To some extent, our persistent zonal flow has been the result of a weak upstream blocking pattern. Upper-leveling in the Gulf of Alaska and a broad trough in the eastern Pacific are a typical characteristic of this block. However, the slow-moving low at the Washington/Oregon coast has prevented a full-on blocking pattern and will help initiate a trend of increasingly unsettled weather. By Saturday, the deeper trough over the EPac will move onshore and produce more noteworthy precipitation in the Southwest.
Dynamically, the trough is likely to pass too far to our south to pack a heavy punch. The saving grace is deep moisture and instability will be associated with its passage, and the orographic snowfall machine should still be able to squeeze decent accumulations out of the system. Snow levels appear to flirt with the valley floor initially, highlighting that any late week precip is likely to begin as rain below the benches (all snow up high though). By Saturday morning, however, cold air streams in and light accumulations (1-3”) are possible in the valley. Ensembles are a little scattered on where the snow level will be, but the deterministic NAM suggests around 5000-7000 ft, remaining below resort bases.
In the case of the NAEFS and SREF ensemble QPF, the majority of the spread is in comparing model families rather than within each one. This leans towards a bimodal solution that can make forecasting based on model means difficult. I’ve chosen to lean on the GEFS and ARW solutions here based on prior experience more than anything. We’ll update tomorrow with hopefully increased confidence!
I would cautiously hang my hat on a forecast of 8-16” of snow for the upper Cottonwoods into Sunday morning and 6-14” for the Park City area, noting that accumulations will land on the lower end for low elevation resort bases and the high end of the range for upper elevation terrain. With the trough tracking so far south, accumulations will decrease sharply to the north, with the Ogden area looking at around 2-8” in higher terrain.