Forecasts and the ensemble¶
Two kinds of forecast¶
Deterministic: a single forecast, from the ECMWF IFS model (9 km in some regions, 0.25° ≈ 25 km in others; see Domains). It is the best single guess and drives every layer on the map after today.
Ensemble: about 50 forecasts from the ECMWF ensemble (ENS), each started from slightly
different conditions. Where they agree the forecast is confident; where they spread apart it is
uncertain. Boards with an ensemble show a Deterministic | Ensemble switch on the Map tab.
Reading the ensemble¶
Switch to Ensemble. The panel offers:
- Hourly / Accumulation
- Hourly: choose
Snowfall,PrecipitationorTemperature, step through the forecast. - Accumulation: total new snow over the next
1d,3d,5dor10dfrom the forecast start (default 5 days), in cm.
- Hourly: choose
- Percentile:
P10 (low),P50 (median),P90 (high). - Probability of snow: the share of ensemble members that give snowfall at each place.
| Percentile | Read as |
|---|---|
| P50 | the middle outcome: half the members give more, half less |
| P90 | a high case: only 1 member in 10 gives more. Use it for "how bad could it get?" |
| P10 | a low case: only 1 in 10 gives less. Use it for "how little could we get?" |
Example
For a pass, the 3-day accumulation reads P10 = 5 cm, P50 = 20 cm, P90 = 45 cm. Expect about 20 cm. There is a real (1 in 10) chance of 45 cm or more, so plan for that if closure matters.
The percentiles are taken at each place separately. A P90 map is not one possible weather scenario; it shows the high case everywhere at once.
The meteogram in ensemble mode¶
Clicking the map in Ensemble mode draws the median line with a shaded P10–P90 band. Snow depth and wind stay deterministic: they have no ensemble members.
How far ahead to trust¶
- Days 1–3: generally reliable for timing and broad amounts.
- Days 4–7: useful for trends; amounts uncertain. Look at the ensemble spread.
- Days 8–10+: indicative only. Treat a single deterministic map with caution.
Mountain precipitation is the hardest part of any forecast. Errors in where the storm hits matter more than the downscaling. See Known limitations.
When does the forecast update?¶
The board rebuilds once a day, using the latest ECMWF run available at build time. Boards with an ensemble also refresh the ensemble products on their own schedule. The Last run time in the header tells you when the board was built; the date slider marks where the forecast starts.
More on how forecasts are produced: Science → Forecasts.