topo^alpin calculates a thunderstorm risk score (0–8 points) from five indicators sourced from the ICON, GFS and ECMWF models. The score is computed per 6-hour block and colour-coded in the meteogram.
Score calculation
| Indicator | Threshold | Points |
|---|---|---|
| CAPE (max across models) | > 500 J/kg | +2 |
| > 200 J/kg | +1 | |
| CIN (min across models) | < 25 J/kg (low barrier) | +1 |
| Lifted Index (min) | < −4 (strong instability) | +2 |
| < 0 (marginal instability) | +1 | |
| Showers (avg across models) | > 0 mm | +1 |
| Convective cloud top | > 7000 m | +1 |
Maximum: 8 points (CAPE 2 + CIN 1 + LI 2 + showers 1 + cloud top 1 + WMO bonus 2, clamped to 8)
Moisture gate
The score is only activated when sufficient moisture is present. At least one condition must be met:
- Precipitation probability > 20 %
- Showers > 0 mm
- Relative humidity > 70 %
Without moisture (e.g. during foehn conditions), the score is set to 0 — high instability alone does not produce thunderstorms.
WMO thunderstorm bonus
When weather models themselves predict a thunderstorm (WMO code ≥ 95) and the weight sum of those models is ≥ 3.0, an additional +2 points are added. This prevents the additive score from masking a clear model signal.
Meteogram display
| Score | Colour | Meaning |
|---|---|---|
| 0–1 | — | No risk |
| 2–3 | Orange | Elevated risk |
| ≥ 4 | Red | Warning |
Data sources
- CAPE: All models (absolute maximum, conservative)
- CIN: GFS only (only model providing convective_inhibition)
- Lifted Index: GFS only
- Showers: ICON EU, ICON D2, GFS (weighted average)
- Convective cloud top: ICON EU, ICON D2 (sentinel value −500 = no convective clouds is filtered)
Thresholds
The CAPE thresholds (200/500 J/kg) are lower than typical flatland values (400/1000 J/kg) because alpine convection triggers at lower energy levels — orographic lift and diurnal thermal cycling reduce the effective trigger threshold.