It depends on the cooker you use, but even a thin walled crappy cooker should be able to cook fine in 30 degree weather. You might jsut need a little more fuel is all.
I have cooked at below zero temperatures with my WSM's (as low as -6) without having to do anything more than simply adding more lit coals when doing the Minion method and opening the vents all the way. No problems at all.
High wind though...that's antoher story with those WSM's. At -6 with high wind, that might not work, but I've never had the gift to try and cook in that weather.
You MUST go ahead and cook. Don't let things like the weather get in the way. What if it starts a surprise rain storm mid-cook? Are you just going to give up?
Personally, I enjoy cooking on both the hottest and coldest days of the year, just to spite the weather. Maybe I'm pissing ol' Mother Nature off though, and that's why weather is so weird lately. Al Gore can take all the credit he wants, but it was I who pissed off that old bag.