The idea in one line
We look at the next 48 hours of forecast data (from Open-Meteo) for your exact location, score four separate winter hazards from 0 to 100, and your risk index is the single highest of those four scores — the most severe hazard, not an average. A higher number means more dangerous winter conditions are expected.
The four hazards we measure
| Hazard | What we look at | Reaches 100 when… |
|---|---|---|
| Snow | Total snowfall over 24 hours, plus whether any single hour is intense (≥ 2.5 cm/h). | About 21 cm (8 in) in 24 h on its own; ~15 cm (6 in) already scores about 70. |
| Ice (freezing rain) | Liquid falling into sub-freezing air — ice accretion, in millimetres. | 6 mm (~¼ in) of ice or more. 1–6 mm scores about 47–80. |
| Cold / wind chill | The lowest “feels-like” (apparent) temperature. | Wind chill of −28 °C (−18 °F) or colder. −18 to −28 °C scores about 55–85. |
| Blizzard | Consecutive hours with strong gusts (≥ 56 km/h / 35 mph), falling snow, and very low visibility (< 400 m). | 3 hours or more of those combined conditions. 1–2 hours scores about 65–80. |
These thresholds are modelled on the kind of criteria the National Weather Service uses, expressed in round numbers; exact official criteria vary by region and local forecast office.
How the four scores become one number
Each hazard gets its own 0–100 score, and the risk index is simply the maximum of the four. We deliberately do not add them up: a single dangerous hazard (say, heavy ice) is dangerous on its own, and averaging it with calmer hazards would hide it.
A worked example
Suppose the forecast for the next 24 hours shows about 18 cm (7 in) of snow, with a peak of 2 cm in the heaviest hour, a lowest wind chill of −10 °C, and no freezing rain or blizzard conditions:
- Snow score: 18 cm is 1.2× the 15 cm reference → 1.2 × 70 = 84 (no heavy-burst bonus, because no single hour reached 2.5 cm).
- Ice score: 0 — no freezing rain.
- Cold score: 0 — −10 °C is milder than the −18 °C threshold.
- Blizzard score: 0.
The risk index is the highest of these — 84 — and because the 24-hour snow total is over 15 cm, the status is labelled a Winter Storm Warning (estimate).
Risk categories
Alongside the number we label the most severe hazard, in this priority order:
| Label | Triggered when (next 48 h) |
|---|---|
| Blizzard conditions | 3+ hours of combined gusts, snow and low visibility |
| Ice storm | 6 mm (~¼ in) or more of ice accretion |
| Winter storm warning | 15 cm (6 in) or more of snow in 24 h |
| Extreme cold | Wind chill of −28 °C (−18 °F) or colder |
| Winter storm watch | 8 cm+ snow in 24 h with at least one hour of ≥ 1 cm/h (building toward warning levels) |
| Winter weather advisory | 8 cm+ snow, or 1 mm+ of ice |
| Wind chill advisory | Wind chill of −18 °C (0 °F) or colder |
| No winter hazards | None of the above in the next 48 hours |
Official warnings always take priority
For locations in the United States we also check the National Weather Service active-alerts feed. If an official winter warning, watch or advisory is in effect, that official alert is shown and takes priority over our computed estimate — the index is then a secondary, clearly-labelled estimate. Outside the US, official feeds vary by country, so we show the computed estimate and link you to the relevant national weather agency.
Limitations — please read
- The index is built from a forecast and is only as accurate as the underlying weather model; conditions can and do change.
- Thresholds are general, round-number approximations. Real NWS criteria differ by region and forecast office — snowier areas use higher snow thresholds.
- It is not an official warning. Always confirm with, and follow, the National Weather Service and your local authorities before making safety decisions.
- Snow and ice amounts are model estimates; local elevation and microclimates can differ from the forecast.
Written and maintained by the Winter Storm Warning editorial team. More about our data sources and independence.