What precip type will it be?? It’s enough to drive a forecaster nuts. It now appears that the near surface layer may warm up just enough on Monday before the precipitation moves in…to perhaps consider plain old cold rain as the precip type? Well, that is what the GFS, the NAM, and extended dev2 RUC13 models suggest. The NAM aside (it is having really bad problems warming the boundary layer too much during the mid-late afternoon amidst shallow cold airmasses), both the GFS (Img 1) and RUC13 (Img 2) look somewhat similar. That said, GFS also has a tendency to overforecast 
temperatures from remnant shallow arctic airmasses as well. On the other hand, the RUC models have done a very good job with surface temperature of recent, even in the 24hr+ time range (some of the experimental RUC versions on the web go out to 36 and even 60 hours). I think Monday will be a little bit warmer than originally thought, but will it be warm enough to sustain plain rain versus freezing rain? Of course, that’s the 64,000 dollar question. Given recent performance of the models, I’d have to lean against a colder solution. There will also be wet-bulb effects when precipitation starts anyway, depending on how dry the surface airmass will be… we will be very near the center of the surface high, so I imagine surface dewpoints will be low.
What’s this upper low going to do?? Man, the 00z run of the Canadian is now showing a really impressive phasing of the the two waves into a big, almost cut-off looking upper low that just sits over the Southwest…thus holding onto precipitation much longer during the day Tuesday. I don’t think anyone (or any model) really knows how this is going evolve! The precipitation progs from the Canadian are still very impressive (see Img 3)