There's a big thread on this very subject in "Horse Talk". You might pick up some tips there Horses and Hurricanes... O.o ...prep? One of the things you might consider, especially if you're right in Sandy's path, is to spray paint your cell phone number, with area code, on both sides of each horse. A Katrina rescuer said it would have made things a lot easier, in trying to find the owners of horses.
Automotive paint works great, won't wash off and will NOT hurt the horse --
The worst is they will look like auction horses until the paint comes off but at least you know they can be quickly returned if they get off the farm.
While I agree with taping information to a halter, halters often get off the horse in a wild escape but the paint won't.
As far as "in" or "out"? It's a crap shoot. We live in an area that sees a lot of straight-line winds and horrendous lightening storms.
I keep the horse routine the same, regardless of weather; in at night, out in the daytime.
Their favorite fence row tree to stand under got wiped out by lightening in a night storm, last Spring. The odds are good they'd've been under it had I left them out. The odds are good I'd've lost all four of them, since that ancient oak came crashing across the fence in our direction
If you're deep into the path of this gawrsh-awful storm, go with your instincts, Pray, and hold your breath. This is going to be a rough and history making ride for many folks