While each mooring provider typically has their own rules the most frequent one I have seen is they "Warn/Require" you to leave the mooring if winds exceed 40 MPH. They are tested to a load limit and they feel that is about when they will take no liability for the moorings.
I've been in The Bight at Norman Island at night when a squall hit with winds in the 30MPH area for a few minutes. It felt as though the aft of the boat was being lifted out of the water by the bimini. Being a pitch black night I can't think of a worst time for 80 or so boats to all cut loose and head for the Sir Francis Drake Channel.
As it was and is typical, you can get some people who will anchor In Side a Mooring field. To keep from banging into other moored boats they have very short scope and they blew away in a few minutes.... saw a few hair raising near misses and I'm sure a few sets of shorts were soiled before it was over.
Don't think running engine to attempt to hold your ground would be effective as the wind changes quickly and you have no effective helm control unless you have water moving by your rudder which in mooring situation you do not have..... just winds slamming you. Probably not a bad idea to have it ready but your going to get blasted by the storm and will have limited if any view at off of what is going on around you.
Most stay below decks and batten down all hatches but prepare for what ever may come and hope it does not. Probably the safest thing to do most of the time. With very few boats in charter set up for center life line use with a harness, going forward to release or even to pick up the lines if they broke away would be dangerous on a heaving wind blown deck with out the safety harness and center life lines in place for attachment.
Winter months are when most fast moving squalls hit the VI's but they can and do happen year around so keep alert by monitoring the weather reports several times a day and always before you bed down for the night,
Most of the time you will have great sailing and very smooth moorings and anchorages.
Fair Winds and calm moorings
I prefer a sailboat to a motorboat, and it is my belief that boat sailing is a finer, more difficult, and sturdier art than running a motor.... Jack London