Forecast Products Overview
LMKwhen supports threshold-based alerting on NWS gridded forecast data via the National Digital Forecast Database (NDFD). Forecasts are extracted per location at ingest time and evaluated against your configured thresholds.
Available products
Temperature
| Product | Unit | Description |
|---|---|---|
| Temperature (high) | °F | Daily max dry-bulb temperature |
| Temperature (low) | °F | Daily min dry-bulb temperature |
| Apparent Temp (high) | °F | Daily max apparent (feels-like) temperature |
| Apparent Temp (low) | °F | Daily min apparent (feels-like) temperature |
Heat & safety
| Product | Unit | Description |
|---|---|---|
| Heat Risk | Index 0–4 | NWS experimental heat risk index |
| Heat Stress (WBGT) | °F | Wet Bulb Globe Temperature — daily peak. NIOSH work/rest reference: 82°F alert limit, 88°F exposure limit |
Wind
| Product | Unit | Description |
|---|---|---|
| Wind Speed | mph | Sustained 10-meter wind speed daily max |
| Wind Gusts | mph | Maximum 3-second wind gust daily max |
Precipitation & winter
| Product | Unit | Description |
|---|---|---|
| Precip Probability (12hr) | % | Chance of measurable precipitation in a 12-hour period, daily max |
| Total Precipitation | inches | Running 3-day accumulation total through each forecast day |
| Snow Accumulation | inches | Daily snowfall total |
| Ice Accumulation | inches | Ice accumulation (seasonally available) |
Humidity
| Product | Unit | Description |
|---|---|---|
| Max Humidity | % | Daily maximum relative humidity |
| Min Humidity | % | Daily minimum relative humidity. RH at or below 15% is a common Red Flag Warning criterion |
Tropical
Tropical products are populated only while the National Hurricane Center has an active advisory affecting your location — outside of an active system they report no value.
| Product | Unit | Description |
|---|---|---|
| Wind Threat | Level 0–10 | Tropical-cyclone wind threat (None / Elevated / Moderate / High / Extreme) |
| Surge Threat | Level 0–10 | Storm surge threat (None / Elevated / Moderate / High / Extreme) |
| Rain Threat | Level 0–10 | Flooding rain threat (None / Elevated / Moderate / High / Extreme) |
| Tornado Threat | Level 0–10 | Tropical tornado threat (None / Elevated / Moderate / High / Extreme) |
| Wind 34kt+ | % | Probability of sustained winds ≥ 34 kt (tropical-storm force) |
| Wind 50kt+ | % | Probability of sustained winds ≥ 50 kt |
| Wind 64kt+ | % | Probability of sustained winds ≥ 64 kt (hurricane force) |
The four threat products use NHC's Hurricane Threats & Impacts scale (None / Elevated / Moderate / High / Extreme) — you pick a minimum level from a dropdown rather than typing a number. See Ordinal threat levels.
Forecast days
Each product covers up to 3 forecast days:
- Day 1 — today's forecast
- Day 2 — tomorrow's forecast
- Day 3 — forecast for the day after tomorrow
Day boundaries follow NDFD convention: 06:00 UTC, not midnight. See NDFD Gridded Forecast Alerts for a full explanation.
Alert behavior
When a forecast value crosses your threshold for a given day, an alert fires. If a later forecast update worsens the value, a new alert fires. If the forecast improves and drops below your threshold, an all-clear is sent.
Each alert tracks forecast continuity — upgrades, downgrades, and reconfirmed values are all recorded in the alert timeline.
Polygon and circle locations
For polygon or circle locations, NDFD values are sampled at the centroid of the shape. Accuracy decreases as polygon size increases. A warning is shown in the alert rule wizard when any in-scope location is a polygon or circle.