Move the giant before the storm arrives
Calming a Flemish Giant starts before the first thunder, because moving a very large rabbit mid-storm adds handling stress to noise stress. Bring his carrier or pen into the quietest interior room, away from windows where lightning flashes and rain hammering are strongest. A Flemish Giant needs a hide big enough to turn around in, so use his usual shelter or a large box with two openings rather than a cramped pet hide sold for small breeds. Lay familiar bedding that smells of his normal enclosure inside it. Settling him while the house is still calm lets him map the space and choose his spot before the noise begins.
Sound, light and vibration all reach a big rabbit
Storm stress in a Flemish Giant comes through several senses at once, so reduce all of them together. Thunder vibrates through floors, and a heavy rabbit resting his whole body on boards feels it plainly. A thick blanket over the pen, leaving airflow, dulls both sound and lightning flashes. A rug or folded towels under the enclosure softens vibration. Low background sound from a radio in another room can mask sudden bangs better than silence between claps. Keep household emergency activity, torches, alarms and shouting, away from his room where possible, and ask children to visit him quietly rather than crowding the pen.
Hay, water and the eating check that matters
Food is both comfort and health monitor for a Flemish Giant during a long storm. Give him a generous pile of his usual hay to burrow into and eat, a heavy water bowl that his size cannot tip, and a small amount of his normal greens once he is settled. Rabbits soothe themselves by chewing, and steady hay intake keeps his gut moving. Check every couple of hours that the hay pile is going down and droppings are appearing. A giant who has eaten nothing for many hours after the storm, sits hunched and grinds his teeth needs veterinary advice promptly, because a frightened rabbit's gut can slow dangerously.
Your presence, without fussing over him
Presence calms a Flemish Giant best when it is calm itself, so sit near the pen reading or talking quietly rather than repeatedly reaching in. Some giants lean against a trusted person's hand through the bars; others want to be left inside the hide until the noise passes, and forcing strokes on a rabbit in hiding raises his heart rate instead of lowering it. Watch his ears and posture for information: ears relaxed sideways and a stretched-out body mean he is coping, while wide eyes, constant thumping and flattened crouching mean the room needs to be made quieter or darker still. Thumping is his alarm signal, so answer it by checking the environment, not scolding.
If the emergency means leaving the house
Evacuation changes the plan for a Flemish Giant, because his size rules out small carriers and last-minute improvisation. Keep a large, sturdy carrier or crate he fits in comfortably, lined and ready, and practise short sits in it with hay so it is not a new object on the worst night. Load him into the vehicle early, cover the crate, and keep it flat and secure so it cannot slide. Bring hay, water and a familiar blanket, and keep the crate away from barking dogs in any shelter or car. Once placed, leave him covered and quiet rather than opening the crate repeatedly for reassurance.
Splitting the storm watch so nobody forgets a check
Long storms and overnight emergencies outlast one person's attention, so the eating checks, water top-ups and quiet visits need to be shared deliberately rather than assumed. Decide in advance who checks the giant at which point, who has the vet's number, and who moves him if the household has to leave. In KinsLe, Care Teams let you assign those storm duties for your Flemish Giant to specific family members or helpers, and each finished check is marked with a timestamp so everyone can see he has been seen to.