Chill in the air will persist – overnight patchy frost possible in the mountain valleys

September 14, 2018 – Below average temperatures for the remainder of the week. Patchy smoke and haze will continue, mainly over Shasta County.

Discussion

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Quasi-stationary longwave upper trough remains over the eastern Pacific and PacNW with the mid/upper jet slicing into central California. Skies are clear except for a band of high clouds associated with the jet arcing into the southern portion of the forecast area. Current temperatures are cooler compared to 24 hours ago, given less cloudiness and lighter winds, and range from the 30s in the mountain valleys to the 50s to lower 60s across the Central Valley. High temperatures will remain below average into early next week as the upper trough lingers, but will warm slightly inland Sunday and Monday as heights/thickness rebound as the trough weakens. Overnight lows will be chilly with patchy frost possible in the mountain valleys. Locally breezy conditions will be possible in the mountains during the afternoons the next few days, especially Saturday as a stronger shortwave moves by to our south.

Extended Discussion (Tuesday through Friday)

Extended period will start out under upper level troughing over the west coast keeping daytime high temperatures below normal and bringing breezy afternoon winds to the Sierra. At this time, it appears any precipitation will remain north of the forecast area. Upper trough shifts east of the state on Wednesday with upper ridging filling in over the west coast. Onshore flow and cool airmass will keep daytime highs well below normal with another afternoon of breezy winds over the Sierra. Models diverge quite a bit for second half of extended period with ECMWF building a ridge over the west coast Thursday and Friday while GFS digs in another trough along the west coast. Both models have been flip flopping between runs so overall confidence in the forecast for this period is lowered. In either case, forecast remains dry through the remainder of the extended period. Will trend with GFS for temperatures keeping daytime highs a little below normal.