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May 23, 2017 – NorCal’s first triple digit heat of the season will continue for portions of the Central Valley again today. Cooling trend begins Wednesday with near to slightly below average temperatures by Friday. Isolated late day showers or thunderstorms possible over the northern mountains on Wednesday.

Discussion

Clear skies across interior NorCal early this morning. Current temperatures are pretty similar to readings from 24 hours ago and generally range from the 40s to around 50 in the mountain valleys to the 60s and 70s elsewhere. The marine layer continues to vary in depth from around 1-1.5K feet helping to sustain the Delta Breeze where the reading at Travis AFB is in the upper 50s.

Little change in temperatures compared to Monday are expected across the region today as the upper ridge moves overhead. However, a stronger Delta Breeze is expected to bring slight cooling to the Carquinez Strait and Fairfield areas and faster cooling to the Sacramento region this evening. Isolated thunderstorm activity will be possible late today in the Sierra to the south and east of the forecast area.

Marine layer deepens tonight as upper low approaches and onshore flow increases. Models showing strong flow through the Delta developing overnight into Wednesday with cooler overnight minimums extending further inland with the Delta Breeze.

Cold front progged to drop from the PacNW into far northern California Wednesday before stalling out. The developing upper trough and stalled front may provide the focus for a few late day thunderstorms across the northern mountains.

The bigger impact for the area will be the significant cooling trend that commences Wednesday and continues through Friday. Many areas are expected to be 15-20 degrees cooler Thursday and Friday compared to today! Additionally, breezy southerly winds will affect much of the region.

Extended discussion (Saturday through Tuesday)

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Ridging builds back in for the holiday weekend with warming temperatures. Warmest temperatures expected across northern Sac Valley where highs will approach the upper 90s by Sunday and Memorial Day. Those with outdoor plans should keep an eye on the forecast as activities may be impacted by the heat.

Onshore flow will keep decent delta breeze going, so expect much cooler conditions in that region and possibly into the Sacramento area.

Weak moisture return and instability under dirty ridge may allow for a few showers and thunderstorms to develop across the Sierra Saturday and Sunday afternoons, but low confidence in just how widespread the activity will be.

For now, have introduced slight chance pops, mainly south of Tahoe. Short wave approaching the coast will shift ridge off to the east for Monday and Tuesday, bringing additional chances for convection across the Sierra. Again, low confidence in this but something to watch going forward.