2023 storm in the Mediterranean Sea
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Storm Daniel, also known as Cyclone Daniel, was a catastrophic tropical cyclone that became the deadliest Mediterranean tropical-like cyclone in recorded history, as well as one of the costliest tropical cyclones on record outside of the North Atlantic Basin. Forming as a low-pressure system around 4 September 2023, the storm affected Greece, Bulgaria, and Turkey with extensive flooding. The storm then organized as a Mediterranean low and was designated as "Storm Daniel." It soon acquired quasi-tropical characteristics and moved toward the coast of Libya, where it caused catastrophic flooding caused by the collapse of two dams, Derna and Mansour, resulting in the flooding of the Wadi Derna river causing catastrophic damage to the city of Derna. After collapsing the two dams, Storm Daniel degenerated into a remnant low. The storm was the result of an omega block; a high-pressure zone sandwiched between two zones of low pressure, with the isobars shaping like the Greek letter omega (Ω).
In Greece, severe rainfall led to flooding that caused more than two billion euros in damage, making it the most costly recorded storm for the country. Libya was hit the hardest, with torrential rains causing two dams near the city of Derna to fail. This resulted in over 5,900 deaths and 7,000 injuries, with at least 8,000 others missing. Libya's vulnerability to such disasters was blamed on its civil war, which damaged critical infrastructure and left it in poor condition before the storm. In the aftermath, several countries along the Mediterranean Sea pledged to provide aid to affected countries.
Meteorological history.
An area of low pressure developed over the Ionian Sea with its surface temperature within the range of tropical transition. On 4 September 2023, it moved inland over the Balkan Peninsula which led to torrential rains, notably over the Thessaly region. The system became a Mediterranean cyclone the following day, and was named Storm Daniel by the Hellenic National Meteorological Service. Daniel transitioned into a Mediterranean tropical-like cyclone on 9 September. During the following days, the system moved southeastward, peaking as a tropical storm with winds recorded by instruments on Metop at .
The storm made landfall near the city of Benghazi in Libya on 10 September. Daniel went east and continued inland before degenerating into a low pressure-area later on, with the storm dissipating by 12 September. Climate warming may also have influenced mid-latitude atmospheric blocking in the summer, which resulted in Storm Daniel and another cold-core low that caused flooding in Spain.
Impact.
Storm Daniel was regarded by researchers from Yale University as the deadliest single flood event to hit Africa in recorded history, with its death toll surpassing flooding in Algeria that killed 3,000 in 1927. It was also regarded as the deadliest storm globally since at least Typhoon Haiyan in 2013, and possibly Cyclone Nargis in 2008.
Greece.
On 5 September, flooding in Thessaly, Greece, killed at least one person. On the same day, the village of Zagora received of rain, 55 times more than the country's average rainfall for the same month. Portaria also recorded a new rainfall record of . Further rainfall could not be measured because the weather station subsequently failed. On 6 September, the Krafsidonas river, which rises at Pelion, overflowed its banks in Volos and destroyed a bridge and a nursing home, while dragging cars, buses, trees, and other debris along its path.
On 7 September, the main motorway between Athens and Thessaloniki was closed and train services between the two cities were suspended. In Thessaly, over 800 people had to be rescued amidst collapsed buildings and bridges and submerged villages. In Larissa, after the rains ended on 8 September, water continued to rise as the Pineios river overflowed its banks to reach a level of , compared to the normal level of . In the Vale of Tempe, the water level rose to about meters, reaching the level of a suspension bridge.
Storm Daniel turned many villages in the low-lying area of Karditsa, in the mainland Thessaly plain, into a lake. Towns and villages affected were Palamas, Proastio, Agia Triada, Megala Kalyvia and Kalogriana that were blocked from water.
One man died in Volos when a wall collapsed on him, and in the nearby Pelion area the body of an old lady was discovered on 6 September while four people were reported missing. At least six villages in and around the Pelion mountain range suffered huge damage.
Since the rainfall started, the Copernicus Programme's Rapid Mapping Service was activated for the flood zone in Greece, in which analysis of the Sentinel-1 data from 7 September revealed an estimated flood area of around . Meteorologists classified the storm as Greece's worst since records began in 1930. The floods in Thessaly, which supplies about 15% of Greece's agricultural production, destroyed the crops for the remainder of the year and caused serious long-term damage as the thick layer of mud made the soil infertile, taking up to five years to become fully functional again. The governor of Thessaly, Kostas Agorastos, told the Hellenic Broadcasting Corporation (ERT) that the storm damage in the region was calculated to be more than €2 billion. By 16 September, the death toll in the country had risen to seventeen. Among those killed were an Austrian couple who were trapped in a holiday home that was washed away by floods in Potistika, near Pelion, on 6 September.
Police banned travel to Volos, certain Pelion villages and the nearby island of Skiathos. Authorities also sent text alerts to inhabitants in other areas of central Greece, on the Sporades islands and on the island of Evia near Athens, warning them to limit their movements outdoors, since flooding was forecast to continue until at least the afternoon of 7 September.
Later that month, the Deputy Climate Crisis and Civil Protection, Evangelos Tournas, reported to the Hellenic Parliament that 110,000 animals were found dead and 135,000 poultry were registered as lost across Thessaly.
Turkey.
During the initial days of the storm, five people were killed in Turkey during floods in İğneada, Kırklareli Province. The deaths happened in the vicinity of İğneada Floodplain Forests National Park in an unlicensed bungalow establishment. Rising waters carried logs from a nearby forestry business and the logs dragged the houses along with them, destroying the buildings and killing the people in the process.
On 6 September, the districts of Ikitelli, Arnavutköy, Başakşehir and Küçükçekmece in Istanbul were flooded due to heavy rainfall. Two died in Başakşehir and Küçükçekmece, with 31 others injured. The victims were a Guinean citizen who was trapped inside his basement apartment in Küçükçekmece, the other was a woman who died after being swept away by the floods in Küçükçekmece. More than 1,750 homes and businesses in the city were affected. The governor of Istanbul, Davut Gül, said the city received rainfall roughly equivalent to what it would expect in the entirety of September in the space of six hours. He also said on social media that authorities would provide accommodation and safety for those affected by flooding.
Bulgaria.
Villages on and near the Black Sea coast in Burgas Province, including Kosti and Arapya, became submerged, forcing evacuations. Three people were swept away after a bridge collapsed in the Tsarevo area, and another person drowned near the town.
Rainfall in Kosti was measured at (420% of the monthly average for September), in Ahtopol it amounted to (350% of the monthly average), and in Gramatikovo to (368% of the monthly average). In Tsarevo, rainfall was expected to set a national record, with of precipitation within 20 hours (40% of the "annual" average). Flooding in the town prompted authorities to declare a state of emergency.
A rare waterspout of approximately was observed in the sea near Tyulenovo in the northeast of the country.
The whole southern region of Bulgaria’s Black Sea coast was affected by the disaster. Most of the rivers in the region burst their banks and several bridges were destroyed, causing serious transporting and rescue problems to over 4,000 inhabitants and tourists, according to tourism minister Zaritsa Dinkova.
Libya.
Preparations.
Osama Hamada, Prime Minister of the Government of National Stability which controls eastern Libya, declared a state of emergency on 9 September and suspended classes as a precaution. The National Oil Corporation also announced a three-day closure of four oil ports including Ras Lanuf, Zueitina, Brega, and Sidra. The facilities in Ras Lanuf, Brega, and Sidra reopened on 12 September, while the port of Zueitina reopened on 13 September.
Derna dam failures.
In Libya, at least 5,923 people were killed, mostly in and around the city of Derna after the Derna (also known as Belad) and Mansour (or Abu Mansour) dams collapsed, releasing an estimated 30 million cubic metres (39 million cubic yards) of water, and causing catastrophic damage across the area after the Wadi Derna overflowed its banks by on each side. 30,000 residents were displaced.
Prior to the storm, residents were prevented from leaving their homes after authorities imposed a precautionary curfew at 10:00 PM on 10 September. Residents recalled hearing loud explosions at the time the dams burst, with video showing the flood reaching Derna shortly before 03:00 EET on 11 September. Prime Minister Hamada stated that residential neighborhoods were swept away. Videos posted to social media showed cars being submerged in the deluge. Four bridges also collapsed, while Hamada's aviation minister Hisham Chkiouat said that Derna looked as if it had been hit by a "tsunami". He also said that 25% of the city had "disappeared", with large parts of the city dragged out to the Mediterranean Sea. 
Hamada's health minister, Othman Abduljalil, said 6,000 people were missing in Derna alone. The mayor of Derna, Abdulmenam Al-Ghaithi, told al-Arabiya that the final death toll in the city could range from 18,000 to 20,000, equivalent to a fifth of the city's population. In September 2024, Libya analyst Anas El Gomati suggested a death toll of between 14,000 and 24,000. Only three of the city's ten districts escaped the flooding, while five out of seven entry routes into Derna were rendered inaccessible. The collapse of bridges along the Wadi Derna effectively split the city into two. Officials said that of the 6,142 buildings in Derna, a total of 1,500 suffered damage, of which 891 were completely destroyed, 211 partially destroyed, and the remaining 398 were submerged in mud. An estimated six square-kilometres (2.3-square-mile) of land in the city was inundated.
Hospitals in the city were rendered inoperable while morgues filled up, prompting bodies to be laid out on sidewalks and in the city's main square. More than 300 bodies were sent to a morgue in Tobruk to cope with the overcrowding. More than 1,000 bodies were later buried in mass graves. Naval teams were dispatched to recover bodies swept out to sea by the floods. Over the succeeding days, at least 200 bodies were found washed up as far as 20 kilometers from Derna. Others were found more than 100 kilometers (60 miles) from the city. Dozens of victims were found trapped in their cars at sea. One survivor was rescued after being found 11 nautical miles (about ~20 kilometers) off the coast of Derna.
The scale of the disaster in Derna was attributed to decades of neglect of the region by the regime of Muammar Gaddafi, followed by the city becoming a battleground during the civil war and concurrent NATO intervention in the 2010s and its resulting political effects that included the establishment of rival governments in the west and east of the country respectively. After Gaddafi's overthrow, the city changed hands four times. The collapsed dams were built by Yugoslav company Hidrotehnika-Hidroenergetika from 1973 to 1977 to control flooding, irrigate agricultural lands and provide water to nearby communities. They were described as clay-filled embankment dams with a height of 75 meters and 45 meters respectively.
The Mansour (or Abu Mansour) dam had a water storage capacity of 1.5 million cubic meters, while the Derna (or Belad) dam upstream had a capacity of 22.5 million cubic meters (1.5 million cubic meters by another source).
The two dams sustained major damage in a storm in 1986, and cracks were reported in both structures in 1998. Derna's deputy mayor said that the dams had not been maintained since 2002 and were not built to withstand such volumes of water. The lack of maintenance occurred despite the allocation of more than 2 million euros for that purpose in 2012 and 2013. However, a Turkish