Observed vs. predicted water level: the coastal anomaly worth watching
Why the gap between NOAA observation and astronomical prediction can matter more than the prediction alone.
Two different numbers
The prediction is primarily astronomical. The observation is what a gauge actually measured. Wind, atmospheric pressure, river flow and ocean setup can move the observation above or below the prediction.
Our anomaly calculation
For a supported NOAA station we take the newest verified water-level observation and compare it with the nearest timestamp in the same-datum prediction series. The difference is shown in feet and never hidden inside a black-box score.
What a positive anomaly means
A positive number means observed water is running above the astronomical prediction at that gauge. It does not by itself mean flooding is occurring.
Why timestamps matter
A prediction from a different hour can make the comparison meaningless. The site therefore reports the observation time, prediction time and retrieval time together.
Use the official alert layer
When coastal flooding is a concern, NWS alerts and local emergency guidance are the authoritative layer. The anomaly is context, not an emergency product.