2009

An active weather week is in store for the whole state, folks. The details of the pattern shift after about day 4 is still highly uncertain, but there is plenty to talk about regarding the next 84 hours…

A very high amplitude blocking pattern has been directing the Pacific jet well north into the Alaskan arctic for the past week. Return equatorward flow on the east side of the block has been pooling a large mass of very cold arctic air over western Canada, and now this mass is beginning to move southward. A retrogressive cutoff low will form and drift southwest from interior British Columbia over the next 24 hours, moving south just inland of the coast until the Oregon/California border, where it will probably attain a bit of overwater trajectory and pick up some moisture. The low itself will usher in dramatically colder temperatures at all levels across the entire state. Though the exact magnitude of the cold temperatures is still somewhat uncertain, all the major models bring the -6 C isotherm at 850 mb south to at least the Sacramento area. The models have recently trended very slightly less cold but much moister with the low, bringing a much higher chance of precipitation statewide. The GFS is picking up on a subtropical disturbance being picked up by the low and rotated into Southern California, potentially to dramatic effect. The real headline, at least initially, will be the very low snow levels in Northern California. Snow or sleet is expected to fall, at least briefly, on the beaches near Eureka and Arcata. Further south, in the Bay Area, freezing levels will lower to around 1000 feet, with accumulating snows above that level (though some flakes are likely at lower elevations than that). Over the Central Valley, freezing levels will generally be in the 500-1000 foot range. However…due to the convective nature of the precipitation and large breaks in the cloud cover, radiational cooling may be able to drop temperatures to near freezing near Sacramento. If this happens…some light snowfall would be possible in the Sacramento Valley during the Monday morning commute.  Even colder air will overspread NorCal later Monday and Monday night, causing temperatures to plummet. Though most of the precipitation will have moved into SoCal by this time, the models are indicating the potential for some lingering showers in the very unstable atmosphere. If any precipitation falls anywhere in NorCal after about 9 PM on Monday, it will fall as snow as the entire atmospheric column will be sub-freezing. Overnight lows on Tuesday morning will bottom out in the 20s in the Sacramento Valley, and Wednesday morning could be even colder if the incoming system is delayed slightly.  That’s another important note: if the next (much warmer) system arrives in the early morning hours on Wednesday instead of later Wednesday, some potential exists for a very rare warm advection snow event in NorCal as overrunning precipitation overspreads a very cold (potentially subfreezing)  surface layer. This is still a ways out, but is worth keeping an eye on for inland valleys and perhaps even the Central Valley. Southern California is likely to see a bigger storm out of this event than NorCal, however. While the main focus in NorCal will be cold temps, low snow levels, and perhaps some decent convective cells, SoCal will likely see heavy precipitation and strong winds beginning late Monday. The dramatic north-south baroclinic instability will produce quite the storm south of Santa Barbara–with rainfall potentially being measured in inches and snowfall in feet. Rain rates will be quite high at times, as well, since the storm is highly likely to have some convective elements.  This is all coming together very quickly, but some flash flooding and strong thunderstorms are not out of the question. Also, blizzard conditions are looking rather likely in the SoCal mountains. As mentioned previously, we get a break (though with very cold and perhaps record-breaking temperatures) on Tuesday and early Wednesday. After that…the models still disagree, and the ECMWF continues to be wetter than thge GFS (though both now have a sig. precip event). Runoff issues may be a serious problem after the heavy snowfall in SoCal on Monday and Tuesday. I suspect that both models may be missing the strength of the Wed/Thurs storm as well as subsequent storms into the weekend, given the powerful-looking low that is forming in the far West Pacific (and this is visibly absorbing the circulation center of former cyclone Nida, which still contains a lot of energy). In short…we have a very active week ahead, and this very dynamic pattern may continue for the foreseeable future…

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