Terrible.
It's winter, and in the Utah mountains. Spikes and poles if she's in the area where the base station is located. There will definitely be spots where the snow is deep, and icy places. Spare set of clothes, stove, jello powder, sleeping bag/bivy, and winter 10 Essentials. Recco and Garmin inReach. Extra phone battery. Phone kept next to body. Exhausting if you have to break trail in deep snow. Potential for falling into a tree well. Snowfields to get lost in. Frozen brook to fall into and freeze to death. Getting dumped on by a tree. Mushy snow early in the hike, which freezes around your boot later in the hike: you lose your boot. Early darkness. Avalanche risk.
I'm kinda freaked out imagining what could go wrong. I think maybe the highest likelihood is coming across a snowfield, and not wanting to turn around, because you think you're almost back to your car. Especially since her family heard from her at 3 pm, though it's possible she didn't start on the hike until afternoon.
If it were me, I'd not go on a trip like this one, except in an experienced group, and they'd better be very experienced, like Sierra Club types or the local 100-year-old climbing club. But it's also probably very beautiful back in that reservoir area.
***Pro safety tip. This is my repeat plea to learn outdoor skills in a long-time established group, with folks who've "done it all", so you're not relying on the echo chamber in your head or like-minded (not to mention, youthful and therefore definitely inexperienced) friends to evaluate risk.
Consider the experienced actor about this time last year who had an accident in the backcountry while not wearing crampons as the sheriff had warned, and also headed in the wrong direction, I suppose from disorientation in the snow). At any rate, even extensive experience can account for very little. But a group with very experienced members adds hugely to safety. There are surely several clubs in the Provo area going out on trips on a Sunday year round. Even then, the risk is not minimal, on any backcountry trip, any season.
So, I'm begging!***
Perhaps there's a backcountry shelter near Tibble Fork Reservoir because the area is busy in summer? A restroom? That would be a stroke of luck.
Mostly, I just think this is a scary situation. I hope they find her, but I worry when I see stories like this.