The Long Swell

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Floods · explainer · 16 October 2026

How a flood is made

Barcelona's wettest day since its complete record began in 1978 came when three things lined up: a sea after its warmest September in 87 years, a pool of cold Atlantic air, and a wind driving moist air onshore.

By Kristjan Lepik

01

Rain

How much is 159 millimetres?

Drawn to scale · 1 m square, 20 cm walls
Barcelona · Fabra observatory · 3 October 2026
159.1 mm
159 litres on one square metre
≈ one full bathtubon every square metre · a bath holds about 150 litres, for scale

On 3 October the Fabra observatory in Barcelona measured 159.1 mm of rain. One millimetre of rain is one litre on every square metre.

So a one-metre square of ground at the gauge took 159 litres in a day. For scale only: a full bath holds around 150.

That is the wettest day at Fabra in its complete record, which starts in 1978.

Rain like that needs three things at once: warm water to feed it, a way to lift it, and something to set it off. Here is how they lined up.

02

Heat

How hot did the summer get?

September was warm across Europe, but the sea was the outlier. The western and central Mediterranean had their warmest September in a record going back to 1940: 2.4 °C and 2.8 °C above the 1991 to 2020 normal.

Map of Europe, 2026 temperature against 1991 to 2020, September: much warmer than normal over France, Iberia, the western and central Mediterranean.
September 2026
MayJunJulAugSep
ColderWarmer· scale ±3 °C, saturates beyond
Each month of 2026 against the same month in 1991 to 2020: sea surface temperature over the sea, air temperature at 2 m over land. ERA5 monthly means, 2026 preliminary.
03

Sea

Where did the water come from?

The western Mediterranean averaged 26.0 °C in September. Normal is 23.6. That is the record. Rain on this coast starts as sea water, lifted into the air.

Temperatures measured: western Mediterranean (Alboran, Balearic, Gulf of Lion), September mean, ERA5; 2026 is the warmest September in a record going back to 1940 (1 of 87). The motion is illustrative: the warmer sea is drawn with a heat shimmer and slightly denser vapour, not to a measured rate.

04

Air

How much water can air hold?

The air held more water too. Across the whole planet, not just Spain, September 2026 carried more water vapour than any September in a record going back to 1940, 6.5% above normal. Warmer air holds about 7% more water for each degree.

6.5%20261940No September in this record reaches it.WATER VAPOUR IN THE AIR, EVERY SEPTEMBER SINCE 1940, % VS 1991-2020

Global total column water vapour, area-weighted, ERA5. About 7% more water vapour per degree C of warming (Clausius-Clapeyron; IPCC AR6 WG1 ch. 8).

05

Pump

What lifted it and set it off?

27 Sep, 00:00 UTCERA5 · measured · 2026 preliminaryECMWF forecast · run 2026-10-16 00:00 UTC
Sea, September vs normalSea, 8 to 12 October vs normal
Cold air, 5 km up
Wind, 1.5 km up
Barcelona
Heights exaggerated · every layer is a measured fieldHeights exaggerated · cold air and wind are ECMWF's model forecast, not measurements AEMET on where the low will sit: “elevada incertidumbre”, high uncertainty.

27 September. Five kilometres up, a pool of unusually cold air is sitting over the Atlantic.

29 September. At its deepest it is 15.9 °C colder than normal for the date, far out over the ocean.

30 September. It swings toward Iberia, still 11.7 °C colder than normal, out in the Atlantic north-west of Galicia.

1 October. It weakens and breaks up, leaving a shallow dip in the air over Spain.

3 October. A kilometre and a half up, the wind off Barcelona blows in from the south-east at 10 metres a second, off a sea that had just had its warmest September in 87 years.

06

Ground

Why does wet ground matter?

The ground is the part still in place. In the second week of October, topsoil from the Ebro delta to Murcia was wetter than normal for the date. Around the Ebro delta it was 55% above normal, the second wettest in 36 years. Ibiza was the exception, drier than usual. Ground that is already wet takes in less of the next rain.

Dry groundsoaks the rain in

Already wetfills, and the rest runs off

River

River

Hour 0 of 24Same rain on both · illustrative · not to scale

feedsliftedfalls ondrains backwarms the sea and the airSeaAirPumpGroundHeat
07

Weekend

What does this weekend look like?

The same ingredients are lining up again in ECMWF's forecast for this weekend: first the cold air and the wind, as the European centre's model sees them. It is their forecast, not ours. Then the map: where floods have hit this coast before, how wet the ground is now, and AEMET's warnings as they are issued.

27 Sep, 00:00 UTCERA5 · measured · 2026 preliminaryECMWF forecast · run 2026-10-16 00:00 UTC
Sea, September vs normalSea, 8 to 12 October vs normal
Cold air, 5 km up
Wind, 1.5 km up
Barcelona
Heights exaggerated · every layer is a measured fieldHeights exaggerated · cold air and wind are ECMWF's model forecast, not measurements AEMET on where the low will sit: “elevada incertidumbre”, high uncertainty.

This weekendThe same layers, in ECMWF's forecast. Run 2026-10-16 00:00 UTC. Not a forecast of ours.

18 October. In ECMWF's forecast, a new cold pool drops over France, about 10 °C colder than normal. Unlike the first storm, its edge reaches over Spain, 4 to 5 °C colder.

19 October. In the forecast, the wind a kilometre and a half up turns onshore at Barcelona, from the south-east at 11 metres a second, over the warm sea.

Alcaraz y Segura: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Alcaraz y Segura: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Altiplano de Murcia: Precipitación acumulada en una hora 20 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Antequera: Precipitación acumulada en una hora 20 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Campo de Cartagena y Mazarrón: Precipitación acumulada en 12 horas 60 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en 12 horas 60 mm, 10-18 00:00 to 10-18 07:59; Precipitación acumulada en una hora 20 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en una hora 20 mm, 10-18 00:00 to 10-18 07:59; Tormentas , 10-17 12:00 to 10-17 23:59; Tormentas , 10-18 00:00 to 10-18 07:59Capital y Montes de Jaén: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Cazorla y Segura: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Costa granadina: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Cuenca del Genil: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Depresión central de Barcelona: Precipitación acumulada en 12 horas 40 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en 12 horas 60 mm, 10-18 00:00 to 10-18 03:59; Precipitación acumulada en una hora 20 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en una hora 20 mm, 10-18 00:00 to 10-18 03:59; Tormentas , 10-17 12:00 to 10-17 23:59; Tormentas , 10-18 00:00 to 10-18 03:59Grazalema: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Grazalema: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Guadix y Baza: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Gúdar y Maestrazgo: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Hellín y Almansa: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Ibiza y Formentera: Precipitación acumulada en 12 horas 100 mm, 10-18 00:00 to 10-18 23:59; Precipitación acumulada en una hora 30 mm, 10-18 00:00 to 10-18 23:59; Tormentas , 10-18 00:00 to 10-18 23:59Ibiza y Formentera: Precipitación acumulada en 12 horas 100 mm, 10-18 00:00 to 10-18 23:59; Precipitación acumulada en una hora 30 mm, 10-18 00:00 to 10-18 23:59; Tormentas , 10-18 00:00 to 10-18 23:59Interior de Alicante: Precipitación acumulada en una hora 30 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en una hora 30 mm, 10-18 00:00 to 10-18 07:59; Tormentas , 10-17 12:00 to 10-17 23:59; Tormentas , 10-18 00:00 to 10-18 07:59Interior de Mallorca: Precipitación acumulada en una hora 20 mm, 10-18 10:00 to 10-18 23:59Interior norte de Valencia: Precipitación acumulada en una hora 20 mm, 10-17 18:00 to 10-17 23:59; Precipitación acumulada en una hora 20 mm, 10-18 00:00 to 10-18 03:59; Tormentas , 10-17 18:00 to 10-17 23:59; Tormentas , 10-18 00:00 to 10-18 03:59Interior norte de Valencia: Precipitación acumulada en una hora 20 mm, 10-17 18:00 to 10-17 23:59; Precipitación acumulada en una hora 20 mm, 10-18 00:00 to 10-18 03:59; Tormentas , 10-17 18:00 to 10-17 23:59; Tormentas , 10-18 00:00 to 10-18 03:59Interior sur de Castellón: Precipitación acumulada en una hora 20 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Interior sur de Valencia: Precipitación acumulada en una hora 20 mm, 10-17 18:00 to 10-17 23:59; Precipitación acumulada en una hora 20 mm, 10-18 00:00 to 10-18 03:59; Tormentas , 10-17 18:00 to 10-17 23:59; Tormentas , 10-18 00:00 to 10-18 03:59La Mancha albaceteña: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59La Mancha conquense: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Levante almeriense: Precipitación acumulada en una hora 20 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Levante mallorquín: Precipitación acumulada en una hora 20 mm, 10-18 10:00 to 10-18 23:59Litoral de Barcelona: Precipitación acumulada en 12 horas 60 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en 12 horas 60 mm, 10-18 00:00 to 10-18 03:59; Precipitación acumulada en una hora 30 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en una hora 30 mm, 10-18 00:00 to 10-18 03:59; Tormentas , 10-17 12:00 to 10-17 23:59; Tormentas , 10-18 00:00 to 10-18 03:59Litoral norte de Alicante: Precipitación acumulada en 12 horas 90 mm, 10-18 00:00 to 10-18 23:59; Precipitación acumulada en una hora 30 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en una hora 30 mm, 10-18 00:00 to 10-18 23:59; Tormentas , 10-17 12:00 to 10-17 23:59; Tormentas , 10-18 00:00 to 10-18 23:59Litoral norte de Valencia: Precipitación acumulada en una hora 30 mm, 10-18 00:00 to 10-18 07:59; Tormentas , 10-18 00:00 to 10-18 07:59Litoral sur de Alicante: Precipitación acumulada en 12 horas 80 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en 12 horas 80 mm, 10-18 00:00 to 10-18 07:59; Precipitación acumulada en una hora 30 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en una hora 30 mm, 10-18 00:00 to 10-18 07:59; Tormentas , 10-17 12:00 to 10-17 23:59; Tormentas , 10-18 00:00 to 10-18 07:59Litoral sur de Castellón: Precipitación acumulada en una hora 30 mm, 10-17 18:00 to 10-17 23:59; Precipitación acumulada en una hora 30 mm, 10-18 00:00 to 10-18 03:59; Tormentas , 10-17 18:00 to 10-17 23:59; Tormentas , 10-18 00:00 to 10-18 03:59Litoral sur de Girona: Precipitación acumulada en 12 horas 60 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en 12 horas 60 mm, 10-18 00:00 to 10-18 03:59; Precipitación acumulada en una hora 30 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en una hora 30 mm, 10-18 00:00 to 10-18 03:59; Tormentas , 10-17 12:00 to 10-17 23:59; Tormentas , 10-18 00:00 to 10-18 03:59Litoral sur de Valencia: Precipitación acumulada en 12 horas 90 mm, 10-18 00:00 to 10-18 23:59; Precipitación acumulada en una hora 30 mm, 10-18 00:00 to 10-18 23:59; Tormentas , 10-18 00:00 to 10-18 23:59Nacimiento y Campo de Tabernas: Precipitación acumulada en una hora 20 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Nevada y Alpujarras: Precipitación acumulada en una hora 20 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Noroeste de Murcia: Precipitación acumulada en una hora 20 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Norte y nordeste de Mallorca: Precipitación acumulada en una hora 20 mm, 10-18 10:00 to 10-18 23:59Pirineo de Girona: Precipitación acumulada en 12 horas 60 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en 12 horas 60 mm, 10-18 00:00 to 10-18 03:59; Precipitación acumulada en una hora 30 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en una hora 30 mm, 10-18 00:00 to 10-18 03:59; Tormentas , 10-17 12:00 to 10-17 23:59; Tormentas , 10-18 00:00 to 10-18 03:59Poniente y Almería Capital: Precipitación acumulada en una hora 20 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Prelitoral de Barcelona: Precipitación acumulada en 12 horas 60 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en 12 horas 60 mm, 10-18 00:00 to 10-18 03:59; Precipitación acumulada en una hora 30 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en una hora 30 mm, 10-18 00:00 to 10-18 03:59; Tormentas , 10-17 12:00 to 10-17 23:59; Tormentas , 10-18 00:00 to 10-18 03:59Prelitoral de Girona: Precipitación acumulada en 12 horas 60 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en 12 horas 60 mm, 10-18 00:00 to 10-18 03:59; Precipitación acumulada en una hora 30 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en una hora 30 mm, 10-18 00:00 to 10-18 03:59; Tormentas , 10-17 12:00 to 10-17 23:59; Tormentas , 10-18 00:00 to 10-18 03:59Prepirineo de Barcelona: Precipitación acumulada en 12 horas 60 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en 12 horas 60 mm, 10-18 00:00 to 10-18 03:59; Precipitación acumulada en una hora 30 mm, 10-17 12:00 to 10-17 23:59; Precipitación acumulada en una hora 30 mm, 10-18 00:00 to 10-18 03:59; Tormentas , 10-17 12:00 to 10-17 23:59; Tormentas , 10-18 00:00 to 10-18 03:59Ronda: Precipitación acumulada en una hora 20 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Serranía de Cuenca: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Sierra Tramontana: Precipitación acumulada en una hora 20 mm, 10-18 10:00 to 10-18 23:59Sierra sur de Sevilla: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Sierra sur de Sevilla: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Subbética cordobesa: Precipitación acumulada en una hora 15 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Sur de Mallorca: Precipitación acumulada en una hora 20 mm, 10-18 10:00 to 10-18 23:59Valle del Almanzora y Los Vélez: Precipitación acumulada en una hora 20 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Valle del Guadalentín, Lorca y Águilas: Precipitación acumulada en una hora 20 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Vega del Segura: Precipitación acumulada en una hora 20 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59Vega del Segura: Precipitación acumulada en una hora 20 mm, 10-17 12:00 to 10-17 23:59; Tormentas , 10-17 12:00 to 10-17 23:59 Barcelona, Maresme, Girona coast: topsoil +23% vs normal, 5th wettest of 36Southern Catalonia, Ebro delta: topsoil +55% vs normal, 2nd wettest of 36Castellon coast: topsoil +32% vs normal, 8th wettest of 36Valencia, central coast: topsoil +29% vs normal, 7th wettest of 36La Safor, Marina Alta: topsoil +18% vs normal, 12th wettest of 36Alicante, Vega Baja: topsoil +28% vs normal, 8th wettest of 36Murcia coast: topsoil +53% vs normal, 5th wettest of 36Malaga, Costa del Sol: topsoil +11% vs normal, 15th wettest of 36Ibiza: topsoil -12% vs normal, 16th wettest of 36Mallorca: topsoil +30% vs normal, 9th wettest of 36 SPAINMEDITERRANEAN SEABALEARIC ISLANDSALGERIABarcelonaValenciaAlicanteMálaga Oct 1982, Tous dam failure, Jucar basin1982Nov 1987, Gota fria, Valencia coast1987Oct 1994, Catalonia, Llobregat and Francoli; Oct 2019, Catalonia and Mallorca1994 · 2019Oct 2000, Valencia and the south-east coast; Oct 2024, Valencia DANA2000 · 2024Sep 2012, Lorca and the south-east2012Dec 2016, December floods, south-east and Mallorca; Sep 2019, Vega Baja DANA2016 · 2019Oct 2018, Sant Llorenc des Cardassar, Mallorca2018Jan 2020, Storm Gloria2020Oct 2024, Malaga, Guadalhorce valley (same DANA as Valencia); Nov 2024, Malaga, second DANA2024

Floods since 1982Damaging floods on this coast, by year. A selection, not a ranking; markers place each at a named town, not its extent. HANZE v2.1 to 2020 (Paprotny et al. 2024); 2024 from named news sources.

Wet ground, nowTopsoil after the first storm, filled where it is among the wettest third of 36 years for the date, darker the higher it ranks; outlined areas are within the usual range. ERA5, top 7 cm, 8 to 10 October.

AEMET's warningsAEMET's rain and thunderstorm warnings for 17 to 20 October, drawn at the level AEMET set. Issued up to 2026-10-15 21:50 UTC. Source: AEMET OpenData (Meteoalerta).

Not a forecast of ours, and no combined score: each layer is shown as its source published it.

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Data: ERA5 (Copernicus Climate Change Service), 2026 preliminary; AEMET (Barcelona Fabra, station 0200E). Compiled by the floods desk.