Don't do chains, socks, or studs, and you're taking a big risk driving the OEM M+S (or worse, the 21" OEMs) up to Brian Head in Feb. The road up to BH is on UDOTs list of Chain/Snowtire Controlled Roads from October-April, and Feb is usually a pretty solid snowfall month:
Utah Department of Transportation,
This may sound like overly cautious advice, but you should just buy a set of Michelin Xi3 Snows for this trip and any future ski trips. Cheap insurance/assurance. Over three winters, I can tell you they offer incredible grip in ice and snow for a tire that has almost no negative effect on dry-weather range (though they make an odd humming sound on wet pavement. They will be worth every penny to protect your family and your 100D investment, and priceless for your peace of mind. My MS snow driving experience echoes
agtdDelirium above, here's why you should ditch the OEM tires and put on dedicated snows, even for this sole trip:
I've got a lifetime of snow-driving experience in VT, CO, UT, CAN, WY and the Sierra Nevada, and 4 years of snow driving in my Model S, (plus two months towing a camper through the winter Rockies with our Model X...different story!). My first winter in the MS75D I drive confidently into an Appalachian snowstorm on the 2-month-old OEM Goodyears, heading up to Snowshoe Resort, a trip we've done multiple times prior. Moving cautiously through the storm at the gentle speed of 35 mph, a herd of deer ambled and jumped out in front of me and I had nothing but luck to get me around and between them...zero traction from the Goodyear M+S that came on the car, couldn't stop, could barely change direction. The OEM tires might as well have been summer tires...the M+Snow designation is not earned. After that trip, I did exhaustive research on every snow tire that could fit the MS and settled on the Michelin Xi3. TMC users will say the Nokian WRs and Blizzaks are also excellent, but I have little recent experience with those brands.
The Xi3 is a Phenomenal all-around tire, great in deep snow, ice, and in wet or dry conditions, although I'd wager its not the very best in any one of those individual conditions. I've easily driven through a Lake-Erie-effect (60"+ in 48 hrs) snow that was deeper than the hood of my MS, where the risk was being unable to see out the windshield...not losing grip or traction (DM me if you want the video proof). The tire gives pretty phenomenal grip on ice, although ratings indicate their are other tires that exceed the Xi3 on pure ice. And I've reluctantly driven them from NC to Florida and back, when I couldn't access my summer wheels from storage on short notice (Falcon Heavy rocket launch), and noticed almost no range loss over the OEM M+S tires. So these tires would serve you well through the BH Utah trip and all through a SoCal winter if you chose to keep them on for another trip to Big Bear/Tahoe/Mammoth. You won't regret it.
Have a great ski trip!
