Efficient Rainfall Situation in Seville
Why does Seville sometimes receive in 24 hours what should rain in 3 months? The answer lies in an exceptional meteorological configuration combining deep low-pressure systems, subtropical atmospheric rivers, and cold upper-level troughs. In this guide you'll learn what these phenomena are, how they form, and why they can transform Seville's dry climate into a devastating flood scenario.
The Rainfall Challenge in Seville
Seville has a continentalized Mediterranean climate: Very dry summers (June-September: nearly 0 mm), annual precipitation of 500-600 mm (modest), rain concentrated in autumn-winter, and rain episodes normally short (a few hours). But occasionally something extraordinary happens: 100-200 liters/m² in 24-48 hours.
What is an Atmospheric River?
An atmospheric river is a narrow band of concentrated moisture that flows through the atmosphere like a river on land, but in the sky.
Dimensions:
- Length: 2000-3000 km (from tropical Atlantic to Spain!)
- Width: 400-600 km (narrow, concentrated)
- Height: 2-3 km from surface
- Moisture: 80-95% humidity
- Water content: 2-3 cm precipitable water
- Speed: 50-80 km/h (SW/S winds)
The Ideal Synoptic Configuration for Seville
For an atmospheric river to discharge over Seville, several ingredients must work together:
INGREDIENT 1: Deep Low West/Southwest
Position: Atlantic, between 35-40°N and 15-20°W (west of Portugal). Depth: Less than 970 hPa. Movement: Slow or stationary.
INGREDIENT 2: Subtropical Atmospheric River
Origin: Tropical Atlantic (15-25°N) or Azores region. Route: From SW, entering through Gulf of Cádiz. Duration: Must remain active 24-72 hours.
INGREDIENT 3: Strong South/Southwest Winds
Sustained speed: 40-70 km/h. Gusts: Can exceed 100 km/h. Direction: South or southwest (180-240°). Persistence: At least 24 continuous hours.
INGREDIENT 4: Cold Upper-Level Trough
Height: 5-9 km above sea level. Temperature: -15°C to -25°C (polar/Arctic air). Creates brutal thermal contrast: +20°C at surface, -15°C at 5 km = 35°C difference generating extreme instability.
INGREDIENT 5: Persistence (Blocking)
The configuration must remain stable for 24-72 hours. This occurs when: blocked circulation pattern, high pressure to north "braking" the low, or very slow-moving/nearly stationary low.
Subtropical Low-Pressure Systems
MEDICANES: Mediterranean "Mini-Hurricanes"
Medicanes (Mediterranean hurricanes) are tropical or subtropical cyclones forming in the Mediterranean. RARE but extremely dangerous.
Characteristics:
- Size: 100-300 km diameter
- Central eye: 10-40 km diameter with calm
- Winds: 90-160 km/h (category 1-2 hurricane equivalent)
- Rain: 200-500 L/m² possible
- Structure: Symmetric spiral with cloud bands
Historical Episodes in Seville
December 1876 - The Great Flood
Seville's worst hydrological disaster. Guadalquivir massively overflowed. Entire city flooded (15% of urban area underwater). Torrential rains for 5 consecutive days.
January 2010 - The Deluge
120 mm in 24 hours in Seville city. Perfect configuration: deep trough + atmospheric river. South winds at 70 km/h with 100 km/h gusts.
How to Predict These Episodes?
5-7 Days in Advance
- Look for: Deep low forming in mid-Atlantic
- Check: Integrated water vapor (IWV > 25 mm = atmospheric river)
12-24 Hours in Advance
- Surface winds: From south/southwest at 40+ km/h?
- Satellite: Subtropical cloud band visible?
- AEMET warnings: Orange or red for rain?
Climate Change Impact
Global warming is affecting these events:
- Warmer Atlantic: +1-2°C since 1900 = more evaporation
- Wetter atmosphere: +7% humidity per °C = more intense atmospheric rivers
- More intense events: When it rains, it rains HARDER
Conclusion
Atmospheric rivers and ideal synoptic configurations are responsible for the most intense rainfall Seville can experience. Understanding these phenomena allows better interpretation of weather forecasts and preparation days in advance.