The NAM forecast initialized at 1800 UTC 29 Jan (Saturday) has an impressive blast of cold air coming into Utah. As discussed in October, the 700-mb temperature at Salt Lake City is usually between -20C and 20C, so I use -20C at 700 mb as an indicator of "really cold air." The 84 hour forecast has sub -20C air covering much of the state of Utah, including about -21C at KSLC. BRRRR!!!!
On the other hand, the GFS forecast initialized at the same time has a somewhat different solution. The coldest 700-mb air comes through 12-h earlier, at 1800 UTC 1 Feb, with the 700-mb temperatures at KSLC only falling to -19C. Cold, but not as far out the tail of the bell curve.
Given the differences between the NAM and the GFS, I thought I would also consult the NCEP Short-Range Ensemble Forecast system (SREF). As I understand it, the SREF is comprised of 21 forecasts produced by four different modeling systems run with differing initial conditions. For the SREF initialized at 1500 UTC (the SREF runs at 0300, 0900, 1500, and 2100 UTC), the mean 700-mb temperature forecast valid at the same time as the NAM forecast above was -23C!
So, it appears an arctic blast is likely, but the severity of the cold air remains uncertain. We'll see if the models begin to converge on a solution over the next day or two.