The GFS has trended slightly farther to the west with the impending Arctic trough over the West Coast. The difference isn’t huge, but the small increase of overwater trajectory will allow a little more moisture to be entrained by the trough and produce a few more showers a bit farther south than the models indicated yesterday. The magnitude of cold air associated with the trough is still forecast to be pretty impressive–with 850 mb temps dropping as low as  -10 C in the Bay Area and -13 C in the Eureka area. Farther south, around Santa Barbara and Oxnard, 850 mb temps could still drop as low as -7 C, which is very unusual in those parts, to say the least. The -3 C isotherm will probably make it to the Mexican border. Up north, in OR/WA, 850 mb temps will be below -13 C. Most of the showers…and there probably will be some…will likely occur towards the beginning of the event, before the coldest air aloft filters in. Even so, would expect very low snow levels across CA (and down to sea level in the PacNW). The Hanford AFD this PM mentioned the similarity between the upcoming event and the cold outbreak of Dec. 20, 1998. On that day, snow fell to sea level in scattered parts of NorCal, and that spectacularly cold December afternoon was the only time that I have ever experienced true snow in the San Rafael hills (even if it was only a brief, non-accumulating shower). As for the possibility of precip with this event: at least scattered showers and possibly an isolated thunderstorm can be expected across NorCal on Thurs. and Fri. The same can be said for coastal SoCal about 24 hours later. Precip amounts will generally be under 0.25 inches, but this could translate to local snow accms. of 1-2 inches in the hills. It is worth noting that the Canadian model does indicate a more westerly track with significantly more precipitation than the GFS or ECMWF do. We’ll see.

Snow level summary: sea level north of Eureka. 500-1000 feet but locally to sea level in flurries in the San Francisco Bay Area. 1000-2000 feet in CentCal. 1500-3000 feet in the far southern parts of CA. Some very cold overnight minimums could occur in places that do not usually see them as a very cold and dry cP airmass moves in from the NE by late in the wknd. Crops and pipes are potentially at risk in this type of pattern. I don’t even think it’s worth speculating what might happen after that, as the models are having a hard enough time in the 1 week period. Looks rather concerningly dry, however, especially considering the current Palmer index designation of D1 (mod. drought) in SoCal. Stay tuned…