A very cold pattern is developing over the West, especially in California. The Arctic front can be seen on satellite progressing southward, with a great deal of cold air cumulus clouds behind it, eventually tapering to cold air stratocumulus where the entire air column is so cold that the atmosphere is actually rather stable (cold at the surface and aloft). Accumulating snows are likely during the next 48 hours in Seattle and Portland, and even along the immediate coastline in WA/OR. In fact, localized moderate accums. to 6 inches are possible as low as sea level where heavy convection develops. Convective shower activity will certainly be the name of the game, and thunderstorms are almost a guarantee as the surface heats up a bit in the afternoon (all the way into the upper 30s!). Lows will drop into the 20s most low elevation areas in the PacNW, with local mid 10s in the valleys and low 30s on the beaches. Now for the CA part of the story…

Eureka will see the best chance of significant shower activity in the state over the next few days. The airmass that moves in will be quite unstable as the temperatures at the surface will take some time to respond to the frigid air aloft, and so some intense showers and isolated thunderstorms are a definite possibility. In general, snow levels near Eureka will be below 500 feet, and to sea level during moderate or heavy precipitation. Even in weaker showers, small hail or sleet could begin to accumulate, even on the beaches. The Bay Area will have a harder time seeing precipitation than Eureka as the system will exhaust much of its already meager moisture before it arrives here. That said, the airmass will still be quite unstable and cold when it arrives here, which still supports the possibility of scattered showers. Snow levels will generally be around 1000 feet (give or take 500 feet), but I certainly believe that brief snow showers or flurries are possible down to sea level, esp. if precip. occurs during the late night or early morning hours. The same can be said for the Sacramento Valley and even further to the south in the San Joaquin Valley. As the system moves south into SoCal, some scattered showers could occur, esp. mountain areas. Snow levels could be below 1500 feet, bringing the possibility of a trace to 1 inch of snow accum. to the higher valleys inland from Los Angeles. Hopefully, the precipitation issues will be better resolved by this time tomorrow, but for the most part this is going to be an event watched on the radar screens and not in the models. This is a very anomalous pattern, and the models still don’t seem to be capturing the convective potential all that well at this point. Once skies clear and winds die down, some record cold overnight lows are likely in Fri/Sat nights, mainly in the Bay Area and Valleys. Lows in the 10s are possible in outlying areas, and low 20s are possible for the colder towns inland. Mid-upper 20s are possible more than one mile from the Bay or the Pacific, with near or slightly below freezing temperatures possible even in urban cores. 

Finally, it is worth mentioning that a little subtropical disturbance is currently advecting some moisture north over CA. This disturbance has popped up only recently, but I believe is may be able to bring some rain to SoCal ahead of the Arctic cold front. Stay tuned…as the most rain in the entire state could actually occur as a result of this small system. A very odd pattern, that’s for sure…