Learn how we turn NWS forecasts and alerts into impact levels for your ZIP code, and what those levels are not
Impact levels are a rules-based estimate. They are not an official forecast, not a probability, and they never replace an official alert or an order from local emergency management.
What goes in
For the point you enter we read the National Weather Service gridded forecast, active NWS alerts that apply to that point, and recent pressure observations from the nearest station. Nothing else.
What comes out
Six areas of life (School & work, Travel & roads, Power & utilities, Health & safety, Home & property, Flooding) across four windows (6, 12, 24, 48 hours). Each cell is one of Low, Moderate, High, Severe, or "Can't assess" when data is missing. We never show "Low" because data is missing.
Thresholds
These values are the exact ones in our code.
| Measure | Thresholds | Levels |
|---|---|---|
| Travel: wind gusts (mph) | 35 / 45 / 58 | Moderate / High / Severe |
| Power: wind gusts (mph) | 45 / 55 / 65 | Moderate / High / Severe |
| Home: wind gusts (mph) | 50 / 58 / 74 | Moderate / High / Severe |
| Travel: snow (in) | 2 / 6 / 12 | Moderate / High / Severe |
| Power: snow (in) | 8 / 14 / 24 | Moderate / High / Severe |
| Travel: ice (in) | 0.05 / 0.1 / 0.25 | Moderate / High / Severe |
| Power: ice (in) | 0.1 / 0.25 / 0.5 | Moderate / High / Severe |
| Flooding: rain in 6 h (in) | 1 / 2 / 3 | Moderate / High / Severe |
| Flooding: rain in 12 to 48 h (in) | 1.5 / 3 / 5 | Moderate / High / Severe |
| Health: heat index (°F) | 95 / 105 / 115 | Moderate / High / Severe |
| Health: wind chill (°F) | 15 / 0 / -20 | Moderate / High / Severe |
Heavy snow of at least 6 in together with gusts of at least 35 mph is treated as a whiteout risk and sets travel to Severe.
Official alerts set a floor
An active official warning puts a minimum level under the matching areas of life, so an estimate can never look calmer than the National Weather Service. Alerts that have expired or not started yet are ignored. We list 22 alert families with floors.
Pressure trend
We compare station pressure over about three hours. A fall of 3 hPa or more is "falling", and 6 hPa or more is "falling fast". With less than about 90 minutes of data we show "unknown".
Limits
Forecasts change and can be wrong. Postal-code locations use the center of the area. Thresholds have not been validated against historical events yet, and we will note here when that is done. If something looks wrong, see corrections.