Very unusual weather in the works for CA

Well, it does finally appear that something genuinely interesting may occur weather-wise in CA. After nearly a year of rather dull and incredibly persistent weather patterns, a significant early-season storm system may affect the entire state next week. Here is the breakdown:

The system in question is a cutoff low that is forecast to develop over Oregon and retrograde south over/off of the coast of CA by next Wednesday. This in itself is quite unusual for this time of year–both the strong jet causing the low to form and the amplified flow causing the low to pinch off and retrograde are against climatology for this time of year. As the low drifts slowly south/southwest over CA, the low will deepen, and there will probably be enough cold air aloft for scattered convective activity (not necessarily confined to the mountains, either). Initially, since the low will have trekked mainly over land at this point, there will not be much mid-level moisture to work with and precipitation will be rather sparse and light. Dry lightning could even be a concern. In addition, as the low approaches, pressure gradients will ramp up dramatically and some strong northerly winds could develop, which will certainly have implications for the ongoing fire situation in SoCal if they develop. After this point, the forecast becomes much less clear…

The models currently disagree as to whether the low will retrograde further westward off the CA coast and then drift 300-500 miles south and stay stationary for a brief period about 750 mi. SW of San Francisco. The models, esp. the GFS, has been trending towards this scenario in the past few runs. If this were to occur, the low would be able to pick up some substantial moisture from the Pacific Ocean. Even less certain is whether the low will be able to draw subtropical moisture up from the south and advect this moisture over CA. There is currently a tropcial disturbance in the E. Pac. that may develop over the next couple of days, so this may be the moisture source the GFS is keying in on. If the low is able to pick up this moisture, the low may be able to bring significant precipitation to almost the entire state by next weekend as it lifts back to the NE. The GFS now indicates that a 150 kt jet streak will be over Central California coinciding with the time of the deepest moisture and the best upper divergence. If this pans out as currently indicated, there will be the potential for strong to severe thunderstorm activity across portions of the state. It is still far too early to be certain about any of this, but it certainly bears watching. I can’t recall ever having seen a system quite like the one the models are depicting, with a northerly origin but transitioning to a much warmer and moister system as it moves south. This is different from the typical low that picks up tropical moisture and advects it over SoCal in the summer; this will be a genuine winter-summer pattern hybrid. Certainly will be interesting to watch its development…

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