Unusual late-spring cutoff to bring interesting weather this week

A sharp upper trough is bringing strong and gusty winds to much of the state today, with some areas seeing gusts in excess of 50 mph. Winds will actually continue to increase in some areas and will remain gusty and occasionally strong through Friday. The initial cold front brought 2 lines of thunderstorms to the western slopes of the far northern Sierras, which looked very pretty from the Valley. No thunderstorms are really expected today (except perhaps for an isolated cell in the far northern mountains once again), but this will change by tomorrow. As the upper low begins to retrograde from the NE by late Thursday (a very unusual track for any system on the West Coast), a chance of deep moist convection will develop over far Southern California and over the entire length of the Sierras. In SoCal, this includes coastal and valley areas, so this may be a pretty notable event for many. I would expect isolated to scattered thunderstorms over all areas in SoCal on Friday and possibly into Saturday. In the Sierras, uniform scattered coverage will be likely on Thursday. On Friday, as the secondary upper low begins to retrograde and ease SW over NorCal, the chance of thunderstorms in the northern half of the state will increase (as it decreases in the south). Coverage will be scattered to widespread in the Sierras with scattered storms developing in the Coastal Range as well. By late afternoon Friday, some isolated storms could drift off the Sierras into the northern Sacramento Valley (north of Chico or so). Saturday then becomes quite interesting for NorCal, as the low continues to drift SW and instability as a result of very steep lapse rates (strong late-May sun angle, anyone?) increases over all areas. I expect scattered thunderstorms to develop over the Sierras, the Coastal Range, and possible the higher terrain of the Bay Area by late Saturday morning, with coverage increasing in the afternoon. Storms will be able to drift into the Sacramento Valley and possibly even into coastal areas, bringing light showers and perhaps a bit of lightning as they decay. By afternoon, I believe there will even be enough instability to  generate some surface-based convective columns in the Valley, so isolated heavy thundershowers in the Valley and even the Bay Area are possible then. On Sunday, the low continues to stick around and mid-level moisture increases a bit. I expect isolated to widely scattered thunderstorm coverage on Sunday in the Valley and possibly in the Bay Area, with numerous thunderstorms in the Sierras. The low begins to shift a bit (possibly offshore) on Monday, so mountain thunderstorms will certainly continue then but possibly even across other areas. Beyond Monday the pattern becomes very unclear. The models are beginning to indicate that some sort of low may reform out over the far eastern Pacific and then come ashore again in NorCal. If this occurs, we are looking at another (and possibly even more significant round) of thunderstorm activity at some point next week. After a bone-dry spring, it is very odd to see significant precipitation chances as we approach the summer months…


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