Alert for a custom data field

Is it possible to set up an alert based off of a custom data field? I've tried an alert for pace, but the calculated pace jumps around a lot due to patchy GPS, and so is impractical. My work around was to try to set up an alert based off of a rolling 5s average pace (there are already custom data fields on the store that report this), but I can't figure out how to set up an alert based off of this. Are Alerts but of the SDK?