Choose the decision time
Agree when you'll check the forecast and contact the operator. Make the final decision early enough that guests can change their departure plans. Some activities also require a decision by the provider on the day.
Don't treat a forecast as permission to ignore an operator's closure or cancellation.
Make the alternative real
Check that the backup venue is open, can fit the group and suits the budget. Keep it near the original area where possible. An indoor idea several hours away isn't a practical replacement.
Know which deposits can move or be refunded. Get those terms before booking the outdoor activity.
Send one clear update
State the chosen venue, meeting point, time and any cost difference. Tell guests whether the original booking has been cancelled or moved.
Keep both options in your Outxie plan while organising, then clearly identify the one that is going ahead. If no suitable alternative exists, postpone. A date change can be a better decision than forcing the group into an activity nobody chose.
Rules, fees and opening hours change. Confirm anything that matters before you rely on it.