topo^alpin computes a weighted ensemble value from the six individual models. The calculation runs live in the browser – when models are toggled off, the forecast updates immediately.
Model Weights
| Model | Weight | Rationale |
|---|---|---|
| ICON EU | 2.0 | Best Alpine topography, full variable set |
| ICON D2 | 2.0 | Highest resolution (2.2 km), convection-permitting |
| AROME | 2.0 | High resolution (1.3 km), strong in Western Alps |
| ECMWF | 1.5 | Leading global model, only precipitation type |
| AIFS | 1.5 | AI-powered, strong for large-scale patterns |
| GFS | 1.0 | Coarser resolution, but longest forecast range |
Temperature (min/max) and gusts are computed as absolute extremes across all active models, not as averages.
WMO Weather Code Voting
Each model provides a WMO code at the block midpoint. topo^alpin selects the most severe code via severity ranking:
| Severity | WMO Codes | Description |
|---|---|---|
| 9 | ≥ 95 | Thunderstorm |
| 8 | 66–67, 85–86 | Freezing rain, snow shower |
| 7 | 71–79 | Snow |
| 6 | 56–57 | Freezing drizzle |
| 5 | 80–82 | Rain shower |
| 4 | 61–65 | Rain |
| 3 | 51–55 | Drizzle |
| 2 | 45, 48 | Fog |
| 1 | 1–3 | Cloudy |
| 0 | 0 | Clear |
Outlier filter: High severity (≥ 8) requires a total weight ≥ 3.0, otherwise it falls back to the next lower level. AROME is excluded (provides no weather code).
Downgrade Rules
- Snow downgrade: Ensemble snowfall < 0.5 cm or fewer than 2 models with snow → snow symbol removed. If sunshine duration > 30% of block duration → snow showers instead of overcast.
- Shower downgrade: < 1.0 mm/6h and ≤ 50% of models → cloudy instead of shower
- Light precipitation: < 0.5 mm/6h and < 40% of models → no precipitation symbol
- Fair weather upgrade: No precipitation and cloud base > 1500 m above location → partly cloudy
- Sunshine upgrade: No precipitation and sunshine duration > 50% of block duration → mostly sunny
- Changeable weather upgrade: Precipitation > 0.1 mm and sunshine duration > 30% of block duration → shower icon (rain or snow showers depending on phase)
Warnings
- Freezing rain: Only ECMWF provides the precipitation type. Displayed when
precipitation_type = 3occurs in at least one hour within the block. - Wet snow: ECMWF
precipitation_type = 6or8.