A rather sudden and significant change in the prevailing weather pattern is currently in store for the entire state of CA. After weeks of dry and rather bland weather in NorCal (though with some admittedly significant precipitation in SoCal), winter is about to make itself known throughout the West. A powerful cold front will blast in from the north late Friday night into Saturday, bringing ushering in an extended period of much colder temperatures and potentially wet weather. There is a connection to frigid Arctic air currently pooling in western Canada, and so temperatures aloft associated with the systems this week will range from moderately cold to very cold. The models continue to differ with respect to the amount of precipitation we will recieve this week (especially because most of it will be convective in nature and therefore highly variable in spatial distribution) and also the magnitude of the cold air. Models have, at various points in the past 72 hours, pointed to anything from a historic cold event with widespread sea-level snow fall to a rather mediocre storm with snow levels not dropping much below 3000 feet. My current opinion, which is close to the current consensus of the models, is that we will see something in between these two extremes. The most recent (12z) GFS shows 850 mb temperatures dropping to -2C over the Santa Barbara coast, -5 C over the Bay Area, and -8 C over the far north coast by Tuesday/Wednesday of next week. 500 mb thicknesses will be relatively even lower than these 850 mb temps would indicate–perhaps sub-520 dm near Eureka. Frozen precipitation will be the main consideration with the systems this week, and will address this issue first. With the currently progged 850 mb temps and thickenesses, I would expect inland snow levels in far NorCal to drop below 500 feet, perhaps reaching the valley floors in the North Coast interior valleys and possibly even in the Northern Sacramento Valley by Monday. On the immediate coast, snow levels will probably be between 500-1000 feet. The Sierra foothills will certainly see some snow, perhaps all snow, during this event. For the Bay Area, snow levels will generally be a little higher–more likely in the 1000-2000 foot range–but these could drop a little lower if 850 mb temps of -5 really pan out. The Central Coast will see snow above 2000 feet, but cold air trapped inland could bring the snow levels down as low as 1000-1500 feet briefly. With cold winter systems such as these, significant convective instability tends to generate locally intense precipitation. Within these heavy precip cores, downdrafts containing colder air from aloft (from the 850 mb level, in fact) often occur, and these can bring the freezing level temporarily down to the surface. Therefore, local showers of frozen precip (hail, sleet, or snow) may occur anywhere in the northern half of the state by Saturday afternoon, and throughout the state by late Monday (though in SoCal lower elevations won’t see much white stuff outside of hail). Precipitation currently looks to be significant but not very heavy, and it will be fairly spread out in time between Saturday and Tuesday. Areas above the snow line (which is actually quite a bit of the state) will see large amounts of snowfall this week, which will help the current water situation somewhat (though the liquid snow-liquid ratio will be so high that the actual water content of the snow will be rather low). Thunderstorms are possible from Eureka to San Diego at times, and any that develop will likely contain lots of small hail (as mentioned above). The models have been extremely inconsistent with what will happen after day 7, but I think it may remain cold and unsettled with periodic precip and low snow levels. Stay tuned.
