2009

After a long quiescent period since the record-breaking October storm, much of California stands to see substantial rainfall and much cooler temperatures this week. Two storm systems from the Gulf of Alaska will approach the state beginning late in the day on Tuesday. The first system is unlikely to bring significant precipitation, and it may be mostly dry even as far north as the Bay Area. The second system, which will approach the coast on late Wednesday, stands a much better chance at bringing  significant weather to at least the northern 2/3 of the state. This cold storm will be supported by a strong jet (160kts) and a large pool of cold air aloft. Baroclinic instability will be significant enough to support the development of a rather intense cold front, which will move into NorCal on Thursday. The front will be preceded by fairly strong winds and will contain a period of heavy to very heavy rain along with some chance for thunderstorms. As the front moves south across the state, the front will weaken, but it will bring locally heavy rains as far south as the southern Bay Area and will probably hold together long enough to bring at least some rainfall to Los Angeles.  Cold air aloft will filter across the entire state behind the front, bringing a brief period of cold-advective convective precipitation in NorCal into Friday. More significantly, this cold air will set the stage for some rather cold temperatures next weekend once skies clear. After this late-week storm, there is very little on the horizon once again, and it does appear (with unusually high confidence) that the Thanksgiving week will be mostly dry statewide and may actually be sunny in many places.  The good news is that these two systems this week will bring lower snow levels (down as low as 3500-4000 feet), and will produce significant accumulations above 5000 feet in the Sierras  (perhaps in excess of 1-2 feet). The models have started to hint at an increasingly prevalent southern branch of the jet stream in the far extended forecast. Although nothing of significance is explicitly indicated by any of the models attm, this sort of pattern evolution is typical of what often occurs during strong El Nino years. Remember–with any El Nino event, and especially with one that has only recently become significantly stronger–the main effects on precipitation in CA do not occur until most of the way through December, if not later.  It is entirely possible that the rest of fall will be relatively uneventful, only for the storm door to swing wide open for the rest of the season. There is a higher than average chance of this happening this winter. The Seasonal Outlook will be updated once the latest ENSO statistics become available this week.