Weather Models: GFS and ECMWF
Ever seen one model predict rain while another says sunshine? Weather models are powerful but imperfect tools. In this guide you'll learn what they are, which to use when, why they sometimes fail, and how to interpret them like a professional meteorologist.
What is a Weather Model?
A weather model is a computer program that simulates the atmosphere using complex physical and mathematical equations.
Main Weather Models
1. ECMWF - The "King"
🇪🇺 European Centre • Resolution: 9 km • Range: 10 days • The most reliable globally • Best for: General prediction reference
2. GFS - Free and Widely Used
🇺🇸 NOAA • Resolution: 13 km • Range: 16 days • Tends to be more "extreme" • Best for: Long-range trends
3. ICON - Innovative
🇩🇪 DWD • Resolution: 13 km (6.5 km Europe) • Range: 7.5 days • Best for: European second opinion
4. HARMONIE-AROME (AEMET)
🇪🇸 AEMET Spain • Resolution: 2.5 km • Range: 48 hours • BEST for Seville at short range
Why Models Fail
The 5 Main Reasons:
1. Atmospheric Chaos (Butterfly Effect)
Small errors amplify exponentially. Reliability drops ~5% per day. Beyond 7 days, little better than chance.
2. Limited Resolution ("Pixelation")
Models divide atmosphere into 9-13 km boxes. Smaller phenomena (individual storms) are "pixelated" or ignored. Storm location ±30 km typical.
3. Insufficient Observations
Few data over oceans, deserts, poles. Satellites help but can't see "inside" clouds or measure wind directly.
4. Simplified Physics
Cloud formation, turbulence, rain are "parameterized" (estimated). Rainfall amount ±50% easily.
5. Smoothed Topography
Models "smooth" mountains. A 1500m range may appear as 800m, changing winds and local rain.
How to Interpret Model Maps
Precipitation Maps
- Colors show intensity (light blue = drizzle, dark blue/green = heavy, red = torrential)
- ❌ Exact amount unreliable (±50% margin)
- ❌ Precise storm location ±20-30 km
- ✓ General trend IS reliable (will rain/won't rain)
Wind Maps
- Arrows show where wind BLOWS TO
- ✓ VERY reliable (better than rain)
- ✓ Direction almost always correct
- ⚠️ Gusts can be 30-50% stronger than sustained wind
Practical Tips
1. NEVER Trust a Single Model
Always compare 2-3 models. If ECMWF, GFS and ICON agree → high confidence. If they disagree → low confidence, wait for updates.
2. Compare Multiple Runs
Models update every 6 hours. Check 2-3 consecutive runs. If all similar → stable = reliable. If each run changes a lot → unstable = unreliable.
3. Use Ensembles
Ensembles are 50+ simulations. If all members agree → 95%+ probability. If widely dispersed ("spaghetti") → low confidence.
4. Reliability by Time Range
- 0-3 days: 90-95% reliable
- 4-7 days: 75-85% reliable
- 8-10 days: 60-70% reliable
- >10 days: ~50% (coin flip)
Recommended Sources
- Windy.com: Excellent visual interface, free
- AEMET: Official forecasts for Spain
- Meteociel.fr: Detailed maps for advanced users
Conclusion
Models are incredibly powerful tools but not magic crystal balls. Remember:
- Models are GUIDANCE, not absolute
- Always compare 2-3 models
- Reliability drops drastically after 7 days
- For Seville: HARMONIE (0-48h) → ECMWF (3-10 days)