I would love to notify guests that our wedding will be outside and with that we will have to go with the flow. We plan on having a tent, but the ceremony area will be untented, so we will have to have a plan B should the weather not cooperate.
The event will not be casual, but we want to remind the girls that they will be on grass the whole night, so may want to pick their shoes wisely.
I have seen poems that state the above in cute ways - of course Im not able to find them now.