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
| Parameter | Example use case |
|---|---|
| Temperature (high) | Freeze risk below 32°F |
| Temperature (low) | Heat stress above 95°F |
| Wind speed | Work stoppage above 25 mph |
| Wind gusts | Property damage risk above 50 mph |
| Precipitation probability | Rain delay planning |
| Snow accumulation | Plowing/maintenance scheduling |
| Relative humidity | WBGT heat stress calculation |
| Wind chill | Cold weather work safety |
| Heat index | Outdoor worker safety |
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 exceeds 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.
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.