From first-hand experience...as soon as it's likely to hit with time to pack up & escape safely. BUT, if hurricane imminent ANYWHERE you live or travel to reach WDW, can cancel reservations with no penalties. AND, if don't wish to cancel, take risk campground will be closed & travel in anyway, Fort closes, Disney GREAT at outting you up in other on-site accommodations to fit your party. In 2017 husband and I arrived Sunday for Fort resv's, closed, put up in All-Star sports with other perks (camper food to concierge & unlimited access to fridge/freezer, extra FPs, etc.) Plus park big 5th wheel & tow vehicle in hotel parking lot with plenty of room - no disconnect! Disney does it RIGHT! All that said, Disney will be in touch days before Florida danger imminent.
You will be better off calling. I think they let you change your plans last minute with no charge the last time a hurricane came through.
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