We were born and raised in the area, and lived there until 5 years ago. We had annual passes to DLR the last two years we were there, and went during cooler weather 60-70 times during that time period.
The only time we ever needed real coats, was late at night in january. I learned over time that it was much easier to dress the kids in several thin layers under a sweatshirt, rather than bring a bulky coat. I would bring knit gloves and hats too. This is when my kids were still really small, from baby to age 9.
But we have pics from a time we went for the Christmas parade on December 22nd....I'm wearing just a hoodie over a t-shirt, my daughter is wearing a jean jacket unbuttoned over a t-shirt, and my son who was under 2 at the time was wearing just a pair of sweatpants, a turtleneck, and a sweatshirt.
If you can pack layers that the kids can peel off (it might be cool in the morning) as the day warms up, and then put back on one by one as the night cools off, it might be easier on you as far as packing is concerned. We live in Boise now so for the first time in my life, I KNOW what real cold is, lol. And we would never need the Winter gear we have here back home.
I wouldn't expect early December to be that cold, honestly, especially not inland where you'll be. At the ocean, it may be breezy and chilly, but
Disneyland is far enough inland to be much warmer.
I wouldn't really expect rain, either. I can't remember one single instance of rain on Thanksgiving or Christmas in the 30 years that I lived there. I lived ON the beach for the first 25 of those and there were Christmases that we went boogie boarding, lol. The rain usually starts to hit in January and February.
I don't know where you are in Utah, but for a weather comparison, I can say that by Halloween or a few days after in Boise, it's already colder than any day I ever experienced in my whole life in Southern California. Being used to snow as you are, I would seriously doubt that you would find a coat necessary on any day of the year at Disneyland.
If you've got soft, thin, comfortable long-sleeved shirts that you can pack for the kids, with a t-shirt to go over it, and a hoodie to go over that, I can't imagine you guys being too cold. You might still want to bring the knit gloves and hats, just to keep everyone extra toasty at night, but like I said, being used to snow and cold weather as you are, you may find that a bit overkill.
hope that helps!
