Kate, I do think we'll get at least low 40's on Thursday night. This looks like a much stronger, more organized weather front than the last couple we've had. You're surely warmer than I am, being out there on the island, but still a 40deg. night will set your plants back some if you don't cover them.
I've been protecting my orchids for all these cold nights that went down below 50 last week. My really warm growers are in our bathroom until things stabilize, Vandas and anybody with blooms or buds is getting the 'spa' treatment in there. (sun through the windows, warm shower daily) The others are in my little greenhouse shelf units. I posted a pic of the setup out there in the "our growing area" thread last Friday. I'm using one big pillar candle for heat inside those and they stay above 50 all night with that, and zipped up of course. All my other tropicals go under the big frost cloth tent, and they've not missed a beat so far.
I also make sure everyone is well watered before we get cold temperatures. That's the best prep you can do. The humidity is pretty low when it's cold here, so when the orchids are indoors, I hang wet towels and placemats around the house wherever the drips don't matter, and I spread them on the countertops, too. If I get really energetic, I can get the windows fogging up inside the house some nights
and then I know I've got it to 'greenhouse' status. Hey, it's healthy for me as well as the plants!