I used to forecast the weather for the Air Force, then the NWS, then I contracted in the Middle East. I was in aviation weather forecasting for 10 years. We forecasted within a 5 mile radius around the airfield and that was measured from the center point of the runway.
Now most civilian runways have more than 1... so I would venture an educated guess it would be from the center of the airport property itself. That's how Im interpreting it.
Appreciate the responses everyone! Good discussion here.
Hello Eliphion. The lat/long coordinates of a US airport's reference point may also be looked up on this website: AirNav: Airport Information