures of the Aegean Sea provided the environment for the extreme precipitation in the coastal regions of Thessaly and Central Greece. The remarkable marine heat wave in the Mediterranean this year will have likely contributed to these precipitation totals, by providing enhanced moisture fluxes.

Flooding Libya - September 10, 2023 After causing severe flooding in Central Greece, storm Daniel moved over the Mediterranean. Over the warm waters of the Mediterranean with sea surface temperatures of up to 27 °C off the coast of Libya, the surface low intensified considerably. Over the open ocean, the structure of the storm started resembling the structure of a tropical cyclone, as the storm developed a shallow warm core. These cyclones are called Medicane as their structure is a mixture of a mid-latitude cut-off low and a tropical cyclone. In the Mediterranean, these storms form most often during the autumn months, when troughs from the mid-latitudes reach the Mediterranean and form cut-off lows. Due to the substantial diabatic heating of the warm ocean waters of the Mediterranean, these low-pressure systems can develop tropical characteristics.

Visible Satellite Imagery of Storm Daniel showing tropical characteristics as the storm made landfall in Libya, 10.09.2023,
source: NASA Worldview

On September 10, 2023, storm Daniel reached the coast of Libya, bringing torrential downpours to the coastal region of Cyrenacia in Libya. The city of Al Bayda received 414 mm of rain within a 24 h-period. Normally, the coastal regions of Libya receive only a few millimeters of rain in September. Further, inland satellite images after the passage of Storm Daniel show standing water in the desert of Libya because of the extreme precipitation. The comparison of satellite imagery before and after the passage of Storm Daniel shows that the vegetation in the coastal regions of Libya is much greener than before.

Visible Satellite Imagery of Northern Libya before (right), 07.09.2023, and after the passage of Storm Daniel (left), 13.09.2023, source: NASA Worldview

The port of Derna received more than 100 mm of rain during the passage of storm Daniel, normally Derna receives only 1.5 mm of rain in September. As a result, one of the most devastating floodings of the new millennium occurred, as two dams along the normally dry Wadi Derna collapsed and brought extreme flooding to the city. Many residential buildings were swept away as the flood water inundated the city, as the Wadi Derna runs straight through the city.

Damaged and destroyed buildings in the port Derna in Libya after the flooding of the Wadi Derna, 10.09.2023,
source: Copernicus - Image of the day, Credit: European Union, Copernicus Emergency Management Service data

Nearly 4,000 buildings were damaged or destroyed because of the flooding. More than 5,000 confirmed fatalities were reported from the city, with more than 10,000 people still missing. Estimates say that up to 20,000 people might have lost their lives as the flood wave moved through the city. Reports suggest that in the war-torn country, neglect has led to the failure of the two dams upstream of Derna along the Wadi Derna. Nevertheless, the precipitation totals due to Storm Daniel in parts of Libya were unprecedented, especially in a month when the country receives hardly any precipitation.

Text: KG
September 09, 2023

September 20, 2023

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