When an alert is first created in Helios (either by Helios itself or from the NWS), an effective time attribute is included that designates when the alert became active. We can list alerts that identify a weather transition by using the effective_min query option:

https://api.helios.earth/v1/alerts?event=road+wetness&effective_min=now-20m

In this case, our search results will include Road Wetness alerts that have been created in the past 20 minutes. Any longer duration alerts will be filtered from our result set.