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June 15, 2018 – Cooling trend continues into the weekend wit below normal temperatures. Mountain showers and t-storms will be possible Saturday into early next week. Temperatures warm above normal next week.

Discussion

An upper level trough is moving into the region today and will deepen through the weekend bringing a cooling trend.

The coolest temperatures this morning are in Delta breeze influenced areas.Winds at Travis AFB at 3 am were 26 mph sustained, gusting to 33 mph. The Delta breeze will continue through the weekend.

High temperatures today will be about 3 to 6 degrees cooler than yesterday, in the upper 80s to lower 90s in the Valley.Temperatures will drop similar amounts on Saturday and again on Sunday, with temperatures below normal levels.

The trough will also bring a chance for afternoon and evening thunderstorms to the mountains. The risk Saturday afternoon should be over the Lassen National Park to eastern Shasta County area, expanding to the Sierra north of I80 by evening as the core of an upper low drops into NorCal. This area expands southward on will continue to make its southward track on Sunday, spreading instability over the remainder of the Sierra Sunday afternoon and evening. Monday looks drier and warmer as the low shifts eastward into Nevada.

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Extended discussion (Tuesday through Friday)

Extended period starts out warmer and dry as an upper low shifts into the Great Basin. Daytime highs Tuesday are forecast to come in a few to several degrees above normal. A short wave trough dropping down the back side of the Great Basin low may bring enough instability for isolated afternoon thunderstorms over the Sierra Cascade crest but models not real consistent on this detail. Daytime temperatures will remain above normal but not overly hot in vicinity of upper low. Little change expected for Thursday with Sierra Cascade crest thunderstorm threat continuing. Upper ridging rebuilds over the west coast by the end of next week for small warming trend and more stable conditions.