Even for the Dendrobiums, depends on which ones. Nobile type, lindleyi, speciosum, kingianum, and probably most of the deciduous ones from the Himalaya area and south-west China might benefit, but there area plenty of Dens from more tropical areas that don't need or want that chill. Time for some research... when in doubt, better to err on the warmer side. (Better to miss the flowering than to kill the plant) Especially since in your area, Mother Nature can surprise with more than forecast cold. Down to the mid-50's F likely is OK for all, but lower than that, some caution is in order. Especially since it is sudden - many orchids can adapt to cold with a gradual cool-down, but are likely to resent a sudden jolt of cold.
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