May 5, 2017 – Cooler with a threat of mainly mountain showers through much of next week. Gusty north winds Sunday.
Short term discussion
Northern California currently under southwest upper level flow between high pressure over the Great Basin and a low pressure trough over the eastern Pacific. The onshore flow has brought much cooler temperatures most areas this morning and will bring daytime highs down to around normal for this time of year.
A shortwave trough pushing northwest through the Pacific Northwest will bring a threat of showers across the northern mountains this evening. Eastern Pacific upper trough splits on Saturday with southern portion forming a low off the central California coast by Saturday afternoon.
Upper level divergence will bring a threat of showers or even isolated thunderstorms over the Sierra Cascade range. Although less likely, can not rule out a threat of showers over the northern mountains as well.
Overall airmass cools still cooler on Saturday bringing daytime highs down to a few to several degrees below normal.
By Sunday, models shift upper low center into southern California. A threat of showers will continue over the northern Sierra northward to about Shasta county and over the San Joaquin valley nearest the low center.
Surface high pressure pushing in north of the low combined with northerly flow aloft will bring breezy north winds on Sunday.
Upper ridge builds over the west coast next Monday bringing daytime highs back up to several degrees above normal. North winds decrease a bit but still remain breezy and still can not rule out a threat of showers over the Sierra around back side of low.
Extended discussion (Tuesday through Friday)
Forecast models continue to be consistent with advertising northern California to be under a shortwave ridge on Tuesday ahead of an approaching disturbance for the middle of next week.
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Conditions will be fairly dry Tuesday with some scattered mountain showers possible.
Chances for showers will become more widespread on Wednesday across the entire forecast area as the next weather making system approaches. Most rain showers will remain along the coast, and it remains questionable at this point as to how much shower activity will affect non-mountainous locations.
Breezy conditions are possible Wednesday afternoon and night as valley pressure gradient strengthens with increasing lower level jet winds associated with the upper level trough.
Chances for rain will continue Thursday and Friday next week as the upper level low lingers over the west coast. High temperatures will start above normal on Tuesday, but will shift to below normal by the end of the week.