Cold and unsettled week ahead for CA

A very complicated pattern continues to evolve for the next 7-10 days. A deepening and retrogressive cutoff low is sliding south and slightly westward just inland over OR/WA. This low is dragging some very cold unmodified Arctic air with it, and will bring about a dramatic change in the weather beginning tomorrow in NorCal and Monday in SoCal. The first and perhaps most noticable change will be the sharp drop in temperatures and the increase in winds, with highs struggling to reach 50 Sunday across much of NorCal. Winds will increase as a result of pressure gradient differences and thermal instability, generally 10-25 mph with some gusts to 30 mph or so in favored places. These brezzes will produce windchills no higher than 45 degrees even in the afternoon, and mostly in the 30s all day. Some showers could begin to develop over the mountains of NorCal by Sunday afternoon, with snow levels around 2500-3000 feet. On Sunday night, even colder air aloft will continue to filter into the state from the north/northeast. 850 mb  temps will drop well below -3 C for much of CA and below 0 C for the entire state. In the Sacramento Valley and areas north and east, 850 mb temps will approach -6 C and in far northeastern CA they will be around -9 C. This translates to very steep lapse rates and very low snow levels for the resultant showers. This system is expected to be much wetter than 48 hours ago, with the NAM and GFS both depicting the potential for some locally heavy convective precipitation at times and in various locations for the southern 2/3 of the state. Some intense showers are possible as instability increases, likely including the potential for lightning and copious amounts of small hail. Snow levels will drop into the 1000-2000 foot range in NorCal by Monday afternoon, and will likely continue to drop until Tuesday morning. It is possible that showers that fall late Monday night or Tuesday morning will remain frozen all the way to sea level, especially in the Central Valley. So…flakes may fall in Sacramento and other Valley locations (including the San Joaquin Valley, as well). Snow levels will rise a bit back into the 1000-1500 foot range by Tuesday afternoon, but could drop again overnight to around sea level inland on Tuesday night/Wednesday morning. Once again, places like Sacramento stand the chance of seeing some brief snow. More likely, numerous locations will see substantial small hail accumulations at times as convective elements move overhead, but any heavy shower or thunderstorm could potentially bring a brief burst of hail/sleet/wet snow to the Valley floor at any time during this event. The inland valleys of SoCal will see some snow from this event, certainly impacting the Grapevine and other roads that may not often be affected by substantial snowfall accumulations. Precipitation amounts are still highly uncertain, given the nature of the system (retrogressive, convective, and stalling). Precipitation could stick around into midweek, and some cold overnight temperatures will chase the precip in the valleys, leading to icy roadways. Another potentially very cold system…with snow levels potentially dropping below 1000 feet…will dive south into CA by next weekend. More on that as it develops…

 All in all, a cold and potentially very unsettled week ahead…