Powerful storm for NorCal this weekend

A fairly weak system today has brought light to locally moderate rainfall over NorCal since this morning, with only a few light showers south of Santa Cruz. More showers are likely over NorCal over the next 24 hours, with possibly a few rumbles of thunder mixed in tomorrow as a vorticity maximum moves ashore. Generally, however, rainfall will be quite light. All eyes then turn to the rapidly developing storm in the eastern Pacific currently 500-1000 miles north of the Hawaiian Islands. This storm, according to all forecast models, will exhibit  classic cyclogenesis through baroclinic processes (with a well-defined warm front, warm southerly inflow on the east side of the low, very cold air aloft wrapping around the west side of the low, a developing jet stream of over 150 kts preceding the low and allowing for divergence aloft and rapid deepening of the surface low, etc). This system will be a real treat to watch develop over the next 48 hours–it will be something more reminiscent of a cyclonic winter storm that might develop over the Plains States (which would, in that location, produce heavy snowfall and blizzard conditions in the north and severe thunderstorms further south along the cold front). The impacts of such a system in CA will be quite different, but potentially very impressive nonetheless. For such a strong and unusually well-developed storm, the models have come into remarkable agreement over the past 48 hours, rapidly deepening the surface low to  about 975 mb as the cyclone reaches a point 500 nm west of San Francisco. The low will be vertically stacked (low pressure at the surface will be directly under the low pressure aloft) and this will eventually lead to the demise of the low as it begins to move ashore (though the models have decreased the amount of weakening that is expected to take place before it makes landfall between Cape Mendocino and San Francisco). It is important to note that a low of this depth is extremely rare this close to the CA coast, and also that the models do tend to underestimate the degree to which cyclonic storms undergoing explosive deepening (bombogenesis) will actually do so. Therefore–I currently expect a powerful storm to impact NorCal later Saturday into Sunday. At the moment, it appears that this storm may be comparable to the January 4th storm of this year (which brought widespread wind damage and flooding to NorCal). There is the potential for this storm to be either significantly stronger or significantly weaker than this benchmark event, however, due to the undertainties regarding the depth of the low and the rate at which it weakens as it moves ashore. Intense rainfall and very strong winds are almost a guarantee at this point, however; it’s just a question of magnitude. Also interesting to note is the thermodynamic profile of this system as it nears the coast–there is a significant chance of dynamically-forced convective activity even well ahead of the cold front. There will be a secondary enhanced potential for thunderstorms as the low core moves ashore and colder air aloft moves in on Sunday. There may be some potential for severe storms at some point this weekend in NorCal. SoCal will see rain out of this event, but not too much–the Transverse Range will see locally heavy rainfall of 2-4 inches and the Central Coast could see an inch or two but most Southern Californians won’t be particularly impressed by this storm. After the weekend, a very warm and mild pattern will develop next week and last for the foreseeable future (whatever that means…).

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