You can use the rain delay feature. Basically have your rain sensor send a rain delay command, with the maximum amount of rain delay time; then when the rain sensor is deactivated, send a rain delay value of 0 to clear it out. The rain delay settings is included in the /cv command. Check the firmware API for example:
https://openthings.freshdesk.com/solution/articles/5000716363-api-document-firmware-2-1-7-current-