The culprit is what meteorologists call a digging trough, which is a trough that is amplifying and "digging" southward rather than progressing from west to east. The loop for the last two days shows this rather well. Note how the upper-level (500-mb) trough (indicated by the black contours) and the surface trough and cyclone move from the northern Gulf of Alaska into the Southwest U.S.
This type of storm evolution is not uncommon over the western U.S. [for you techies, there is considerable discussion of this in a recent paper by U atmospheric scientists Lareau and Horel (2012)]. Troughs that tend to do this are typically diffluent, meaning that the flow spreads out on their downstream (eastern) side. As is the case above, there is typically strong northwesterly flow on the back (western) side of the upper-level trough, which decelerates and spreads out on the downstream (eastern) side.
The Wasatch Mountains will see some snow showers today and tonight, but it looks like a dust-on-crust event with perhaps 2-4 inches. Areas of southern and central Arizona will get more precipitation (in terms of snow-water equivalent). For more on the weather near Tucson, see Bob Maddox's MadWeather blog.
The weekend storm continues to look good. Keep your fingers crossed.
Addendum @ 9:55 AM
It's not too often that you see a winter storm warming for the Tucson area and a blizzard warning for areas as far south as the Mexican border, but that is the case today.