December is usually safe- the worst driving is in the spring when the snow is wet enough to pack your treads and make your tires essentially bald. The canyon only closes all day a dozon times a year or so, but that's when you want it.If you're staying up there, you win big, assuming you're not interlodged. Other times they open up after a few minutes to an hour or so wait, the worst problem is the four cups of coffee you drank for breakfast.
A 4WD is always best but expensive to rent. You can always park at the lower lot and take the bus. Although I've never done this, even when I had my FWD Honda Civic. The line forms for miles when it's closed for blasting. There's an AM radio station, about 560 (or 650?-it's on signs as you near the canyon) that gives you updates and can help you call it. Sometimes, but not very often, there's a Highway Patrol officer at the mouth of the canyon turning 2 wheelers away. Otherwise, all the 2 wheelers slide around all the way up the canyon to the amusement of the locals. But anyone from New England or the Midwest has done enough winter driving to do fine. It's the Californians who are really comical.
When there's a big huge dump, Alta is half closed anyway. Of the powder playgrounds,usually only Wildcat and Rustler Ridge will be open at first. But that's still a lot of territory. The blue and green stuff is always open, except for the part of Main Street beneath the Baldy chutes. The patrol will be working hard to stabilize Baldy so that Ballroom and the Shoulder can be opened, sometimes late morning, after lunch or even the next day. Supreme and the Backside sometimes don't open till afternoon if at all, and you have to "settle" for Greely (check the map, I'm not giving directions!) Devil's Castle won't open till a day or two later- it's usually closed in the early season anyway because it takes so much snow to cover it.
So, It's a judgement call and really, a guess- but you can't lose. Lots of people peel off at the sight of a three mile line and head for Big Cottonwood- that's usually open, very close and gets the same snow. Solitude's got Honeycomb Canyon that's heaven- I haven't been there in years but now they have a new lift back there so no more long run-out. Brighton is also supposed to be great, but as an Altaholic, I would't really know.
And 49's right about PC resorts. Interstate 80 is almost never closed, although it did close once last year in a blizzard around Christmas at Parley's Summit. Shouldn't be a problem- if it is ,you don't want to be out anyway. Canyons has pow everywhere, PCMR has Jupiter Bowl and DV, well , we don't talk about DV on a powder day except to say my best day last season was DV, late March, 30" of fluff so light a handfull, when dropped, blew away before it hit the ground- and the same twenty people, over and over, all day...
A general rule- if the storm is from the northwest, the frontside gets pasted- Alta, Bird, etc. If from the south, Park City gets most of it.