After a couple of days of sublime snow conditions and ideal weather, major changes are now underway that will bring much warmer and wetter weather to the Wasatch.
A narrow filament of moisture, known as an atmospheric river, is being pulled from the subtropics and into California over the next couple of days, with the leftovers after passage over the Sierra Nevada and other western ranges penetrating into Utah. The evolution of this atmospheric river is well captured by the latest SSMI/AMSRE-derived total precipitable water imagery from the Space Science and Engineering Center at the University of Wisconsin-Madison.
In this loop, subtropical moisture is pulled northward just west of Hawaii where there is strong southerly and southeasterly flow between the subtropical high centered east of Hawaii and a subtropical low near the dateline (Long = -180). This moisture curves anticyclonically (clockwise) around the subtropical high and then spreads eastward toward California as the westerly flow is enhanced by the digging cyclone over the North Pacific.
A lot of large-scale weather features are coming together to bring you this weekend's weather. For example, without the extraction of moisture from the tropics west of Hawaii, the atmospheric river would likely be much less potent. It does pay to examine the entire Pacific Basin when forecasting for Utah.
Update 1:35 PM Dec 17
The subtropical low west of Hawaii is an example of a Kona Low. See Simpson (1952), Morrison and Businger (2001), and Otkin and Martin (2004).