Vittrad

Long underwear might be a bit much of you are not going to spend a lot of time outside. That being said, I've been slogging around in subzero temperatures for the past few days and I've been wearing long underwear and/or tights under all my trousers for the past week (my commute involves a 1/2 a mile or so walk and yesterday when I left the house it was -9F).

If you have a good coat, scarf, hat that covers the ears, gloves, thick soled shoes, and wool socks and you should be just fine. Be sure to dress in layers because you'll want to peel that stuff off in the hotel.

FainaAgain is right about being able to wiggle your toes, I made that mistake the other day, my feet were in serious pain for a bit there until they warmed up. Today I wore my lose snow boots (as it was snowing when left the house) with the same thick wool socks and it made all the difference in the world.

TravelandTunes

Hat, gloves, scarf, warm coat.

TheWeasel

Layers - yeah, they're great when done properly, but you have to be able to put them on and take them off as the temps vary. Emphasis on the "take them off" part - it's no good to be sitting around sweating because you've got long underwear on and then go outside all wet and sweaty. As others have said, unless they're planning some major outdoors time, long underwear is overkill - unless they're elderly and get cold easily, and the conference rooms are cold and drafty. Then maybe they'd be more comfortable with an extra layer indoors, but that's totally independent of the outside temp.

dfrostnh

Agree that long underwear is probably unnecessary unless they expect to spend time outdoors. Conference rooms can be too warm but also sometimes too cold. Just spent time at a conference near LA and was uncomfortably cold in one conference room (and I'm from NH). Plan to wear jackets or sweaters than can be removed if too warm but they also might need that jacket.
Renting all wheel or front wheel drive car is a good idea. I think front wheel drive handles better in skids. Hats, scarves and gloves are important even when getting into a cold car. My preferred gloves are wool knit with an inside lining. Dressy looking knit gloves are pretty much worthless, esp the kind that stretch to fit your hand. Regular shoes are fine for conference. Shoes or boots that will keep your feet dry as posters have described are good for outdoor wear. Nothing worse than leather shoes and slushy streets/sidewalks. Drink plenty of water on the flight. Airplane air is too dry. I took Airborn before my flight (anti-cold remedy found in drugstores and supermarkets) which contains extra Vitamin C, etc. Don't forget lip balm.

tuckerdc

Earlier poster mentioned it, but it bears reiterating: If there is bad weather (snow/freezing rain) and your parents do not have experience in driving in such stuff, then they should NOT get behind the wheel. The reason we see all those accidents and darned chain reaction calamities during the infrequent episodes of weather down south is because of inexperience. And it only takes the first stomp on the brakes to create catastrophe.

highflyer

nytraveler mentioned windburn and I'd like to add moisturizer to your lists.

Even with a scarf I find my face gets burned quickly by the wind and recommend plenty of face cream. Keep as much of your face covered as possible. I haven't resorted to a balaclava yet but I might!