NDFD Gridded Forecast Alerts
The National Digital Forecast Database (NDFD) is a gridded dataset of NWS forecast values — temperature, wind, precipitation, and more — for every point in the US, updated multiple times daily.
LMKwhen lets you set threshold-based alerts on these forecast values. When a forecast for your location exceeds your threshold, you receive an alert.
Available forecast parameters
Products are grouped by category in the rule wizard's tile picker. See Forecast Products Overview for the full catalog with units.
| Parameter | Example use case |
|---|---|
| Temperature (high) | Heat stress above 95°F |
| Temperature (low) | Freeze risk at or below 32°F |
| Apparent Temp (high/low) | Feels-like extremes for outdoor crews |
| Heat Risk | NWS heat-health index at or above Major |
| Heat Stress (WBGT) | Work/rest scheduling at or above 82°F |
| Wind Speed | Work stoppage at or above 25 mph |
| Wind Gusts | Property damage risk at or above 50 mph |
| Precip Probability (12hr) | Rain delay planning at or above 70% |
| Total Precipitation | 3-day running total at or above 3 in |
| Snow Accumulation | Plowing / maintenance scheduling |
| Ice Accumulation | Winter storm prep (seasonally available) |
| Max Humidity | Mold / moisture-sensitive operations |
| Min Humidity | Fire danger at or below 15% (Red Flag) |
| Tropical threat grids | Hurricane wind / surge / rain / tornado threat |
| TC wind probabilities | Chance of 34 / 50 / 64 kt winds |
How thresholds work
You set a threshold value for a parameter (e.g. wind gusts above 45 mph). LMKwhen checks the NDFD forecast for each of your locations. When the forecast first crosses your threshold for a given day, an alert fires.
The alert does not re-fire on every forecast update — only when the threshold is newly crossed or the forecast worsens significantly.
Choosing a threshold direction
Each product's tile pre-selects the comparison direction that matches the metric, so you rarely need to change it:
- Maxima, accumulations, and probabilities default to at or above
(
≥) — e.g. high temperature, wind gusts, total precipitation, precip probability, WBGT. - Minima default to at or below (
≤) — e.g. low temperature and Min Humidity (you want to know when humidity drops to a fire-danger level, not when it rises above one).
You can still change the operator on any generic product if your use case needs the opposite direction.
Ordinal threat levels
The four tropical threat grids (Wind, Surge, Rain, Tornado) don't take a free-typed number. They use the National Hurricane Center's Hurricane Threats & Impacts (HTI) categories, and the wizard shows a dropdown of those levels instead of a text box. You pick a minimum level and the rule fires when the forecast reaches or exceeds it — the same "at this level or higher" behavior as SPC categorical risk.
The five HTI tiers share the same names across all four threats, with threat-specific detail in parentheses:
| Level | Tier | Wind | Surge | Rain | Tornado |
|---|---|---|---|---|---|
| 0 | None | < 39 mph | little/none | little/none | not expected |
| 4 | Elevated | 39–57 mph | > 1 ft | localized | a few |
| 6 | Moderate | 58–73 mph | > 3 ft | moderate | several |
| 8 | High | 74–110 mph | > 6 ft | major | many |
| 10 | Extreme | 110+ mph | > 9 ft | extreme | outbreak |
Tropical products only carry a value while the National Hurricane Center has an active advisory affecting your location. The TC wind probability products (34 / 50 / 64 kt) are ordinary percentages and use the numeric threshold path.
Understanding forecast timing
:::info NDFD uses a 06:00 UTC day boundary This is one of the most common sources of confusion. NDFD does not use midnight as the day boundary — it uses 06:00 UTC. :::
What this means:
- Day 1 = the current forecast day, starting at 06:00 UTC
- Day 2 = tomorrow's forecast day, starting at 06:00 UTC the following day
In US local time (Central):
- 06:00 UTC = 1:00 AM CST / 2:00 AM CDT
So after midnight but before approximately 1–2am local time, LMKwhen still considers it the current Day 1 forecast period. This matches how NWS forecasters think about forecast periods.
Why does this matter?
If you're expecting a "today" high temperature forecast at 11pm local time, you're still in Day 1. The forecast day doesn't roll over until approximately 1–2am local time, not at midnight.
Forecasts update throughout the day
NDFD grids are updated multiple times daily as NWS forecasters refine their models. LMKwhen ingests updated forecasts on a regular schedule. If a forecast worsens after your initial alert, a new alert fires. If it improves and drops below your threshold, an all-clear is sent.