April 27, 2018 – Isolated showers/thunderstorms spreading across NorCal this afternoon and evening mainly north of Marysville. Shower threat shifting southward to north of Sacramento on Saturday and Sunday. Showers possible over the Sierra Cascade range early next week, otherwise dry and warmer.
Upper low now centered off the NorCal coast will be primary weather feature over the next several days. This low is forecast to lift slowly northeast into the Pacific Northwest bringing cooler temperatures to the north state through early next week and isolated showers or thunderstorms to parts of the forecast area.
For today, daytime highs will drop from several degrees above normal yesterday to slightly below normal today. A strong delta breeze this morning should be decreasing slowly through this afternoon. The upper low center moves close enough to the coast to bring a threat of showers to the coast range this afternoon and stability progs indicate enough instability for a slight threat of thunderstorms.
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Upper low center shifts onto the coast tonight and Saturday spreading the shower and thunderstorm threat eastward. Models have been fairly consistent over the last few days in keeping any precipitation generally north of interstate 80 with the best chance of thunderstorms north of about Chico. Little change is expected in the overall forecast Sunday as the upper low slowly shifts eastward with the upper trough axis expected to shift east of the state by the afternoon.
Both GFS and ECMWF indicating a shortwave dropping down the back side of the low and into norcal on Monday. This might result in continued showers or thunderstorms over the Sierra but the remainder of the CWA should remain dry with daytime highs warming back up to near normal.
Extended discussion (Tuesday through Friday)
Cyclonic flow will likely persist over northern California on Tuesday as a broad upper trough remains over the Great Basin. This will bring a chance of mountain showers and possibly a couple of thunderstorms.
High pressure returns to the region by Wednesday next week as the upper trough shifts eastward. Look for temperatures across the Central Valley to rebound into the 80s for the middle of next week.