The problem is this. There is a tongue of very warm air forecast to move across Utah late today and tonight in advance of a closed low that is approaching Utah from the southwest. The warm tongue is forecast to extend across southern and central Utah by 6 PM MST this afternoon.
As a result, Salt Lake is not in the heart of the warm air this afternoon and our 700-mb temperatures only make it to about +15 or +16ºC. That's unseasonably warm and good enough for temperatures in the low to mid 90s, but not enough for the big 100.
The warm tongue then moves over us late this evening and tonight. By mid-day tomorrow, cooler air (cooler being relative here!) associated with the closed low is already starting to move in and 700-mb temperatures are dropping. By noon tomorrow, they are already back to +15ºC again.
Timing is everything. In this case, it appears we'll just miss out on the phasing of the warmest airmass and afternoon heating needed to get to 100. Of course there's always "hope" that the models are wrong. Hope here being purely for the psyche factor of 100. The mid and high 80s are far more desirable.