Flood · Greece and Libya
Storm Daniel: from Thessaly to Derna
In early September 2023 a low pressure system cut off from the jet stream stalled over Greece. For three days it poured rain on the mountains of Pelion and the plain of Thessaly: one village recorded about 750 mm in 24 hours, the most ever measured in Greece. The storm then drifted south over the warm sea, took on the shape of a tropical cyclone and struck eastern Libya, where two dams above the city of Derna gave way in the night.
4–11 September 2023 · Thessaly, Greece; Cyrenaica, Libya · ERA5
750mm
Rain in 24 hours at Zagora
Pelion, 5–6 September: the highest daily total ever measured in Greece.N1
357mm
ERA5 at Volos, 4–7 Sep
The grid point nearest the city; gauges in the hills measured far more.
Part one: the flood of Thessaly
On 4 September a cut-off low settled over western Greece, held in place by high pressure to its north: an omega block, named for its shape on the upper-air maps.S1 Moist air from the warm Aegean was driven into the Pelion mountains and the hills around the plain of Thessaly, and it rained, almost without pause, until 7 September.
Rivers burst their banks across the plain, Greece's breadbasket; villages were cut off for days and large areas of farmland were left under water. Seventeen people died in Greece, and the storm also killed people in Bulgaria and Türkiye.N1
What the grid sees
357mm
ERA5's total at Volos over four days, against about 750 mm in a single day at Zagora, 25 km away. The heaviest rain fell on the steep slopes of Pelion, which a 28 km grid cell averages with the sea and the plain.
Part two: the medicane and Derna
On 7 September the low left Greece and moved south over the Ionian Sea. Over water some 2–3 °C warmer than usual it gained a warm core and a ring of thunderstorms round a clear centre: a medicane, a Mediterranean hurricane.S1 On 10 September it reached the coast of Cyrenaica.
The mountains behind the coast wrung out the rain: Al Bayda measured 414 mm in a day.N2 In the night of 10 to 11 September two dams on the Wadi Derna, above the city of Derna, collapsed. The flood swept whole districts into the sea; thousands of people died and thousands more were missing.N1
One storm, many causes
A study of Daniel traced how the warm sea fuelled both phases, and found that the toll in Libya owed as much to the neglected dams and to the conflict that left them unrepaired as to the rain.S1 Attribution studies found climate change had made such rain more likely and more intense, by up to ten times in Greece and fifty times in Libya, though with large uncertainty.S1
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References
News
- N1World Meteorological Organization. Storm Daniel leads to extreme rain and floods in Mediterranean, heavy loss of life in Libya. September 2023.The rain records and the toll in Greece and Libya.
- N2NASA Earth Observatory. Torrential rain wreaks havoc in Libya. September 2023.Al Bayda's rain and satellite views of Derna.
Studies and guidelines
- S1Flaounas, E., Dafis, S., Davolio, S., et al. (2025). Dynamics, predictability, impacts and climate change considerations of the catastrophic Mediterranean Storm Daniel (2023). Weather and Climate Dynamics, 6, 1515–1538.The storm's dynamics, the warm sea, the factors behind its impacts and climate change.